Object preference by walking fruit flies, Drosophila melanogaster, is mediated by vision and graviperception

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1 2494 The Journl of Experimentl iology 23, Published by The ompny of iologists Ltd doi:.242/jeb.4749 Object preference by wlking fruit flies, rosophil melnogster, is medited by vision nd grviperception lice. Robie, ndrew. Strw nd Michel H. ickinson eprtment of iology, liforni Institute of Technology, M 38-78, 2 E. liforni lvd, Psden, 925, US uthor for correspondence ccepted 7 pril 2 SUMMRY Wlking fruit flies, rosophil melnogster, use visul informtion to orient towrds slient objects in their environment, presumbly s serch strtegy for finding food, shelter or other resources. Less is known, however, bout the role of vision or other sensory modlities such s mechnoreception in the evlution of objects once they hve been reched. To study the role of vision nd mechnoreception in explortion behvior, we developed lrge ren in which we could trck individul fruit flies s they wlked through either simple or more topologiclly complex lndscpes. When exploring simple, flt environment lcking three-dimensionl objects, flies used visul cues from the distnt bckground to stbilize their wlking trjectories. When exploring n ren contining n rry of cones, differing in geometry, flies ctively oriented towrds, climbed onto, nd explored the objects, spending most of their time on the tllest, steepest object. fly s behviorl response to the geometry of n object depended upon the intrinsic properties of ech object nd not reltive ssessment to other nerby objects. Furthermore, the preference ws not due to greter ttrction towrds tll, steep objects, but rther chnge in locomotor behvior once fly reched nd explored the surfce. Specificlly, flies re much more likely to stop wlking for long periods when they re perched on tll, steep objects. oth the vision system nd the ntennl chordotonl orgns (Johnston s orgns) provide sufficient informtion bout the geometry of n object to elicit the observed chnge in locomotor behvior. Only when both these sensory systems were impired did flies not show the behviorl preference for the tll, steep objects. Supplementry mteril vilble online t Key words: rosophil melnogster, vision, locomotion, serch, Johnston s orgn, grvity. INTROUTION ll motile orgnisms explore their environment for resources using n rry of specilized sensors. Indeed, the need to serch effectively for food, mtes nd shelter in complex world hs undoubtedly been mjor selective pressure in the evolution of sensory systems (Ntion, 28). One recurrent theme in the explortory behvior of nimls is the use of different sensory modlities for different stges of serch sequence (usenbery, 992). For exmple, olfctory or visul cues my guide n niml towrd gol over long distnce wheres tctile nd gusttory cues re more importnt once n niml is in contct with potentil resource. In prt becuse of its importnce s genetic model orgnism, much is known bout how the fruit fly, rosophil melnogster, uses combintion of visul, olfctory nd mechnosensory cues while flying in serch of food. Like mny insects, however, fruit flies serch their environment not only while flying, but lso while wlking. s in flight orienttion, wlking flies rely on robust nd innte visul-motor behvior termed object fixtion to nvigte towrd conspicuous fetures within their environment. Horn nd Wehner, inspired by Reichrdt s work with fixtion in flying flies (Reichrdt nd Poggio, 975), demonstrted tht rosophil will turn towrds, nd mintin course towrds, prominent visul object (Horn, 978; Horn nd Wehher, 975). s hs been demonstrted more recently, flies will mintin heding towrd n object fter it disppers (Struss nd Pichler, 998), or even return to course towrd n object fter it disppers nd distrcter object is trnsiently presented (Neuser et l., 28). If given choice, flies preferentilly pproch the closest object, judgment mde using the motion prllx of the object s imge on the retin nd not expnsion cues (Götz, 994; Schuster et l., 22). Together, this work demonstrtes the sliency of visul objects in the locl explortory behvior of wlking rosophil. One phenomenologicl constrint in pst reserch on object fixtion in wlking flies is tht they re typiclly tested in such wy tht they cn never ctully rech the visul objects to which they orient. Götz nd collegues demonstrted tht when choosing between set of unttinble visul objects, flies show preference for nerest object, but do not demonstrte ny prticulr innte preference ccording to fetures such s size or shpe (Götz, 994). However, it is likely tht once n niml reches nd wlks onto n object it will use dditionl sensory cues to ssess whether tht object wrrnts further explortion. For exmple, mechnoreceptors sensitive to grvity or posture could inform n niml bout the surfce topology of the object it is exploring. Recent studies hve shown role for the Johnston s orgns (JO), chordotonl orgns locted in the ntenne of rosophil, in the detection of grvity in severl lbortory ssys (rmstrong et l., 26; ker et l., 27; Kmikouchi et l., 29; Sun et l., 29), but the role of grvity in ethologicl spects of locomotion, such s explortory behvior, is unknown. In this study we exmined the role of vision nd grviperception in shping the explortory behvior of freely wlking fruit flies. Rther thn studying the pproch of flies to virtul or unttinble objects, we llowed them to explore lrge ren contining ctul three- THE JOURNL OF EXPERIMENTL IOLOGY

2 Object preference in fruit flies 2495 dimensionl (3-) fetures while we trcked their locomotor behvior using simple mchine vision system. We found tht while flies re pproching objects they show little preference for different shpes of visul trgets. On reching the trget, however, they demonstrted cler preference for tll, steep objects. This preference ws mnifest by much longer residency times on tll, steep objects, which ws becuse of prepondernce of long periods during which they cese wlking. nimls lcking either visul informtion or with impired grvittionl sense still exhibited preference for tll, steep objects, but nimls with both impirments showed no preference. These results demonstrte the role of visul nd mechnosensory modlities in the explortory behvior of rosophil. MTERILS N METHOS Flies ll experiments were performed on 3-dy-old mted femle fruit flies, rosophil melnogster Meigen, selected from lbortory popultion descended from 2 wild-cught femles. The flies were mintined t 25 nd mbient humidity (2 4%) on 6 h:8 h light:drk cycle. One dy before ech experimentl tril ws performed, we nesthetized the flies on cold plte held t 4. The wings of the flies were clipped between the first nd second cross-veins, pproximtely hlf the length of the wing. If fly s grvittionl sense ws to be impired, it ws lso done t this time, by immobilizing the joint between the second nd third ntennl segments with UV-cured glue (udick et l., 27). The flies were llowed to recover with food overnight nd then deprived of food, but not wter, 4 h before the experiments were performed. ll experiments were performed during the evening pek in their circdin ctivity cycle (Shfer et l., 24). The flies were plced into individul vils with wter source nd llowed to cclimte to experimentl light levels for t lest 3 min prior to experiments. Ech fly ws used only once, nd ll trils consisted of single fly recorded for min. Wlking ren In order to study the behvior of flies exploring topologiclly complex environment we developed lrge, free-wlking ren. The ren consisted of 24.5 cm dimeter blck disk surrounded by 24.5 cm tll bcklit cylindricl pnorm of rndomized blck squres with 5% filling probbility which provided bckground visul stimulus (Fig. ). s viewed from the center of the ren ech squre subtended 5 deg. The pper printed with the pnorm ws bcklit by circulr rry of eight 35 W hlogen lights (Fig. ). Flies were mintined within the ren using therml brrier, which proved esier to regulte nd much more effective thn either wter mot or wll coted with Fluon TM (..R. nd M.H.., unpublished dt). Most flies pproched the therml brrier nd turned wy; rre experiments in which flies did escpe over the brrier before the end of the min tril were discrded. Two versions of the ren were used in these experiments simply due to methodologicl improvements tht were mde during the course of the study. ren ws equipped with wter heted therml brrier nd pssive cooling system (Fig. ) wheres ren 2 ws equipped with n electriclly heted therml brrier nd n ctive cooling system (Fig. ). lthough both systems worked, the ctive electricl system is esier to fbricte nd permits more precise control of surfce temperture. ll trils were performed in ren 2 unless noted otherwise. In ll cses in which identicl tretments were performed in rens nd 2, we verified the dt were indistinguishble nd the results were pooled in subsequent nlysis. In ren (Fig. ), the therml brrier ws.64 cm high round the pltform. It consisted of cylindricl luminum wlled chmber heted by 55 recirculting wter. The pinted luminum surfce fcing the ren ws ~38. n rry of four PU (computer processing unit) fns blowing room ir onto the bottom of the crylic ren floor pssively mintined the floor temperture. The surfce temperture profile of the ren floor ws 24 t the center nd rose grdully to 26 t distnce of 2 cm from the therml brrier, Recirculting hot wter bth Plug Therml brrier Neoprene insultor 5 cm Plug Fn Thermistor PI controller Therml brrier TE module E F Het exchnger Rditor 75 deg 36 mm 6 deg 23 mm mer 45 deg 6 mm IR-LE Hlogen light rry 3 deg mm Fig.. Experimentl pprtus. () Top-down view of the ren with bcklit pnorm. The therml brrier is depicted in red. () schemtic side view of the fly visuliztion setup. Ner-IR LEs (light-emitting diodes) mounted with the cmer bove the ren, nd two of the eight hlogen lights rrnged in circulr rry re depicted. () schemtic verticl cross section of ren with pssive cooling. Recirculting hot wter hets the therml brrier nd four PU fns cool the wlking pltform (only one is depicted). () schemtic verticl cross section of ren 2 with ctive cooling. The therml brrier is strip of glvnized steel wrpped in rope heter nd insulted from the wlking pltform by lyer of neoprene. The wlking pltform in ctively cooled by PI-controlled rry of four thermoelectric modules with wter-cooled het sinks (only one is depicted). (E) The two rrngements of cones in the ren. The ren floor is shown in grey for illustrtion purposes only; the floor nd cones were both pinted mtte blck. (F) The color-code convention used for the cones of equl lterl surfce re. The ngle between the bse nd lterl surfce, nd the height, re noted below ech cone. THE JOURNL OF EXPERIMENTL IOLOGY

3 Robie,.. Strw nd M. H. ickinson beyond which the temperture rose rpidly to 3 s mesured by thermocouple. In ren 2 (Fig. ), the therml brrier ws flush with the top surfce of the ren floor. It consisted of.2 mm thick, 24 mm wide bnd of glvnized steel wrpped with thin electric rope heter (OmegLux, Stmford, T, US), powered by vrible trnsformer (Stco, yton, OH, US) in open loop. The ren floor ws insulted from the therml brrier by thin strip of neoprene. The ren floor ws constructed of.6 cm thick luminum plte with four circulr thermoelectric (TE) modules (TE Technology, Inc., Trverse ity, MI, US) bolted to the underside, ech with wtercooled temperture exchnger. thermistor, mounted t the center of the underside of the floor, provided input to proportionl integrl derivtive (PI) controller driving the four TE modules in prllel with set point of 25. The surfce temperture of the ren floor vried by less then s mesured by non-contct infrred thermometer (OmegScope, Stmford, T, US). Flies were introduced into the ren by plcing them into blck vil with neck tht fitted securely into 3 mm hole in the ren floor. Ech fly ws llowed to crwl up the vil nd out onto the surfce of the ren, thereby voiding the effects of mechnicl gittion cused by spirting flies with mouth pipette. fter the fly entered the ren, the hole ws plugged with stopper tht ws flush with the ren floor. Flies tht did not enter the ren within min were discrded. Of the 9 individul trils ttempted for this study with this loding method, only flies (6%) filed to enter the ren by crwling up nd out of the blck vils. Thus, there is no evidence tht our dt re bised by indvertently selecting ginst flies with wek grvitxis behvior. In trils using flies tht hd their ntenne mnipulted (which do exhibit reduced negtivegrvitctic response) we gently tpped the nimls into the ren from bove. The floor of the ren ws wshed with detergent nd rinsed between ech tril. Fly visuliztion nd trcking t were collected using digitl cmer mounted 48 cm bove the ren floor with 72 nm high pss opticl filter (R72, Hoy Huntington ech,, US; Fig. ). The flies were visulized using ner-ir (infrred) light, which reflects well off of the fly s cuticle, nd the ren floor ws pinted mtte blck to mximize contrst. In ren, we used cmer (Scorpion, Point Grey, Richmond,, nd) with 6 2 pixel resolution. Imge stcks were collected t frmess nd nlyzed in rel time by custom softwre progrm developed in MTL (Mthworks, Wlthm, M, US). In ren 2, we used cmer with pixel resolution (622F, sler, Exton, P, US). Using this cmer, imges were collected t 2 frmes s nd nlyzed in rel time using Motmot, open source cmer softwre written in Python, using the FlyTrx plug-in (Strw nd ickinson, 29). oth trcking progrms determined the fly s two-dimensionl (2-) position nd body orienttion with 8 deg mbiguity bsed on bckground subtrction. The imges of the flies in our movies re pproximtely ten pixels long nd five pixels wide. For ech frme, cropped imges of pixel region round the fly (used for testing utomted lgorithms) were sved long with the 2- coordintes of the fly, body xis ngle nd time stmp. single full resolution imge of the ren ws lso sved. ll dt were collected in ren 2 unless otherwise noted. Empty ren To exmine the role of visul input on bsic locomotor ctivity, 66 individul flies were trcked within n empty ren (i.e. void of the conicl objects), surrounded by the rndom checkerbord pnorm. Hlf the flies were tested under lit conditions (45 lux mesured t the center of the ren) nd hlf tested in complete drkness. To chieve these conditions, we replced the trnslucent cylinder with n opque blck cylinder nd ll mbient light ws eliminted from the room (mesured illuminnce lux). Exmple trjectories nd speed profiles re shown in Fig. 2,. We present exmples tht re representtive of the dt nd hve n ren crossing in the fifth minute in order to show the difference in the speed profiles of flies in light versus drk conditions. ren with objects To test the effect of more complex topology on the flies explortory behvior, we plced four right ngle cones of equl lterl surfce re but of differing heights nd slopes in the ren. The geometric dimensions of these cones nd the color code tht will be used throughout the pper to identify cone type re shown in Fig. F. Under these conditions, we performed 45 trils (2 in ren nd 25 in ren 2). Ech object (pinted blck to mtch the floor nd llow visuliztion of the flies while they were on the object) ws plced in one of four fixed loctions, mking squre within the ren, but the reltive order ws rndomized between trils (Fig. E). The objects were wshed with detergent nd rinsed between trils. To test whether the ssessment of the objects by the Velocity (mm s - ) Time (min) Fig. 2. Exmple trjectories nd corresponding velocity plots. Ech min trjectory is plotted in gry with the fifth minute plotted in blck. The speed profile for tht sme period is plotted on the right. Trils run in drkness re shown with gry bckground. In trjectories with cones present, the footprint of ech cone is indicted ccording the color scheme in Fig. F. Representtive trces where chosen for the following cses: () empty ren with lights on, () empty ren in drkness, () four cones with lights on, nd () four cones in drkness. THE JOURNL OF EXPERIMENTL IOLOGY

4 Object preference in fruit flies 2497 flies ws bsolute or reltive, in one set of experiments we removed the tllest, steepest object nd rrnged the cones in the sme grid leving one spot empty (Fig. E) in 24 trils. To test the role of visul input on object explortion we performed nother 45 trils in complete drkness (2 in ren ren nd 25 in ren 2). Exmple trjectories nd speed profiles re shown in Fig. 2,. To test the role of grvittionl senstion on object explortion we performed 4 trils with flies whose ntenne were immobilized t the joint between the second nd third segments. Finlly, to test the combined effect of the sensory mnipultions, we performed 4 trils using flies with immobilized ntenne in complete drkness. t nlysis The positionl nd orienttion dt were recorded in rel time but were post-processed using custom softwre written in Python (www.python.org) nd MTL (Mthworks, Wlthm, M, US). ll trils were reviewed by exmining the stored video record with the trcking dt superimposed. ny trils with gross trcking errors (e.g. fly position ws lost) were discrded nd not included in the enumertion of tril numbers used for nlysis. Of 266 trils recorded for this study, only six were discrded for trcking errors. For ech tril with cones present, the loctions of the cones were digitized nd used to determine the periods of the tril in which fly ws exploring ech cone. ecuse of the cone steepness nd the centrl position of the cmer, flies exploring the fr side of cone could hve been incorrectly clssified s off cone with the use of simple digitiztion bsed on the footprint of the cone. To prevent this, the digitized footprint ws expnded such tht fly whose center did not pper to be within the footprint of the cone, but ws indeed on the cone ws correctly clssified s on cone. The ssignment of on or off cone ws mnully checked ginst the sved video for ech tril. In trils without cones present or off cone, the 2- position of the fly ws smoothed with Klmn smoother (Kevin Murphy s Klmn filter MTL toolbox) nd used to clculte trnsltionl speed nd totl distnce trveled. For trils with cones present, we clculted the 3- position of the fly on the cones using the trcked 2- positions, model of the 3- structure of the ren, nd stndrd pinhole cmer model. The 3- model of the ren ws creted from the known geometry of the ren nd cones nd hnd digitiztion of the cone positions in ech tril. The surfce of this model ws extruded by mm s n pproximtion for flies own height bove the floor. Through ech 2- fly position on the clibrted imge plne of the cmer, we projected ry (from the 3- loction of the pinhole cmer model center) nd intersected it with the extruded 3- model of the ren to find the estimted 3- position of the fly. We clculted the 3- positions for second time with fly height of 2 mm nd used the mgnitude of the difference between the two z-position dt sets s n estimte of the error in the 3- positions. The 3- position of the fly ws smoothed with Klmn smoother using the error estimte to ssign the uncertinty in the observtion dt. We evluted the qulity of the 3- position estimtes on the tllest, steepest cone (nd the stop wlk ssignment described below) by recording simultneously with second cmer mounted directly over this cone, nd found tht both the 3- estimte nd stop wlk ssignment were ccurtely determined. The temporl structure of the flies locomotor ctivity cn be corsely modeled s discrete bouts of wlking nd stopping (Mrtin, 24). We mnully ssigned wlks nd stops in subset of dt (both on nd off cone) bsed on the smll formt imges. Using these clssifictions s ground truth, we defined stops nd wlks bsed on velocity (3- velocity when on cone) using dul threshold: when the velocity ws bove the high threshold (2.5 mm s ) the fly ws clssified s wlking nd when the velocity ws below the low threshold ( mm s ) the fly ws clssified s stopped. When the velocity ws between the two thresholds it mintined its previous clssifiction until the second threshold is crossed. This Schmitt trigger voids rpid chnges in clssifiction cused by single threshold bsed definition. We lso defined the minimum wlk durtion to be. s (two frmes t 2 frmes s ) to void misclssifying s wlks the trnsient center of mss movements ssocited with grooming. We defined the minimum stop durtion to be. s to void incorrectly ssigning s stops the brief decrese in trnsltionl speed ssocited with shrp turns nd puses. Using these criteri, we determined the percentge of time ech fly spent wlking or stopped nd the durtion of ech wlk nd stop bout, s well s the men nd mximum trnsltionl speeds during ech wlk bout. We set mximum wlking speed threshold of 5 mm s to filter out rre events in which the wing-clipped flies jumped within the ren. On cone locomotor ctivity sttistics were only clculted for trils performed in ren 2, in which we estimted 3- velocity. dditionlly, we used the estimted fly z- positions to determine the height t which ech stop ws performed when the flies were on cone. The body orienttion mbiguity ws resolved using vrition of the Viterbi lgorithm in which orienttion flips nd wlking rpidly bckwrds were penlized (rnson et l., 29), nd we then clculted men ngulr speed during wlking periods. Using method for estimting position nd orienttion error bsed on trjectory segments of constnt velocity (rnson et l., 29), we found the orienttion trcking error to be.5 degrees for the off cone dt. s cn be seen in supplementry mteril Movies nd 2, the orienttion trcking is highly ccurte nd it is unlikely n expert humn could do better. Sttistics Much of our dt were not normlly distributed (nor trnsformble to norml distribution) therefore, throughout the pper we present the distribution of results using box-nd-whisker plots in which the centrl line (colored mgent when on colored bckground) indictes the medin, the box outlines the interqurtile rnge of the dt, nd the whiskers encompss the rnge from minimum to mximum vlue, excluding ny outliers. Outliers (indicted by smll cross) re vlues tht re more thn.5 times the interqurtile rnge below or bove the 25th or 75th percentiles, respectively. We used vrious sttisticl tests in the nlysis of our dt; depending upon the ssumptions of the tests met by the dt, we lwys used the most powerful test possible. If the dt were independent nd norml, we used heteroscedstic t-test. If the dt were independent but ny of the sets being compred were not norml, then we used Mnn Whitney U-test. In some cses our dt were not independent becuse fly cn only be in one loction of the ren t time. If the dt were not independent we used Wilcoxon signed rnk test, nd finlly if the dt hd lrge number of tied scores we used Kolmogorov Smirnov test. Neither the Wilcoxon or Kolmogorov Smirnov tests require tht the dt be norml. In ll cses where dt were being compred multiple times we used onferroni correction for multiple comprisons to djust the P-vlue ppropritely. ll sttisticl nlysis ws performed using SPSS (SPSS Inc, hicgo, IL, US). To report the results of our significnt tests we use letter code where the groups lbeled with the sme letter re not significntly different. group cn hve more thn one lbel which indictes THE JOURNL OF EXPERIMENTL IOLOGY

5 Robie,.. Strw nd M. H. ickinson tht it is not significntly different from ny of the groups lso lbeled with ny of those letters. For experiments with cones present, we compred the results of the experiments within tril type, compring effect of cone type in given tril condition. Throughout the pper we indicte the results of within tril type hypothesis testing with blck lowercse letters. For exmple, the results of compring the encounter rtes in Fig. 5 re indicted with lowercse letters showing tht the blue, green nd yellow cones re not significntly different, nor re the yellow nd ornge cones, e.g. the blue nd green cones re significntly different thn the ornge cone. When multiple tril conditions were tested (such s different sensory mnipultions) we lso compred the results cross tril type, compring effects of tril conditions on the response to ech cone type. We denote the results of cross tril type hypothesis testing with uppercse letters (colored to highlight which cone type is being compred). We only compre the sme cone type cross different tril conditions. For exmple, the results of compring the percentge of time spent on the blue cone cross trils with different sensory mnipultions in Fig. 7 re indicted with uppercse blue letters showing tht pnels, nd re significntly different, but pnel is not significntly different from pnel or. RESULTS Visul input modultes locomotor behvior To test how visul input or the lck thereof ffects locomotor behvior, we trcked individul strved flies s they explored the lrge free-wlking ren for min (N 66). Hlf the trils were performed in complete drkness (except for the ner-ir light used by the trcking system). Smple trjectories nd trnsltion speed profiles re shown in Fig. 2. Using the trcked x y position nd body orienttion of the fly, we clculted bsic sttistics of wlking behvior (Fig. 3). Without visul cues (i.e. in the drk) flies trveled longer totl distnce, not becuse they trveled t higher men speed, but becuse they spent more of their time wlking (Fig. 3,,). In lit rens, flies reched higher mximl speeds but spent less time wlking (Fig. 3,). The trjectories of flies in lit rens ppered strighter thn the trjectories of flies in drk rens (Fig. 2), n observtion tht ws confirmed by compring the men ngulr speed of the flies under the two conditions (Fig. 3E). The differences in bsic locomotor behviors due to visul input were for the most prt conserved in flies (N 9) exploring the floor of n ren with 3- objects present (Fig. 3 E). In the presence of cones, flies spent more of their time wlking while on the ren floor (Fig. 3), nd wlked t higher men speed (Fig. 3) thn they did when the cones were bsent. uriously, this cone-dependent chnge in behvior ws present even in conditions of drkness when the flies could not see the cones. This result suggests tht some mechnicl effect of encountering cone stimultes generl locomotor ctivity with time constnt tht lsts longer thn fly s immedite interction with the object. Flies spend more time on tllest, steepest cone To determine how topologiclly complex environment influences the explortory behvior of flies, we trcked individul flies for min in n ren with four cones of equl lterl surfce re but differing height nd slope (Fig. F). s illustrted by the trjectory in Fig. 2 nd supplementry mteril Movie, the presence of the cones qulittively ltered the overll explortory behvior in the ren. Flies pper to orient towrds cones from distnce nd, once encountered, climb on top of them. To test if prticulr cones were more ttrctive thn others, we mesured the percentge of the min tril tht the flies spent on ech cone, s well s the ren floor (Fig. 4). For simplicity, we will often refer to the cones by the color codes indicted in Fig. F. Thus, the blue cone is the tllest, steepest cone; the green cone is the next tllest, steepest cone, etc. It is importnt to note, however, tht these colors re simply code for cone shpe; the ctul color of the cones ws blck in ll Totl distnce trveled (m) Men speed while wlking (mm s ) Mximum speed while wlking (mm s ) % Time wlking Men ngulr speed (deg s ) E Empty ren Lights on Lights off ren with cones Lights on Lights off Fig. 3. Visul informtion influences the bsic sttistics of wlking. The two leftmost box plots in ech pnel show dt for flies exploring n empty ren in the light (white, N 33) nd in the drk (gry, N 33). The two rightmost box plots show dt from flies exploring the floor of the ren with cones present in the light (white, N 45) nd in drkness (gry, N 45). () The totl distnce trveled by individul flies during min tril. () The men speed while the flies were wlking. () The mximum speed clculted while the flies were wlking. () The percentge of time the flies spent in the wlking stte, normlized for the totl time spent on the floor of the ren when cones were present. (E) The men ngulr speed clculted while the flies were wlking. Sttisticlly comprisons were mde using heteroscedstic two-smple t-tests unless the dt were not normlly distributed in which cse the Mnn Whitney U-test ws used (,). sterisks indicte significntly different distributions (P<.5 with onferroni correction) between the indicted pirs of dt; crosses denote outliers. THE JOURNL OF EXPERIMENTL IOLOGY

6 Object preference in fruit flies 2499 experiments. From inspection of Fig. 4, it is cler tht the flies spent much more time on the tllest, steepest cone (blue). s shown in Fig. 4, the time spent on the tllest, steepest cone ws significntly lrger thn for ll other cones nd even lrger thn the time spent on the ren floor when it is normlized for re. This strong, differentil response to the tllest, steepest cone is not consistent with wht would be expected from rndom wlk explortion of the ren surfce s the cones ll hd identicl surfce re. Flies mke bsolute judgment of cone geometry Flies might spend more time on the tllest, steepest cone becuse of some bsolute sensory cue they perceive bout this object or, lterntively, they might mke reltive ssessment by compring it with other objects in the ren. To test whether the flies preference for the tllest, steepest cone ws bsolute or reltive, we removed the blue cone (Fig. E) nd repeted the experiments. When the blue cone ws bsent, the flies did not spend significntly more time on the green cone thn they did when the blue ws present (Fig. 4,E). These results suggest tht the flies response to the slope nd height of ech cone is bsolute, nd is not mde by reltive comprison. However, when the blue cone ws bsent, flies did spend slightly more time on the remining cones, s evidenced by expnded interqurtile rnges for the green nd yellow cones in Fig. 4E. Such bis is expected becuse, without the blue cone present, the flies hd more time to encounter nd explore the other three cones in the ren. To tke this effect into ccount we creted pseudo removl dt from the results of the originl four-cone experiments by excluding ll segments spent on the blue cone nd scling the remining time to be % (Fig. 4,F). The results of the pseudo removl experiment were not significntly different from those in the rel removl experiment (Fig. 4E,F), supporting our conclusion tht flies preference is medited by n bsolute mesurement of cone geometry. Flies encounter cones with equl frequency The flies my hve spent more time on the tllest, steepest cone becuse they oriented towrds it more frequently (i.e. it ws more ttrctive from fr), or becuse, once encountered, they tended to spend more time on it before returning to the ren floor. In order to test between these two lterntives, we clculted the flies rte of encountering ech object. Fig. 5 shows the individul encounter rtes for ech fly to ech cone, rnked by totl number of encounters from highest to lowest. lthough there ws lrge rnge of encounter rtes cross individuls, when we exmine the percentge of encounters for ech cone type, it is cler tht the popultion encountered ech cone type with equl probbility. The one exception ws the shortest, brodest cone (ornge), which the flies encountered t slightly lower rte thn the blue nd green cones (Fig. 5). These results do not indicte whether the flies encountered the cones by chnce, s in rndom wlk. However, s cn be seen by compring locomotor trjectories in the presence nd bsence of cones (Fig. 2), the presence of the objects in the ren strongly structured the flies explortory behvior. Subjectively t lest, it Individul flies 45 2% b c d e Floor E Individul flies 24 Rel removl,, b b Rel removl Floor F Individul flies 45 Pseudo removl b c d Floor Pseudo removl Fig. 4. Flies spend more time on tllest, steepest cone. olor-coded (see Fig. F) horizontl br grphs show the percentge of the min tril tht ech fly spent on the four cones nd the floor of the ren (white). The dt re rnked by the time spent on the blue () or green (,) cone. () t for trils with ll four cones present (N 45). () t from trils in which the tllest, steepest (blue) cone ws removed from the ren (N 24). () Pseudo removl dt creted by scling the dt from () fter excluding visits to the blue cone (see text for detils; N 45). ( F) The distributions of the dt in,, nd respectively, re shown fter normlizing for re of the surfces being explored. The results of sttisticl tests re indicted with letter code; groups lbeled with the sme letter re not sttisticlly different nd group cn hve more thn one lbel, indicting tht group(s) with ny of the sme letter re not significntly different (for more detils see methods). cross tril sttisticl tests compre given cone type cross experimentl conditions nd the results re denoted with uppercse letters of the color indicting the cone type being compred (color code from Fig. F nd uppercse blck letter ren floor). omprisons were mde using Mnn Whitney U-test with onferroni correction, P<.5. Within tril sttisticl tests compre the different cone types in given experimentl condition nd homogenous groups re denoted with lowercse blck letters. omprisons were mde using Wilcoxon s signed-rnks test with onferroni correction, P<.5. THE JOURNL OF EXPERIMENTL IOLOGY

7 25.. Robie,.. Strw nd M. H. ickinson Individul flies 45 pproch ngle Encounters min one Fly % pproches 3 2 Rel cone dt in light % Encounters Pseudo cone dt,b b Rel cone dt in drk E pproch ngle (deg) Fig. 5. Flies encounter objects of differing geometry t similr rtes. () Horizontl br plots indicte the encounter rtes of ech cone type for ech fly, rnked ccording to totl encounter rte. () ox plots show the percentge of encounters for ech cone type (N 45). See Fig. F for color code. The Wilcoxon signed rnk test for non-independent, non-norml dt ws used to compre groups (P<.5 with onferroni correction for multiple comprisons). See Fig. 4 for explntion of letter codes for homogeneous groups; crosses denote outliers. () The frequency distribution of pproch ngles to ll cone types in the light. () The frequency distribution of pproch ngles to pseudo cones footprints creted from the dt set in which no cones were present. (E) The frequency distribution of pproch ngles to ll cone types in the drk. ppered s if the flies often wlked towrds the cones. To quntify this effect, we exmined the flies body orienttion reltive to the tngent of the circumference of the cone footprint in the frme before they encountered ech cone. There is cler pek in the histogrm of the bsolute vlue of pproch ngle ner 9 deg (Fig. 5), suggesting tht flies mde directed pproch to the cones rther thn rndomly encountering them. We compred this to pseudo cone dt, in which we nlyzed fly trjectories from trils with no objects in the ren s if there hd been cones present. These control dt show no distinct pek in pproch ngle s in the cse with rel cones present (Fig. 5). Furthermore, we exmined the distribution of pproch ngles when there were rel cones present, but under drk conditions (Fig. 5E). These pproch ngle dt resemble our pseudo cone condition, supporting the conclusion tht the flies orient towrd the cones using visul cues. These nlyses, together with the results showing tht flies exhibit little Individul flies Normlized residencies frction 45 5 Time (min) s Residency durtion (s) umultive sum of normlized residencies s % Residencies >3 s Residency durtion (s) b c c THE JOURNL OF EXPERIMENTL IOLOGY Fig. 6. Flies exhibit long residency times on the tllest, steepest cone. () Ech row represents the time series dt of single fly (N 45). The color (see Fig. F) indictes the identity of the cone the fly resides on nd white spces indicte periods spent on the ren floor. () Normlized frequency distribution of log of the residency durtions by ll flies on ech cone type from dt plotted in. () umultive sums of the normlized frequency distribution of ll residency durtions by ll flies. The inset shows the distribution of the percentge of individul flies residency times longer thn 3 s. Sttisticl comprisons were mde using Kolmogorov Smirnov twosmple test (P<.5, with onferroni correction); crosses denote outliers. See Fig. 4 for explntion of letter codes for homogenous groups.

8 Object preference in fruit flies 25 preference in encounter rte (Fig. 5), suggest tht flies ctively orient towrds objects, but do not demonstrte preference bsed on the geometry of the objects on their retin. The slightly lower encounter rte with the ornge cone suggests, however, tht there my be lower limit for detection of this cone from distnce. Given tht the percentge of encounters did not vry cross the three tller, steeper cones, ny difference in object ttrctiveness during pproch cnnot underlie the preference for the tllest, steepest cone. Incresed residency time on tllest, steepest cone fter ruling out encounter rte s the cuse of the flies preference to spend time on the tllest, steepest cone, we next exmined how long the flies remined on ech object once they hd reched it. Fig. 6 shows time series plots indicting ech fly s loction throughout the tril nd the residency times on ech cone type. From the dt in Fig. 6, it is cler tht the flies visits to the blue cone were much longer thn their visits to ny of the other cones, nd longer even thn most periods of exploring the ren floor (Fig. 6). We hve plotted the normlized popultion distributions of residency durtions on ech cone type in two wys. First, we plotted the normlized histogrms of the log of residency time for ll flies on ech cone type in Fig. 6, which shows tht the flies distribution of residencies on the blue cone were shifted towrds higher vlues. Second, we plotted the cumultive sum of the popultion dt for ech cone type in Fig. 6, which shows lrger portion of long durtion residencies on the blue cone thn ny other cone type. The inset in Fig. 6 shows the frction of ech individul fly s residencies tht were longer thn 3 s for ech cone type. The results show prepondernce of long residency times on the tllest, steepest cone (blue). lthough 3 s ws somewht rbitrry choice, the reltionship holds over rnge of thresholds for long residency. The results of Fig. 3 suggest the flies perform n bsolute rther thn comprtive mesure of cone geometry. To further rule out role for short-term memory in the ssessment of cones, we lso exmined the residency durtions on given cone type prsed ccording to the previous cone visited. s shown in supplementry mteril Fig. S, the type of cone visited immeditely before hd no effect on the distribution of residency times, indicting tht dwell time on prticulr cone does not depend on the prior history of cone visits. b c d e,b b,c c d Individul flies %,,,b b,c c c,d,,b,b,b b 8 8 Individul flies Fig. 7. Sensory mnipultions influence flies preference for the tllest, steepest cone. Horizontl br grphs show the percentge of the -min tril tht ech fly spent on ech of the four cones nd the ren floor (s in Fig. 4). ox plots show distribution of dt fter dividing by the surfce re, which ws identicl for ech cone. () Intct flies in the light (N 45). () Intct flies in complete drkness (N 45). () Flies with ntenne immobilized in the light (N 4). () Flies with ntenne immobilized in complete drkness (N 4). See Fig. 4 for description of sttisticl nlysis nd explntion of letter codes for homogenous groups. rosses denote outliers. THE JOURNL OF EXPERIMENTL IOLOGY

9 252.. Robie,.. Strw nd M. H. ickinson Sensory modlities involved in preference for tllest, steepest cone Together, the results in Fig. 6 show tht the flies, once they reched the tllest, steepest cone, remined on it for longer thn the other cones. To investigte wht sensory modlities underlie this preference, we repeted the experiments on flies with deficits in their visul nd grvittionl senses. We impired vision simply by running trils in complete drkness (flies could still be seen by the ner-ir-sensitive trcking cmer becuse of 85 nm lighting), nd we impired grvittionl sense by gluing the joint between the second nd third ntennl segments, mnipultion tht disrupts the function of the Johnston s orgn (udick et l., 27; uistermrs et l., 29; Sun et l., 29). It is importnt to note tht the flies with immobilized ntenne exhibit robust locomotor behvior during explortion s mesured by our bsic sttistics (compre supplementry mteril Fig. S2 with Fig. 3). Fig. 7 shows the percentge of time spent on ech cone type rrnged ccording to Punnett squre of the two sensory bltions. Intct flies in the light exhibited the norml behvior, s seen in Fig. 4 (Fig. 7). Interestingly, flies with either sensory mnipultion (visul, Fig. 7; mechnosensory, Fig. 7) showed firly typicl responses to cone geometry, wheres flies with impirments of both visul nd grvittionl senses exhibited gretly diminished preference for the blue cone (Fig. 7 nd supplementry mteril Movie 2). These results suggest tht either vision or ntennl mechnosensory modlities lone provide cues sufficient to estblish fly s preference for the tllest, steepest cone. Only with both modlities compromised were the flies unble to ssess the properties of the tllest, steepest cone nd thus unble to exhibit preference. ecuse we could not ssume priori tht the sme behviorl chnge (incresed residency time) underlies the behvior of flies tht hd undergone sensory mnipultions, we exmined the cone residency durtions for these flies. Fig. 8 shows tht flies with either single sensory mnipultion exhibited long durtion residencies on the blue cone. However, the flies with grvittionl senstion impirment but intct vision (Fig. 8) showed significntly stronger responses to the two tllest, steepest cones (blue nd green) compred with intct flies in the light b c c, b c c,b b,c c, c b b,c c c % Residences >3 s % Time stopped b b,b,b b,b b,c c c % Residences >3 s % Time stopped Fig. 8. Sensory mnipultions influence residency times. The percentge of residency durtions tht were longer thn 3 s under four experimentl conditions. See Fig. F for color code. () Intct flies in the light (N 45). () Intct flies in complete drkness (N 45). () Flies with ntenne immobilized in the light (N 4). () Flies with ntenne immobilized in complete drkness (N 4). Sttisticlly significnt differences within nd cross trils were determined using the Kolmogorov Smirnov two-smple test (P<.5, with onferroni correction); crosses denote outliers. See Fig. 4 for explntion of letter codes for homogenous groups. Fig. 9. Sensory mnipultions ffect the percentge of time spent stopped on cones. olor-coded (see Fig. F) box plots indicte the percentge of time stopped on ech surfce of the ren, with white indicting the ren floor. () Intct flies in the light (N 25). () Intct flies in complete drkness (N 25). () Flies with ntenne immobilized in the light (N 4). () Flies with ntenne immobilized in complete drkness (N 4). Sttisticlly significnt differences within nd cross trils were determined using the Mnn Whitney U-test (P<.5, with onferroni correction); crosses denote outliers. See Fig. 4 for explntion of letter codes for homogenous groups. THE JOURNL OF EXPERIMENTL IOLOGY

10 Object preference in fruit flies 253 (Fig. 8), not distinguishing between these cones, nd exhibited lrger rnge of responses to ll the cones. This my indicte tht the visul mechnism used to ssess the qulity of cone is less ccurte thn the mechnisms using the grvittionl sense. ltertion of locomotor pttern during object explortion fter hving demonstrted tht flies cn use either visul or mechnosensory cues to ssess the geometry of the objects they re exploring, we next wnted to determine how flies use this informtion to lter their locomotor behvior. The flies explortory behvior in the ren cn be modeled s periods of wlking nd stopping. We pplied our behviorl definition of wlks nd stops to the flies trjectories (see Methods) nd quntified the percentge of time stopped on ech surfce of the ren (ll four cones nd the floor; Fig. 9). ecuse we ssigned ll frmes of ech trjectory s either wlks or stops, the percentge of time wlking is the inverse of the stop dt nd is not shown. The flies with unimpired vision nd intct grvity senstion (Fig. 9) were stopped for the mjority of the time they were on the blue cone. This ws lso true of the flies with single sensory impirments, (visul, Fig. 9; grvittionl, Fig. 9). onversely, flies with both visul nd ntennl mechnosensory impirments (Fig. 9) spent significntly less of their time on the blue cone in stopped stte. Intct flies in the light nd the single sensory bltion flies ll spent less time stopped on the yellow nd ornge cones thn the blue cones. y contrst, the flies with both visul nd grvittionl sense impirments could not distinguish between ny of the cone types s mesured by their time stopped. Flies of ll types consistently spent the mjority of their time on the ren floor wlking rther thn stopped. The intct flies locomotor pttern in the light shifted to higher percentge of time stopped when residing on the cones, with the lrgest shift on the blue cone. Thus, this increse in percentge of time stopped is likely responsible for the lrge percentge of time spent on the tllest, steepest cone. The frequency of stops did not chnge in systemtic wy ccording to cone type (..R. nd M.H.., unpublished dt), however the durtion of stops did vry ccording to the type of cone the fly ws exploring. The cumultive sums of the percentge of stop durtions for ll stops by ll flies re presented in Fig., with the portion of ech individul fly s distribution of stop durtions tht ws longer thn s shown in the inset. Fig. shows tht intct flies in the light performed lrger percentge of long stops on the blue cone thn they did on the yellow nd ornge cones. Flies with single sensory impirments, visul (Fig. ) nd ntennl mechnosensory (Fig. ), still exhibited significntly more long stops while exploring the blue cone thn the yellow nd ornge cones. Their responses to the cones were not significntly different thn those of the intct flies in the light. y contrst, flies with impirments in both their visul nd grvittionl senses (Fig. ) performed few long stops on ny of the cone types, indicting their inbility to sense cues tht would llow them to ssess cone s geometry. Flies perform stops t the top of tllest, steepest cone Hving determined tht the ssessment of cone geometry plys role in structuring locomotor behvior, we were interested in where the flies stopped on the cones. The colored histogrms in Fig. show the frction of stops performed by the flies on ech cone t given elevtion, nd the elevtions of long stops (defined in Fig. ) re shown by the superimposed blck histogrms. The intct flies umultive sum of normlized stops % Stops > s % Stops > s,b b,c c ,b b b % Stops > s % Stops > s,,b b b Fig.. Sensory mnipultions influence distributions of stop durtions. Ech pnel shows cumultive sums of the normlized distribution of stop durtions, with insets showing the percentge of individul flies stop durtions tht were longer thn s (see Fig. 6). See Fig. F for color code. () Intct flies in the light (N 25). () Intct flies in complete drkness (N 25). () Flies with ntenne immobilized in the light (N 4). () Flies with ntenne immobilized in complete drkness (N 4). Sttisticlly significnt differences within nd cross trils were determined using the Kolmogorov Smirnov two-smple test (P<.5, with onferroni correction); crosses denote outliers. See Fig. 4 for explntion of letter codes for homogenous groups Stop durtion (s) THE JOURNL OF EXPERIMENTL IOLOGY

11 254.. Robie,.. Strw nd M. H. ickinson Elevtion of stop (mm) Fig.. Flies tend to stop t the top of the cones. Horizontl br grphs show the frction of ll stops (colored) nd long stops (blck) tht were performed t given elevtion. Ech column represents the stops on given cone type, color code s in Fig. F. The dshed blck line in ech column is the height of the top of tht cone; stop elevtions cn be tller thn the height of the cone becuse we included the flies body height ( mm) in our 3- model. Ech row is different sensory condition: () intct flies in the light (N 25), () intct flies in complete drkness (N 25), () flies with ntenne immobilized in the light (N 4) nd () flies with ntenne immobilized in complete drkness (N 4). In, the top bin of the green nd yellow histogrms is truncted t 5% for presenttion purposes; the rel vlues re 56% (green) nd 65% (yellow) Frction of stops in the light clerly hd preference for stopping t the top of the tllest, steepest (blue) cone, nd the long stops were lso primrily t the top of the cone (Fig. ). These flies lso performed the mjority of their stops t the top of the green nd yellow cones but not the ornge cone. Flies with single sensory mnipultions (Fig., visul; Fig., mechnosensory) lso performed the mjority of their stops t the top of the blue, green nd yellow cones, but not the ornge cone. This indictes tht flies using either visul or ntennl mechnosensory informtion cn still loclize the top of the cones. The flies lcking both visul or ntennl mechnosensory informtion show less pronounced preference for stopping t high elevtions. ISUSSION We developed lrge ren to study the locomotor behvior of wlking rosophil in both simple nd topologiclly more complex environments. The role of vision in structuring locomotor behvior ws pprent even when 3- objects were bsent from the ren (but surrounding visul pnorm ws present). Flies exploring n empty, drk ren spent more time wlking, trveled greter distnce, nd followed more convoluted pths thn flies exploring lit ren (Figs 2, 3). While exploring n environment contining set of cones, flies spent substntilly more time on the tllest, steepest cone even though ll cones hd the sme lterl surfce re (Fig. 4). one removl experiments suggest tht the flies ssess object geometry vi some bsolute mesure nd not by comprison with other objects (Fig. 4). The incresed time spent on the tllest, steepest cone is the result of longer residency times once the object is encountered (Fig. 6) nd not greter frequency of pproches (Fig. 5). The incresed residency times re in turn explined by shift in the distribution of stop durtions towrds long stopping intervls (Fig. ). These long stops occur t the top of the cone (Fig. ). Experiments conducted in complete drkness nd with flies whose ntennl mechnosensory function ws impired indicted tht flies cn use either visul or mechnosensory cues to ssess cone geometry (Figs 7 ). Only if both modlities re impired do the flies demonstrte no preference for tll, steep objects (Figs 7 ). We delibertely designed these experiments using objects tht control for lterl surfce re, nd s consequence two potentilly slient fetures of geometry, slope nd height, were positively correlted. Thus, in none of our experiments could we distinguish between the flies response to slope nd height. It is clerly of interest to determine which of these two fetures of object geometry re encoded by the visul-medited nd mechnosensory-medited mechnisms. We re currently crrying out experiments long these lines, but such nlyses re beyond the scope of the study described here. Visul stimuli influence the sttistics of locomotor behvior This work corrobortes erlier studies showing tht slient visul informtion cn structure the locomotor behvior of wlking fruit flies (Figs 2, 3) (ulthoff et l., 982; Götz, 98; Götz, 994; Götz nd Wenking, 973; Horn, 978; Neuser et l., 28; Schuster et l., 22; Struss nd Pichler, 998; Struss et l., 997). Furthermore, we hve shown tht the presence or bsence of visul stimuli cn chnge fundmentl chrcteristics of wlking behvior such s mximl trnsltionl speed, wlking bout durtion, nd men ngulr speed (Fig. 3). The observed chnges in the sttistics of wlking behvior re probbly the result of visul reflexes, such s object fixtion nd both rottory nd trnsltory optomotor responses (Götz, 975; Götz, 98; Götz nd Wenking, 973; Klmus, 964; Ktsov nd lndinin, 28; Zhu et l., 29). Indeed, ll nimls depend on externl cues in order to mintin THE JOURNL OF EXPERIMENTL IOLOGY

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