KARYOTYPE ANALYSIS OF THE SEA-URCHIN PARACENTROTUS-LIVIDUS (ECHINODERMATA) - EVIDENCE FOR A HETEROMORPHIC CHROMOSOME SEX MECHANISM

A consistent diploid number of 2n = 36 was determined for the sea urch in Paracentrotus lividus from the Gulf of Palermo by analysis of mitot ic chromosomes of both early developing embryos and male gonads. The h aploid number n = 18 was determined by counts of spermatocyte bivalent s at diakinesis. A heteromorphic chromosome sex mechanism of the XY ty pe is likely present in this species. This is indicated by the occurre nce of a chromosomal pair, pair No. 2, which is heteromorphic in both morphology and size in about 50% of the mitotic figures (metaphases an d anaphases) of embryos. In addition, heteromorphism of the same pair of chromosomes occurred during spermatogonial metaphases in the five m ale specimens investigated here. The detection of a low chromosome num ber (2n = 36) compared to other echinoids (2n = 42 to 44), a heteromor phic chromosome sex system and the involvement of three chromosome pai rs in nucleolar organization (NORs) provide evidence of the specializa tion of the P. lividus karyotype.