Evolution of Zebrina Pendula Var. Quadricolor by Centric Fusions: Evidence From Karyotype

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1 Caryologia International Journal of Cytology, Cytosystematics and Cytogenetics ISSN: (Print) (Online) Journal homepage: Evolution of Zebrina Pendula Var. Quadricolor by Centric Fusions: Evidence From Karyotype K. Lalithambika Bai & P.I. Kuriachan To cite this article: K. Lalithambika Bai & P.I. Kuriachan (1981) Evolution of ZebrinaPendula Var. Quadricolor by Centric Fusions: Evidence From Karyotype, Caryologia, 34:3, , DOI: / To link to this article: Published online: 30 Jan Submit your article to this journal Article views: 64 Citing articles: 1 View citing articles Full Terms & Conditions of access and use can be found at

2 CARYOLOGIA Vol. 34, n. 3: , 1981 EVOLUTION OF ZEBRINA PENDULA VAR. QUADRICOLOR BY CENTRIC FUSIONS: EVIDENCE FROM KARYOTYPE K. LALITHAMBIKA BAI and P.I. KURIACHAN Botany Department, University of Kerala, Kariyavattom, Trivandrum , India SUMMARY - The karyotype of Z. pendula var. quadricolor (2n = 23, n.f. = 28) is shown to have 6M+ 7A+10T chromosomes. This variety is suggested to have evolved from Z. pendula, having 4M + 8A + 12T chromosomes, by two simultaneous centric fusions, involving two T chromosomes, the satellited A chromosome and another A chromosome, resulting in the two additional M chromosomes and the newly formed satellited A chromosome of Z. pendula var. quadricolor. The satellited A chromosome of the variety is suggested to have been formed from the short arms of the two A chromosomes of Z. pendula. INTRODUCTION Zebrina pendula Schnizl. (Commelinaceae) forming part of the " wandering jew ", is grown in gardens throughout the world for its beautifully striped leaves. Besides the species proper, BAILEY (1960) has recognized a variety - Z. pendula var. quadricolor Bailey (Synonyms: f. quadricolor, Voss.; Tradescantia quadricolor, Hort. ex Vilmorin; T. multicolor, Hort.) also in the species. This variety is characterized by leaves with metalic green undertone and striped with green, red and white. Though the cytology of Z. pendula has received considerable attention in the past (DARLINGTON 1929; SIMONDS 1954; SHARMA 1955; VENKATESWARLU and RAO 1963; MATTSSON 1971; LALITHAMBIKA BAI 1980) chromosome number of Z. pendula var. quadricolor was reported only recently (LALITHAMBIKA BAI and KuRIACHAN 1980). The details of the karyotype of the variety and evidence in favour of its evolution from Z. pendula by centric fusions are presented here. MATERIALS AND METHODS Stem cuttings of Z. pendula var. quadricolor were obtained from plants grown in the Department of Botany, University of Kerala, Trivandrum; Lalbagh Gardens, Bangalore and a private residence in Mysore. The variety is sterile and is propagated only vegetatively. In order to avoid any possible confusion owing to the possibility u

3 328 LALITHAMBIKA BAI and KURIACHAN of variation among the plants, karyotype was studied from single stem cutting from each of the collections. The root tips were treated in M aqueous solution of 8-hydroxyquinoline at about YC for 2 h., fixed in acetic alcohol ( 1: 3) with a trace of ferric acetate and squashed in 1% acetocarmine. Measurements of chromosomes were made from 3 selected metaphase plates in each material. Only chromosomes 7 and 13 in the complement could be identified with certainty. Measurements of only these are given individually. Others were arranged into 3 groups namely M, A and T chromosomes and only the limit of variation in length of the groups and the average length of chromosomes in each group are given. Relative chromosome length (RCL) of the chromosomes was calculated as its percentage of the total length of the diploid complement (TCL), multiplied by ten. RESULTS All the three materials of Z. pendula var. quadricolor showed 2n = 23 chromosomes in root tip cells (Fig. 1). The karyotype consists of 6 large metacentric, 7 acrocentric and 10 telocentric chromosomes. The chromosomes range between (..Lm and 2.6 (..Lm in length (Table 1) with a mean TCL of TABLE 1 - Details of somatic chromosomes of «Z. pendula» var. «quadricolor». Chromosome group Chromosome number Limit of Average r value variation in chromosome (long arm/ length (p.m) length (p.m) short arm) M A T (..Lm. The chromosome 7, which is also the shortest in the complement, showed normal anaphase separation (Fig. 2). Its long arm is satellited. The satellite is largely heterochromatic, exhibiting a wide range of variation in Fig Photomicrographs of somatic chromosomes in collections of Z. pendula var. quadricolor. Fig. 1, - A root tip cell from Bangalore material (2n = 23). Fig Anaphase in a root tip cell of Trivandrum material showing the satellited A chromosome at both the poles. Figs Setellited A chromosome in root tip cells of collections from Trivandrum (Fig. 3) and Bangalore (Fig. 4, 5).

4 CENTRIC FUSIONS IN ZEBRINA PENDULA VAR. QUADRICOLOR 329

5 330 LALITHAMB!KA BAI and KURIACHAN morphology. In some cells it is very much contracted to form a short segment stainable with acetocarmine, at the distal end of the chromosome arm (Fig. 3) and in other it is stretched to several 1-1m in length (Figs. 4, 5). Two beads that take stain, one close to the chromosome arm and another almost at the tip of the satellite are visible in satellites of medium stretching, whereas these are not very clear in highly contracted satellites. In the highly stretched ones a third tiny stainable bead is also visible at the middle of the satellite. This variety is very shy in flowering. Flowers were not observed in the stock materials at Bangalore and Mysore at the time of collection or in cutting Figs. 6 and 7. - Photomicrographs of PMCs of Z. pendula var. quadricolor. Fig n = hv 7u 5,. Fig n = hv + 8u + 3,. The arrows show the nucleolus organizing small univalent. of these grown at Trivandrum. The stock material cultivated in the Botany Department flowered only once during the last 2 years and produced only one inflorescence. Twenty seven PMCs at diakinesis or first metaphase stage of meiosis could be obtained from this and in three of these the chromosomes were well spread out so that the multivalents, bivalents and univalents could be clearly seen. These PMCs showed lln+ 1r in one, lrv-t 7n+ 5r in another (Fig. 6) and lrv+8n+3r (Fig. 7) in the third. One of the univalents in these PMCs was the smallest chromosome in the compliment. Invariably the nucleolus was attached to it. In the remaining 24 PMCs, the chromosomes were not well spread out. Therefore the associations in these could not be clearly studied. However in all of these also the smallest, nucleolus organizing chromosome remained as a univalent. Stainability test with acetocarmine showed that 89.8% of the 5012 pollen grains were not stainable and hence sterile.

6 CENTRIC FUSIONS IN ZEBRINA PENDULA VAR. QUADRICOLOR 331 DISCUSSION The karyotype of Z. pendula is known to have 2n = 24 chromosomes, of which 4 are metacentrics, 8 acrocentrics and 12, telocentrics (DARLINGTON 1929; MATTSSON 1971; LALITHAMBIKA BAI 1980). The karyotype of Z. pendula var. quadricolor, which consists of 6M + 7 A+ 1 OT chromosomes looks markedly distinct from Z. pendula in the number and morphology of Fig Karyogram of somatic chromosomes of Z. pendula var. quadricolor (Mysore). chromosomes. However, it has been suggested that Z. pendula (2n = 24) has evolved from forms with n.f. = 28 by centric fusions and the loss of 4 centromeres (JoNES 1978). In Z. pendula var. quadricolor the number of M chromosomes is greater by two and the numbers of A and T chromosomes are lesser by one and two respectively than in Z. pendula, though both the taxa have the same n.f. value. This would suggest that Z. pendula var. quadricolor with two additional M chromosomes has evolved from Z. pendula by centric fusions between two A and two T chromosomes. From a comparison of the karyotypes of Z. pendula var. quadricolor (Fig. 8) and Z. pendula (Fig. 9) it is seen that the longest satellited acrocentric (Chromosome 5) and one of the two acrocentrics with very small short arms (Chromosome 11 or 12) of the latter are absent in the former. Z. pendula var. quadricolor ha.s in turn a satellited acrocentric (Chromosome 7), which is not seen in Z. pendula and only one A chromosome with very small short arm

7 332 LALITHAMBIKA BAI and KURIACHAN (Chromosome 13). An analysis of the relative lengths of these chromosomes (Table 2) revealed that the short arms of chromosome 7 and 13 of Z. pendula var. quadricolor are almost equal in length to the short arms of chromosome 11 and 12 of Z. pendula. Though the long arm of chromosome 7 in Z. pendula var. quadricolor is slightly longer than the short arm of chromosome 5 of Z. pendula, both are satellited. Therefore chromosome 7 of Z. pendula var. quadricolor seems to have evolved by a union of the short arm of either Fig Karyogram of somatic chromosomes of Z. pendula (Moovattupuzha). chromosome 11 or 12 of Z. pendula to the short satellited arm of its chromosome 5. As chromosome 13 in Z. pendula var. quadricolor is more similar in length to chromosome 12 than chromosome 11 of Z. pendula, it may be inferred that it is chromosome 11 of Z. pendula which is involved in this centric fusion. The greater length of the long arm of chromosome 7 of Z. pendula var. quadricolor in comparison with the short arm of chromosome 5 of Z. pendula may be due to the greater length of the segment of the long arm of chromosome 11, taking part in the formation of chromosome 7 of Z. pendula var. quadricolor than the segment of the short arm of chromosome 5 of Z. pendula, severed from the satellited end, by the break prior to the centric fusion. No forms having 5 M chromosomes and cytologically intermediate between Z. pendula and Z. pendula var. quadricolor are known. This may

8 CENTRIC FUSIONS IN ZEBRINA PENDULA VAR. QUADRICOLOR 333 TABLE 2 - RCL of the long and short arms of chromosome 5, 11 and 12 of «Z. pendula» * and chromosomes 7 and 13 of «Z. pendula» var. «quadricolor». Long arm Short arm Chromosome Limit of Mean Limit of Mean variation (!J,m) (!J,m) RCL variation (!J.m) (!J,m) RCL Z. pendula 5(A) ( ) ( ) 1.5+Sat 8 ll(a) ( ) ( ) (A) ( ) ( ) Z. pendula var. quadricolor 7(A) ( ) 1.9+Sat 10 ( ) (A) ( ) ( ) * Adopted from LALITHAMBIKA BAr (1980). suggest that the two centric fusions that lead to the evolution of the variety have occurred from four simultaneous breaks as shown in Fig. 10, though confirmatory evidence for this hypothesis has to be obtained from a study of the variety from its entire range of distribution. The authors are not aware of any other record of karyotype evolution among plants by similar multiple breaks and centric fusions associated with origin of a new taxon. It is known that some of the spontaneous chromosome rearrangements in the animal kingdom that have established themselves in evolution are probably 3-or 4-break ones (WHITE 1969). TheM chromosomes in Z. pendula have been found to pair often with A or T chromosomes to form one or two multivalents during meiosis (DAR LINGTON 1929; MATTSON 1971; LALITHAMBIKA 1980; JONES 1978). Two of the three PMCs of Z. pendula var. quadricolor examined in this study revealed one quadrivalent along with bivalents and univalents. Owing to the rarity of flowering in the variety and the paucity of well spread out PMCs, it could not be clearly determined whether more quadrivalents will be formed during meiosis in this variety and also whether the observed quadrivalent is formed from the M chromosomes originally present in Z. pendula or from the two additional M chromosomes formed by the two centric fusions described here. Detailed meiotic studies of this variety might throw light on the above points as well as on the question whether the long arms of chromosome 5 and 11 of the parent species have united to form one M chromosome, or these have united to the two T chromosomes involved in

9 334 LALITHAMBIKA BAI and KURIACHAN the centric fusions. The free centromere resulting from one of the centric fusions might have been lost during successive cell divisions. The satellited chromosome 7 formed by the other centric fusion amounts to 1.5% of the TCL in Z. pendula var. quadricolor. The small nucleolus organizing univalent invariably present in the observed PMCs of the variety is, in all probability, T T T T 0 ~ _J rn_ ~ SA. --+~ ~ v I A ~ I M SA 11 A.... M A Original 4 Centric fusions chromosomes Simultaneous resulting in in Z. pendula breaks quadricolor type Fig A model of the simultaneous centric fusions leading to the formation of Z. pendula var. quadricolor type from Z. pendula. this satellited chromosome. It would also have perhaps been eliminated during the course of evolution, had the new taxon reproduced sexually following meiotic division, wherein it is found always as a univalent. However the variety is totally seed sterile. It is propagated vegetatively and the chomosome 7 shows normal behaviour during somatic divisions. These factors helped retention of this new chromosome in the variety. Acknowledgements. - The authors are thankful to Prof. C.A. NrNAN, Head of the Department of Botany and Dean, Faculty of Science, University of Kerala for encouragements and suggestions in the preparation of the paper. The senior author is also thankful to the U.G.C. Delhi for the award of a Teacher Fellowship, which made the investigation possible.

10 CENTRIC FUSIONS IN ZEBRINA PENDULA VAR. QUADRICOLOR 335 REFERENCES BAILEY L.H., The standard cyclopedia of Horticulture. Vol. III: 3540, The Macmillan Company, New York. DARLINGTON C.D., Chromosome behaviour and structural hybridity in Tradescantia. Jour. Genet., 21: ]ONES K., Mechanisms of chromosome change. Nucleus, 21: LALITHAMBIKA BAI K., Cytological studies on Zebrina pendula Schnizl. M. Phil dissertation, University of Kerala (unpublished). LALITHAMBIKA BAI K. and KuRIACHAN P.I., A new chromosome count in Zebrina Schnizl. Curr. Sci., 49, No. 3: MATTSSON 0., Cytological observations within the genus Zebrina. Bot. Tidssk., 66: 189. SHARMA A.K., Cytology of some members of Commelinaceae and its bearing on the interpretation of phylogeny. Genetica, 27: (Cited from VENKATESWARLU and RAO 1963). SIMMONDS N.W., Chromosome behaviour in some tropical plants. Heredity, 8: (Cited from VENKATESWARLU and RAO 1963). VENKATESWARLU ]. and RAo P.N., Meiosis in diploid and colchicine induced tetraploid Zebrina pendula Schnizl., a structural hybrid. Cytologia, 28: WHITE M.].D., Annual Review of Genetics, 3: 75-98, Ed. Roman H.L. Inc. Palo Alto, California, U.S.A. Received 24 July 1980; revision accepted 26 April 1981