XY chromosome sex systems in the neogastropods Fasciolaria lignaria and Pisania striata (Mollusca : Prosobranchia)

The mitotic and meiotic chromosomes of the neogastropods Fasciolaria lignar ia (L. 1758) (Fasciolaridae) and Pisania striata (Gmelin 1791) (Buccinidae) have been analyzed. Both species display 70 chromosomes in spermatogonial mitoses and 35 bivalents in the corresponding spermatocytes. In both F. lig naria and P. striata pair 35 was found to be heteromorphic in the males and homomorphic in the females, thus; suggesting that an XY(male)/XX(female) m echanism of chromosomal sex determination is operating. Data obtained from combined C-banding and chromomycin A, (CMA,), mithramycin (MM), and DAPI st aining suggest that the differentiated Y chromosome of F: lignaria does not possess heterogeneous DNA. The possible origins of XY chromosomes in these neogastropods are discussed in the light of the current views of sex chrom osome evolution.