Chromosome Polymorphism and Bivalent-Forming Triploid and Tetraploid Tradescantia Pallida (Commelinaceae)

C 1998 The Japan Mendel Society Cytologia 63: 191-197, 1998 Chromosome Polymorphism and Bivalent-forming Triploid and Tetraploid Tradescantia pallida (Commelinaceae) Armando Garcia-Velazquez Especialidad en Genetica Instituto de Recursos Geneticos y Productividad, Colegio de Postgraduados, Montecillo, Mex., Mexico 56230, C.P. Accepted February 12, 1998 Summary Karyotypes and meiotic chromosome pairing were studied in natural autotriploid and autotetraploid Tradescantia pallida. At both ploidy levels chromosomes appeared as metacentrics, being larger at the triploid than at the tetraploidy level (9.6-15.3 ƒÊm and 7.5-10.0 ƒÊm, respectively). T pallida exhibits polymorphism with respect to number and position of secondary constrictions. The tetraploid had three large satellites and a small-spheric one. While the triploid had two large satellites and one small-spheric. Both the triploid and tetraploid showed reduced multivalent pairing for triploid, only one trivalent was seen in 32.8% PMC's while 67.2% PMC's had 6 bivalents plus 6 univalents. In the tetraploid, bivalent pairing was very predominant, 87.72% PMC's had 12 bivalents. The presence of a bivalent-forming system in these polyploids, most probably a genetic one, is suggested. Key words Chromosome polymorphism, Autopolyploids, Bivalent-forming, Tradescantia. Tradescantia pallida (Rose) D. R. Hunt Cay. (Syn. Setcreasea purpurea) is a member of the tribe tradescantieae of the family Commelinaceae. The members of the tradescantieae are predomi- nantly Mexican in origin and based on x=6 fairly symmetrical chromosomes (Owens 1981). In the Commelinaceae family there are few bivalent-forming tetraploids including Tradescantia cymbispatha, T Standleyii (Kenton and Drakeford 1990), T ambigua Mart., T burchii, D. R. Hunt, and T crassifolia group. T ambigua is a South American representative of the genus. There is no data to explain why these self-compatible species are bivalent forming (Owens 1981). It is likely that the diploid-like meiosis in most, if not all, other natural poliploids is genetically regulated, since with- out such a control precise bivalent pairing in these polyploids having several sets of related genomes, would not be achieved (Jauhar 1977). Although allopolyploidy has provided the basis for the evolution of many plant species including Commelinaceae (Kenton 1981, Jones 1974, 1977), the super-imposition of a precise genetic control on chromosome pairing could be critical for con- ferring meiotic and hence, reproductive stability in sexually reproducing polyploids. Polyploids are usually classified as autopolyploids, segmental allopolyploids or true allopolyploids (Stebbins 1971). Such a classification is based on fairly much of information, but most of it deal with chromosome pairing in a hybrid or an individual that is known or presumed to have given rise to a polyploid. The information is generally lacking in natural polyploids. The polyploids consisting of only bivalents are considered allopolyploids, while those of mul- tivalents are treated as autopolyploids or segmental allopolyploids (Jackcson and Casey 1980, 1982). The evolutionary processes of many plant species involve hybridization and polyploidy. An understanding of the genomic relationships between the parental species and the derived polyploids is important, first so that the evolutionary processes may be elucidated and second, for pragmatical reasons (Alonso and Kimber 1981). A complicating factor in polyploidy classification is the presence in some populations and species, of genes affecting homoeologous chromosome pairing. The best known of these is the dominant Ph gene in the hexaploid and tetraploid wheats that prevents homoeologous pairing, and 192 Armando Garcia-Velazquez Cytologia 63 only homologous pairings are observed in normal plants (Okamoto 1957, Riley and Chapman 1958). Ph-like but not necessarily dominant genes have been reported for diploid wheats and Ph- like effects have been suggested for other plants (Avivi 1976, Jauhar 1977, 1993). The occurrence of diploid (2n= 12), triploid (2n= 18) and tetraploid (2n=24) in Tradescantia pallida could be indicative of autotetraploidy via unreduced gametes. There are several cases in the Commelinaceae in which unreduced gametes are produced. In Rhoeo spathacea Garcia (1991, 1995) observed the production of diplandrogynous autotetraploids, when both male and female gametes are unreduced. The similar situation was reported by Jones (1976) in T cymbispatha in which several ploidy levels occurred. Xerophytes of the genus Tradescantia showing their greatest morphological diversity are located along the Gulf of Mexico. Tradescantia pallida (Rose) D. R. Hunt is one of these species and plants were located in Ocampo and Ciudad Victoria in the south of Tamaulipas (23•Ž44'N, 39•Ž08'W), Mexico, at about 400 m altitude. The investigation of triploid and tetraploid cytotypes is the subject of this paper. Materials and methods Vegetative material of Tradescantia pallida (Rose) D. R. Hunt was collected from plants grow- ing in their natural habitat. Propagules were potted and cultivated in the greenhouse. All plants produced pink flowers. Cytological methods The standard cytological techniques used have been described in detail elsewhere (Garcia 1995). Root tips were pretreated with 2 mM 8-hydroxyquinoline at 18°C for 9 hr before fixing in ethanol : acetic acid (3 : 1, v/v). Karyotypes were directly drawn from metaphase plates at •~3200 magnification by using a drawing attachment. Determination homology in the karyotypes was based on chromosome length, centromere position and presence of satellites. Meiotic analysis in PMC's For examination of meiosis, anthers were squashed directly into 1.8% propionic orcein and heated gently to spread the chromosomes. Meiosis at MI, AI and TI stages was observed. Pollen fertility was estimated by scoring the percentage of well-filled, stained grains after squashing in propionic orcein. Cells were photographed on Kodalith film using 64•~ and 100 •~ Zeiss planapo chromatic and Neufluar immersion lenses. Results and discussion Two different chromosome numbers were recorded in the plant material of this study. Out of the six plants, collected in Ocampo, Tamaulipas, one exhibited 2n= 18 and the other five 2n=24. Thus, triploidy and tetraploidy might occur with a basic chromosome number of x=6 as has been reported in the tradescantieae (Hunt 1993). The triploid formation in T pallida even when a rare event, occurs in nature, probably being produced by a cross between 4x and 2x or between 2x and 4x. Several authors have concluded that Tradescantia pallida is a species having diploid, triploid and tetraploid chromosome numbers on the basis of its karyotype (Martinez 1978, Owens 1981, Jones and Kenton 1984). Karyotype and idiogram Based on Feulgen stained chromosomes in root tips treated with 8HQ the karyotype of T. pallida consists of 18 or 24 metacentric chromosomes and they showed a graded series of lengths. As expected for an autotriploid and autotetraploid, the chromosomes could be arranged into groups of 1998 Chromosome Polymorphism and Bivalent-forming Triploid and Tetraploid 193 Fig. 1. Drawing of somatic chromosomes of Tradescantia pallida. a) Tetraploid, 2n = 4x = 24, b) Triploid 2n=3x=18. Scale 10 ƒÊm. three and four on the basis of total length, arm ratio, and position of secondary constrictions (Fig. 1a, b). Two groups can be clearly specialized according to secondary constrictions: they show some polymorphism among putative homologues. In the karyotype of the triploid plant (Fig. 1b) one group includes two metacentrics bearing secondary constrictions in the short arms (Nos. 13, 15) and thereby large satellites. The other group includes a large metacentric chromosome with a small spheric satellite (No. 18). In the tetraploid karyotype (Fig. 1a) there are three chromosomes with large chromosome satellites (Nos. 22, 23, 24), while a small-spheric satellite is located in the short arm of chromosome 7. The standard karyotype of Tradescantia pallida consists of homologous and perfectly pairable elements (Martinez 1978, Owens 1981, Jones and Kenton 1984, Hunt 1993) at both diploid (2n= 12) and tetraploid (2n=24) levels. Obviously, in the present cytotypes perfect homology is lost for those chromosomes with satellites. Hunt (1993) has indicated that there is not any cytological diversity as all the Tradescantia species have large metacentric chromosomes even in the north of the trans-Mexican volcanic belt, where the genus shows its greatest morphological diversity. However, the present study on the triploid and tetraploid Tradescantia pallida suggests that a cytogenetic differentiation occurs probably owing to its geographical distribution. In the described karyotypes for triploid and tetraploid cytotypes of Tradescantia pallida, in ad- dition to chromosome homologous groups there is one group of heteromorphics made up by secondary constrictions. Determination of their evolutionary relationship must await further work. Meiotic behavior Pachytene chromosomes of Tradescantia pallida are not suitable for karyotype analysis. How- ever, at early prophase some multivalent association could be examined. Table 1 presents the chromosome associations in one triploid plant and five tetraploid plants of T pallida. In 1807 PMC's analyzed at MI of triploid plant its high frequencies of bivalents and univalents (5.67 per cell in both chromosome associations) contrast with the smaller number of triva- lents per

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