C 1997 The Japan Mendel Society Cytologia 62: 157-162, 1997 Chromosome Numbers of Species of Atriplex Section Obione (Chenopodiaceae) and Their Relation to Taxonomy H. Flores-Olveraand P. Mercado-Ruaro InstitutodeBiologia, Universidad Nacional Autonoma deMexico. A.P. 70-233, C.P. 04510, DelegacionCoyoacan, D.F. Mexico AcceptedMarch 5,1997 The Chenopodiaceae have a basic chromosome number of x = 9 (Turner 1994), polyploids being fairly common (McArthur and Sanderson 1983). Most species of Atriplex are diploids with 2n = 18, but tetraploids are present (Gustafsson 1973). There are few chromosomal investigations like those of Gustafsson (1973) or Stutz and Sanderson (1979) on Atriplex containing evolutionary interpretations. For species belonging to section Obione (Gaertner) C. A. Meyer, there are reports of 2n = 18 and 2n = 36 (Bassett 1969, Bassett and Crompton 1971, Flores-Olvera 1994b, Frankton and Bassett 1970, Love and Love 1982, Nobs 1975, 1978). Four papers have reports on meiotic chromosome numbers (Flores-Olvera 1994b, Frankton and Bassett 1970, Nobs 1975, 1978), but only one (Flores-Olvera 1994b) presents photographs showing the chromosome figures. Section Obione comprises 32 species of annual and perennial North American herbs. Plants are erect to prostrate, leaves presenting Kranz structure, monoecious with pistillate flowers lacking a perianth, fruits completely surrounded by bracteoles, and seeds with embryos with radicle pointing upwards. The species are distributed from southern Canada to the Caribbean islands and the Venezuelan coast, with a diversification area in southwestern United States (Flores-Olvera 1994a). The taxonomy of this group is unsatisfactory in spite of the classifications of Standley's (1916), Hall and Clements' (1923) and Ulbrich's (1960), based only on morphological data. Although the proposal from Hall and Clements (1923) has been much accepted to our days, it is necessary to adequate it according to new sources of taxonomical evidence. The purpose of this paper is to provide preliminary chromosomal data toward a better classification of Atriplex section Obione. Materials and methods The chromosome counts were made in pollen mother cells (PMC) from immature anthers obtained from floral buds fixed in the field in Farmer's solution (3 : 1 ethanol-acetic acid). They were squashed in a drop of 1% aceto-carmin solution. A drop of Hoyer's solution was used in positioning the coverglass in order to get temporary slides. The slides were observed with a Zeiss Axioscope microscope, and the best fields were photographed with a Contax camera. Table 1 shows a list of the species studied and their localities. Herbarium vouchers are deposited at the National Herbarium of Mexico (MEXU); duplicates are being distributed to several North American herbaria. Results Table 1 lists the chromosome numbers of Atriplex sect. Obione obtained during this study 158 H. Flores-Olvera and P. Mercado-Ruaro Cytologia 62 Table 1. Chromosome numbers of species belonging to Atriplex sect. Obione, with references. Nobs (1975, 1978) reported chromosome numbers based on counts of plants cultivated in the experimental gardens of the Carnegie Institution at Stanford, California without collector. * Species which chromosome numbers are first reported in this paper 1997 Chromosome Numbers of Species of Atriplex Section Obione 159 Table 1. (continued) and those known from the literature. Chromosome numbers are reported for 24 species, representing 75% of the section. Most of the species are diploid with 2n =18 (Figs. lc, d, f-1, Table 1), but A. argentea (Fig. la), A. cordulata (Fig. lb), A. coronata and A. leucophylla (Fig. le) are tetraploids with 2n = 36. While most of the species show a constancy in their chromosome number, A. argentea has both diploid and tetraploid numbers. All tetraploid species examined during the present study formed bivalents at meiosis, occasional multivalents being noticed (Figs. lb, e). The diploid species formed bivalents only (Figs. 1c, d, f). 160 H. Flores-Olvera and P. Mercado-Ruaro Cytologia 62 Fig. 1. Meiosis in species of Atriplex. a. A. argentea, metaphase I, n=18. b. A. cordulata, diakinesis, n=18. c. A. elegans, diakinesis, n = 9. d. A. elegans, metaphase I, n = 9. e. A. leucophylla, metaphase I, n=18. f. A. linifolia, diakinesis, n=9. g. A. minuscula, metaphase II, n=9. h. A. muricata, metaphase I, n=9. i. A. parishii, metaphase I, n=9. j. A. pueblensis, diakinesis, n = 9. k. A. tampicensis, diakinesis, n = 9. 1. A. texana, metaphase I, n = 9. Bars = 5 ƒÊm. Discussion The basic chromosome number for section Obione is clearly x = 9, as shown in Table 1. The four tetraploid species: Atriplex argentea, A. cordulata, A. coronata and A. leucophylla occur in southwestern United States, the area of greatest diversity of the section. Atriplex cordulata 1997 Chromosome Numbers of Species of Atriplex Section Obione 161 and A. coronata are endemic to this region, while A. argentea and A. leucophylla have broader distributions in North America and Mexico. The presence of two ploidy levels in A. argentea perhaps relates to its great morphological variability and wide distribution (southern Canada to northwestern Mexico) in different environments. All members of the A. pentandra group (sensu Hall and Clements 1923) are diploid. Although the count for A. leucophylla a tetraploid, is interesting its taxonomic affinity to A. pentandra group is still not clear. In fact, this species was placed in different groups by Watson (1874) and Standley (1916). Even for Hall and Clements (1923) its relation with A. pentandra group is uncertain. Although both Atriplex truncata and A. saccaria of the A. truncata group (sensu Hall and Clements 1923) are diploid, they appear morphologically dissimilar to the group of A. pentandra. The chromosome number for the other two species of the A. truncata group remains unknown. The only two species of the A. argentea-A. coronata group (sensu Hall and Clements 1923) are tetraploid. In the A. pusilla group (sensu Hall and Clements 1923), however, A. cordulata is tetraploid while A. minuscula and A. parishii are diploid. These are the only chromosome numbers known for the A. pusilla group; counts of the other three species are necessary before meaningful conclusions can be drawn. According to the chromosome number, a group containing the tetraploid species could be recognized, including the A. argentea-A. coronata group, A. leucophylla and maybe the A. pusilla group (sensu Hall and Clements 1923). The morphological evidence also favors the relationship of the A. argentea-A. coronata group with the A. pusilla group (Flores-Olvera 1994a). Atriplex leucophylla, however, has many morphological peculiarities that possibly allow its recognition as a monotypic group. Another monotypic species in section Obione is Atriplex powellii. This diploid species has been classified as a distinctive group by Standley (1916), Hall and Clements (1923) and Ulbrich (1960) on account of its morphological differences. On the other hand, it is possible to propose that polyploidy has played an important role in the evolution of section Obione, as could be the hybridization processes whose effects in the evolution of Atriplex were suggested by Stutz (1978) for perennial species. Structural mutations such as inversions, translocations, fusions, etc. have not been reported for the diploid species of Atriplex. The meiotic figures herein presented corroborate the absence of these events in species of section Obione, since only normal bivalent formation was found in the diploid species analyzed. A high frequency of multivalents in the tetraploid species of Atriplex was observed by Nobs (1975), who suggested that autopolyploidy has been the primary event in the evolution of polyploids. In contrast with Nobs's results, we found a low frequency of multivalents suggesting an allopolyploid origin. This is in accordance with Grant's (1981) proposal that polyploids in vascular plants predominantly originate by allopolyploidy. Summary The meiotic chromosome numbers for 18 species of Atriplex section Obione (Chenopodi- aceae) are presented. This is the first report for Atriplex abata, A. elegans, A. linifolia, A. minuscula, A. parishii, A. pueblensis, A. tampicensis, A. texana and A. thornberi. Photographs of the meiotic figures of 12 species are presented. All the species are diploid with 2n = 18, except for Atriplex argentea, A. cordulata, A. coronata and A. leucophylla tetraploids with 2n = 36. The basic chromosome number (x = 9) for the section is confirmed. Atriplex argentea, with the broadest geographic distribution, has at least two ploidy levels. All the tetraploid species occur in southwestern United States, the area of greatest diversity of the section Obione. Chromo- some numbers appear to have taxonomic value and allopolyploidy might have played an important role in the evolution of the section. 162 H. Flores-Olvera and P. Mercado-Ruaro Cytologia 62 Acknowledgments We thank Javier Valdes, Fernando Chiang, Alfonso Delgado and Helga Ochoterena for reviewing the manuscript, and Dr. B. L. Turner for the critical reading and suggestions. For logistic support in field work we thank Javier Valdes, Steve Boyd and Tim Ross. We appreciate the help of Sara Fuentes and Ivonne Sanchez del Pino in the lab work. References Bassett, I. J. 1969. In: IOPB Chromosome number reports XXI. Taxon 18: 310. - and Crompton , C. W. 1971. In: IOPB Chromosome number reports XXXIV. Taxon 20: 786. Flores-Olvera, H. 1992. Taxonomia del grupo Atriplex pentandra (Chenopodiaceae). Anales Inst. Biol. Univ. Nac. Mexico, Ser. Bot. 63(2): 155-194. - 1994a. Sistematica de Atriplex secciOn Obione (Chenopodiaceae) en Norteamerica. Tesis Doctor en Ciencias. Facultad de Ciencias, UNAM, Mexico, D. F. 1-196. - 1994b. A new species of Atriplex (Chenopodiaceae) from saline soils of central Mexico. Novon 4: 242-245. Frankton, C. and Bassett, I. J. 1970. The genus Atriplex (Chenopodiaceae) in Canada. II. Four native western annuals: A. argentea, A. truncata, A. powellii, and A. dioica. Can. J. Bot. 48: 981-989. Grant, V. 1981. Plant Speciation. 2nd edn. Columbia University Press, New York. Gustafsson, M.