Chromosome Studies on Hydromystria laevigata (Hydrocharitaceae)

Abstract

Chromosomes of individuals from two Argentine populations of Hydromystria laevigata (Willd.) A. T. Hunz. were analyzed for numbers, meiotic behavior, and karyotype. It has n = 14 and 2n = 28. Both populations have regular meiosis and show no significant statistical differences between them. The karyotype is similar in both populations: asymmetrical and bimodal; it is formed by: 4 m pairs + 6 sm pairs + 4 st pairs. No satellites were found. Hydromystria is a monotypic genus of Hydrocharitaceae. The only species, H. laevigata, is an aquatic plant that grows in Mexico, West Indies, and South America (Hunziker, 1981, 1982). Some authors include Hydromystria within Limnobium (Dandy, 1959; Cook et al., 1974; Cook & Urmi-Konig, 1983), while others recognize them as distinct monotypic genera although closely related (cf. Hunziker, 1981, 1982). No comprehensive cytological study has been carried out on H. laevigata, although some chromosome counts have been published. Geitler (1938; sub Trianea bogotensis Karst.) reported 2n = 26-28, and the same author (1940) found 4n = 56 in counts on colchicine-induced tetraploid root cells. Recently, Urmi-K6nig (in Cook & Urmi-Konig, 1983) reported five different chromosome numbers for this plant [sub Limnobium laevigatum (Willd.) Heine]: 2n = 26, 27, 28, 29, 30. In the present contribution, the somatic and gametic numbers of two Argentine populations of H. laevigata are reported, as well as their meiotic behavior and karyotype. MATERIALS AND METHODS The materials came from two morphologically identical populations (hereafter named P1 and P2) collected in the following localities: Argentina, C6rdoba: P1 = Punilla: Cabalango, Bernardello, Weigel & Moscone 359 bis, 29 Aug. 1982; P2 = Calamuchita: Emblase Rio III, camino a la segunda Usina, Moscone, Carrizo & Moscone 1, 24 Oct. 1982; we analyzed 35 plants of P1 and 16 of P2. Voucher specimens have been deposited in the herbarium of Museo Botdnico de C6rdoba, Argentina (CORD). Mitotic chromosomes were examined in root tip squashes; the root tips were fixed in 1: 3 acetic acid: ethanol mixture after pretreatment in a saturated solution of p-dichlorobenzene in water for 5-7 hours at room temperature, and then stained with alcoholic hydrochloric acid carmine (Snow, 1963) for at least 12 hours. Karyograms were prepared from microphotographs, using the terminology introduced by Levan et al. (1 964). Two parameters were used: the arm ratio and the centromeric index. The chromosomes were first arranged according to their increasing arm ratio and then according to the decreasing order within each group. The idiogram is based on ten metaphase plates (five from each population). Meiotic behavior was observed in pollen mother cells obtained by squashing young anthers fixed in 1: 3 acetic acid: ethanol and stained with acetic carmine. All the permanent mounts were made with Bradley's method (1948). The statistical methods followed Sokal and Rohlf (1979); as regards the t-test, the levels of significance considered were 0.05 and 0.01.