Karyotypes and sex ratios in populations of Eubranchipus grubii (Dybowski, 1860) and Branchipus schaeferri Fischer, 1834 (Anostraca) from Poland
Abstract
Two populations of the fairy shrimp Eubranchipus grubii (Dybowski, I 860) and one population of Branchipus schaefferi Fischer, 1834 from Poland were studied cytogenetically. The aim was to confirm their diploid chromosome numbers and chromosome morphology, as well as to reveal the distribution of nucleolar organizer regions (NORs) and the course of meiosis.
The first species exhibited 24 small chromosomes in both sexes, while the latter showed 2n = 20 in both males and females. In E. grubii, female meiosis involved a "diffuse stage" characterized by decondensation of chromatin, and positive heteropycnosis (after staining by Giemsa) of one bivalent.
The morphology of the plates of the second meiotic division suggests the chromosomes in both species are monocentric (prevailingly bi-armed). Nevertheless, constitutive heterochromatin has not been visualized by C-banding.
Silver staining revealed 3 pairs of nucleolar organizer regions (NOR) in B. schaefferi, while in E. grubii the detection of NORs failed. In contrast to some previous studies reporting supernumerary chromosomes (B chromosomes) as sex ratio distorters in B. schaefferi, we failed to observe any B chromosomes in the populations under study.
We suggest the fluctuation of sex ratios as found in the local populations, would rather be influenced by the environment.