COMPARATIVE-ANALYSIS OF KARYOTYPES AND NUCLEOLAR ORGANIZER REGIONS INDIFFERENT POPULATIONS OF MYTILUS-TROSSULUS, MYTILUS-EDULIS AND MYTILUS-GALLOPROVINCIALIS

The cytotaxonomical characteristics of Mytilus trossulus, M. edulis an d M. galloprovincialis were studied using karyometric analysis and sil ver staining. The karyotype of M. trossulus, reported here for the fir st time, consists of six metacentric and eight submetacentric-subteloc entric chromosome pairs. The occurrence of five metacentric chromosome pairs in M. galloprovincialis differentiates this taxon from M. tross ulus and M. edulis which both have six metacentrics. The number of sub metacentric to subtelocentric chromosome pairs was variable between po pulations and between taxa. We suggest that this variability has arise n from differential chromosome condensation rather than from structura l change. Intercomparison of chromosome relative length showed strong similarity among the three taxa. Chromosomal nucleolar organizer regio ns (NORs) were identically located on two submetacentric-subtelocentri c chromosome pairs in all three taxa. However, M. trossulus showed a c haracteristic difference in the occurrence of NORs on one metacentric pair, which was absent in M. galloprovincialis and M. edulis. Variabil ity in the number of NORs per cell was observed in all populations stu died. We conclude that the three taxa of Mytilus studied here cannot b e differentiated by a single karyological character,but that a combina tion of karyological characters is virtually diagnostic.