CHROMOSOMES OF MELANOPSIS-DUFOURI FERUSSAC, 1823 (CAENOGASTROPODA, MELANOPSIDAE)

The most frequent chromosome numbers were 2n = 36 and n = 18 in Melano psis dufouri Ferussac inhabiting the Ullal de Baldovi, an ancient spri ng of the Albufera lake (Valencia province), and the small stream Ramb la de Gaibiel (Castellon province) in Spain. Subtelocentric and subtel o-telocentric chromosomes predominated in the sets of snails from both populations. Pericentric inversions were basic factors in the interpo pulational differentiation of karyotypes. Intrapopulational karyotype polymorphism (specimens with 2n = 36, 2n = 34, rarely Zn = 35) and mos aic molluscs occurred in the Ullal de Baldovi population as a result o f small chromosome elimination. The rather frequent appearance of one or two non-homologue small chromosomes in hypodiploid nuclei 2n = 35, 2n = 34 was obviously connected with fragmentation of chromosomes. Kar yological findings are in accordance with phenotypic diversity of M. d ufouri from the Ullal de Baldovi population.