Chromosome diversity in Mediterranean and Antarctic euphausiid species (Euphausiacea)
Résumé
Chromosome number and morphology were studied in gonadal tissue from eight euphausiid species, using an air-drying technique and Giemsa staining. Among Mediterranean species, haploid chromosome numbers were: n = 19 in Euphausia hemigibba, Euphausia brevis, and Nematoscelis megalops, n = 18 in Euphausia krohni, and n = 11 in Nyctiphanes couchi. Among Antarctic species, haploid chromosome numbers were: n = 17 in Euphausia superba, n = 20 in Thysanoessa macrura, and n = 13 in Thysanoessa vicina. Chromosome morphology was further studied for three species after chromosome measurements: Euphausia hemigibba karyotype showed 17 metacentric and 2 submetacentric chromosome pairs, Euphausia krohni 17 metacentric and 1 submetacentric chromosome pairs, and Nematoscelis megalops 17 metacentric and 2 submetacentric chromosome pairs. Three species sharing the same chromosome number of n = 19, i.e., Euphausia hemigibba, Nematoscelis megalops, and Meganyctiphanes norvegica were distinguished by a different slope of the decrease in chromosome size and by the presence/absence of metacentric and submetacentric chromosome pairs differing in number and position. Chromosome morphology of other species was derived from the observation of meiotic metaphases II. A majority of metacentric chromosomes was also displayed. Cytotaxonomic differences in haploid chromosome number allow the discrimination of morphologically neighboring sympatric species (e.g., Meganyctiphanes norvegica and Nyctiphanes couchi; Thysanoessa vicina and T. macrura). Euphausiids share common chromosomal features which clearly separate them from other eucarids, i.e., a narrow range of haploid chromosome number, a large chromosome size, and a majority of metacentric chromosomes.