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Chromosome Numbers and a New Model for Karyotype Evolution in Ruppia L. ( Ruppiaceae)

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Chromosome Numbers and a New Model for Karyotype Evolution in Ruppia L. ( Ruppiaceae) Aquatic Botany, 45 (1993) 1–13 Elsevier Science Publishers B.V., Amsterdam Chromosome numbers and a new model for karyotype evolution in Ruppia L. ( Ruppiaceae) S. Talavera, P. Garcia-Murillo and J. Herrera Departamento de Biologia Vegetal y Ecologia, Universidad de Sevilla, E-41080 Sevilla, Spain ( Accepted 8 October 1992) Go to table of contents Digitalizado por Biblioteca Botánica Andaluza ABSTRACT Talavera, S., Garcia-Murillo, P. and Herrera, J., 1993. Chromosome numbers and a new model for karyotype evolution in Ruppia L. (Ruppiaceae). Aquat. Bot., 45: 1-13. Populations of three species in the genus Ruppia that inhabit the western part of the Mediterranean region were studied caryologically. All three species present bimodal karyotypes. Ruppia drepanensis Tineo ex guss. (2n = 20) has a karyotype asymmetry of the 2C type. Ruppia cirrhosa (Petagna) Grande and Ruppia maritima L. (both with 2n =40) present a 2B asymmetry. Observations of meiosis of microspore mother cells revealed that R. drepanensis forms ten bivalents at diakinesis and Metaphase I. The other two species exhibit 20 bivalents. The long arm of subtelocentric, large chromosomes in all three species shows no sign of chiasmata at meiosis. INTRODUCTION With only about eight species, the genus Ruppia has a nearly worldwide distribution (Den Hartog, 1981). All of these species are hydrophytes from hypersaline or brackish waters. Studies on species of this genus have dealt with their anatomy (Sauvageau, 1891; Monoyer, 1928; Tomlinson, 1982), morphology (Graves, 1908; Luther, 1947), ecology (e.g. Verhoeven, 1979, 1980; Brock, 1981, 1982; Vollebergh and Congdon, 1986), flower develop- ment (Posluszny and Sattler, 1974) and pollination biology (Gamerro, 1968; McConchie, 1982; Cox, 1983, 1988; Haynes, 1988; Les, 1988; Cox and Knox, 1989) . Taxonomic studies have also contributed to the knowledge of this ge- nus (e.g. Mason, 1967; Jacobs and Brock, 1982; Talavera and Garcia-Mu- rillo, 1987). Ruppia seems to be closely related to Potamogeton (including Groenlan- dia), the main difference being that it bears bisexual flowers with two sta- mens, while in Potamogeton there are four. Other traits such as stipules fused Correspondence to: S. Talavera, Departamento de Biologia Vegetal y Ecologia, Universidad de Sevilla, E41080, Sevilla, Spain. © 1993 Elsevier Science Publishers B.V. All rights reserved 0304-3770/93/$06.00 2 S. TALAVERA ET AL. to the leaf blade, pollination taking place on the surface of the water, and elongated pollen grains, relate Ruppia species to those in the subgenus Coleo- geton of Potamogeton. As a result, some authors have included Ruppia in the Potamogetonaceae together with Potamogeton and Groenlandia ( Thorne, 1981; Dahlgren et al., 1985 ), while others refer the genus to a separate family (Takhtajan, 1980, 1986; Cronquist, 1981, 1988). Information on the cytotaxonomy of the genus has been provided by Reese (1962, 1963), Van Vierssen et al. (1981) and Marchioni-Ortu (1982) for European populations, and by Snoeijs and Van der Ster (1983 ) for Australian plants. Except for Reese's (1962, 1963) observations on the idiogrammatic formula of Ruppia maritima L., there are few data on the karyotypes and behaviour at meiosis of other species. Together with information on the three species studied here, we present a review of the literature on Ruppia. MATERIAL AND METHODS Root tips and flower buds from plants growing in the western part of the Mediterranean region were used for this study. Buds were fixed for 24 h in Carnoy's solution (100% ethanol/glacial acetic acid 3 : 1), then transferred to 70% ethanol and stored at 4°C. Root tips were pre-treated with 0.002 M 8- hydroxyquinoline at 4°C for 3—4 h, then fixed and stored with flower buds until study. Material was stained with hydrochloric acid—alcoholic carmine (Snow, 1963) at 30°C for 4—7 days. Anthers or roots were then placed in a drop of 45% acetic acid on a slide, squashed, and observed. We followed Levan et al. (1965) for the idiogrammatic formula and chromosome nomenclature, and Stebbins (1938, 1971) for karyotype symmetry and chromosome size. Plant vouchers are deposited at the herbaria of the Departamento de Biologia Vegetal y Ecologia, Universidad de Sevilla (SEV and SEVF). RESULTS AND DISCUSSION Variations in chromosome number As shown in Table 1, western Mediterranean populations of Ruppia are either diploid or tetraploid. Populations of Ruppia drepanensis Tineo ex Guss. are diploid with 2n = 20 (Figs. IC, ID, 2C, 2D) and n =10 (Figs. 3A, 3B, 4A, 4B) . Populations of R. maritima L. var. maritima are tetraploid with 2n = 40 and n =20 (Figs. 1B, 2B ), and the same is true for R. maritima var. breviros- tris Ag. and Ruppia cirrhosa (Petagna) Grande (Figs. IA, 2A, 3C—3F, 4C— 4F). Although Murbeck (1902) and Graves (1908) reported 2n= 16 for R. maritima L., all subsequent counts have demonstrated that species in this genus present 2n = 20 or 2n = 40. Values of 2n = 18, 2n = 30, and 2n = 60 have CHROMOSOME NUMBERS OF RUPPIA 3 also been reported on occasions (see below). Ruppia polycarpa Mason, Rup- pia megacarpa Mason, Ruppia tuberosa J.S.Davis and Tomlinson and R. dre- panensis Tineo ex guss. present 2n = 20 (Table 1) . Only a southern Australian population of R. tuberosa was reported by Snoeijs and Van der Ster (1983) to have 2n = 30 (3 x ), and another population in New Zealand studied by Mason (1967) and Carstairs (1982) 2n = 18 and n= 9, respectively. The dis- agreement between the earlier and these more recent counts might be ex- plained either on the basis of aneuploidy in the studied plants, or because of incorrect counts resulting from the small size of some chromosomes in this genus since small chromosomes may often remain undetected if they are un- der larger ones (Reese, 1962 ). Some populations of R. maritima L. are dip- loid (2n = 20) and others are tetraploids (2n = 40) . All studies on R. cirrhosa (Petagna) Grande (Table 1) have reported n =20 and 2n = 40, so it seems to be mainly a tetraploid. Only Reese (1962) has reported hexaploid popula- tions of this taxon from northern Germany. According to the literature and to our own data, the basic chromosome number in the genus Ruppia is x= 10. Among the species for which data are available, four are diploid, one is a tetraploid, and one (R. maritima) pre- sents both diploid and tetraploid populations. Diploid populations occur in cool regions of northern Europe and Canada, while tetraploids inhabit warm or temperate areas from the Mediterranean region and Japan. Hexaploids (of R. cirrhosa from Germany with 2n = 60; Reese (1962)) and triploids (of R. tuberosa from southern Australia with 2n = 30; Snoeijs and Van der Ster (1983)) are very rare. Size and morphology of chromosomes, and karyotype asymmetry Reese (1962) established that R. maritima has a pair of chromosomes much larger (about 4 ,um long) than the remaining pairs (ranging from 0.5 to 2.5 ,u m) . He also indicated that large chromosomes had the centromere near the distal end, and a satellite on the longest arm. Among the small pairs, five had the centromere at a submedian position, and the remaining four were so small that they were termed `punktformig' (i.e. dot-like). Furthermore, Reese (1962) reported that tetraploids and hexaploids had two and three pairs of relatively large chromosomes, respectively. These observations have been verified by Gamerro (1968) and Van Vierssen et al. (1981) . The same pat- tern has also been observed in R. megacarpa and R. tuberosa by Snoeijs and Van der Ster (1983 ). The karyotype of R. drepanensis (Figs. 1C–1E ) shows a pair of subtelocen- tric chromosomes (the ratio of the long arm to the short one is 3.3) with a secondary constriction at the long arm (Fig. 1 D ). According to Stebbins (1938 ), this pair should be considered medium-to-large (4.5–5 ,um) . All the remaining pairs are small, their sizes ranging from 1.2 ,um to 2 ,u m. In two TABLE 1 Chromosome numbers in species of the genus Ruppia Taxon n 2n Locality Reference Ruppia maritima var. maritima 20 Canada Taylor and Mulligan (1968) 20 40 Japan Harada (1956) 10 20 N. Germany Reese (1961, 1962, 1963) 20 The Netherlands Van Vierssen et al. (1981) 40 S. France Van Vierssen et al. (1981) 40 Spain, Cadiz (SEVF) This study 20 Spain, Almeria (SEV114537) This study 40 Spain, Sevilla (SEV113519) This study 40 Spain, Sevilla (SEV113512) This study 40 Spain, Guadalajara (MA346158) Cirujano (S. Cirujano, unpublished data) 20 Spain Aedo and Fernandez Casado (1988 ) 20 Italy, Sardinia Marchioni-Ortu (1982) var. brevirostris 20 N. Germany Reese (1961, 1962, 1963) 40 S. France, Camargue Van Vierssen et al. (1981) 20 Italy, Sardinia Marchioni-Ortu (1982 ) 40 Italy, Sardinia Marchioni-Ortu (1982) 40 France, Corse This study var. longipes Hagstrom 40 Spain, Cadiz Van Vierssen et al. (1981) Ruppia cirrhosa 40 N. Germany Reese (1961, 1962, 1963) 60 N. Germany Reese (1962) 40 S. France, Camargue Van Vierssen et al. (1981) 40 Spain, Cadiz (SEV 113555) This study 40 Spain, Huelva (SEV 113552) This study 20 Morocco, Tanger (SEV128702) This study 40 Italy, Sardinia Marchioni-Ortu (1982) var. occidentalis 40 Canada Love and Love (1981) 39 Argentine, Patagonia Gamerro (1968) Ruppia drepanensis (sub R. cirrhosa) 20 Spain Cirujano (1982) (sub R. cirrhosa) 20 Spain Castroviejo (1983) (sub R. cirrhosa) 20 Spain Cirujano (1986) 20 Italy, Sardinia Marchioni-Ortu (1982) 20 Spain, Cadiz (SEV113597) This study 10 Spain, Cadiz (SEV113595) This study 10 Spain, Huelva (SEV113585) This study 20 Spain, Malaga (SEVF) This study Ruppia polycarpa 9 New Zealand Mason (1967) 20 S. Australia Brock (1982) 18, 20 S. Australia Carstairs (1982 ) Ruppia tuberosa 20 S. Australia Brock (1982) 20, 30 S.
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