Sex is determined by sex chromosomes in Littorina saxatilis (Olivi) (Gastropoda, Prosobranchia)

Abstract

The karyotype of Littorina saxatilis was determined as 2n = 34 in male and female individuals from Galician (NW Spain) populations. The chromosome number and morphology observed in our sample is similar to that previously reported for this species, with the exception of a new heteromorphic chromosome pair. This chromosome pair was consistently found in all males and half of the embryos, implying an involvement in chromosomal sex determination. Analysis or diplotene cells from males showed a monochiasmate heteromorphic bivalent which could be related to the putative X and Y sex chromosomes. In order to reveal any possible banding pattern on mitotic and meiotic chromosomes, untreated and C-banded (CB) chromosomes were stained using Giemsa (G), 4,6-diamidino-2-phenylindole (DAPI) and Chromomycin A3 (CMA). Only CB-G and CB-DAPI allowed identification of one positively stained region in prophase cells. Silver-staining, to detect active rDNA, allowed detection of one silver precipitate in prophase cells, which may correspond to at least one pair of main nucleolar organizer regions (NORs) localized on one chromosome pair. The region revealed with CB-G and CB-DAPI was found to coincide spatially with the silver precipate, which would suggest that both techniques detect the nucleolar structures.