COMPARATIVE ANALYSIS OF KARYOTYPES AND NUCLEOLAR ORGANIZER REGIONS IN DIFFERENT POPULATIONS OF MYTILUS TROSSULUS, MYTILUS EDULIS AND MYTILUS GALLOPROVINCIALIS

Abstract

The cylotaxonomical characteristics of Mytilus tros-sulus, M. edulis and M. galloprovincialis were studied using karyometric analysis and silver staining. The karyotype of M. trossulus, reported here for the first time, consists of six metacentric and eight submetacentric-subtelocentric chromosome pairs. The occurrence of five metacentric chromosome pairs in M. galloprovincialis differentiates this taxon from M. trossulus and M. edulis which both have six meta-centrics. The number of submetacentric to subtelo-centric chromosome pairs was variable between populations and between taxa. We suggest that this variability has arisen from differential chromosome condensation rather than from structural change. Intercomparison of chromosome relative length showed strong similarity among the three taxa. Chromosomal nucleolar organizer regions (NORs) were identically located on two submetacentric-subtelocentric chromosome pairs in all three taxa. However, M. trossulus showed a characteristic difference in the occurrence of NORs on one metacentric pair, which was absent in M. galloprovincialis and M. edulis. Variability in the number of NORs per cell was observed in all populations studied. We conclude that the three taxa of Mytilus studied here cannot be differentiated by asingle karyological character, but that a combination of karyological characters is virtually diagnostic