MULTIPLE SEX-CHROMOSOME SYSTEM AND OTHER KARYOLOGICAL CHARACTERIZATIONS OF PTEROTRACHEA-HIPPOCAMPUS (MOLLUSCA, MESOGASTROPODA)

Two modal diploid numbers of chromosomes were found for Pterotrachea h ippocampus Philippi (Mollusca: Mesogastropoda) collected from the Gulf of Palermo in 1990: 2n = 31 and 32 for males and females, respectivel y. This, along with other karyological characteristics such as the occ urrence of a trivalent configuration at diakinesis and two types of me taphase-II spreads in spermatocytes, supports the notion that a X1X2Y male/X1X1X2X2 male sex mechanism operates in the species investigated here. Silver nitrate procedure revealed an intraindividual variation i n the Ag-staining pattern occurring in this species. The majority of t he chromosome pairs displayed terminal and/or interstitial heterochrom atic blocks after C-banding.