A COMPARATIVE-STUDY OF THE CHROMOSOMES OF MYTILUS-EDULIS AND MYTILUS-GALLOPROVINCIALIS

The chromosomes of the marine mussel Mytilus edulis, from Whitsand Bay, SE Cornwall and Robin Hood''s Bay, N Yorkshire [UK], were compared with those of the Mediterranean mussel, M. galloprovincialis, from Venice [Italy]. Chromosome counts made on cell preparations from gill tissues showed that both species have a diploid chromosome number of 28. In M. edulis and M. galloprovincialis the karyotype consists of six metacentric pairs (Arm ratio = 1-1.7)and a variable number of submetacentric (A.r., 1.7-3) and subtelocentric (A.r., 3-7) pairs. There was a marked degree of similarity between the karyotypes of both species in terms of chromosome size. Mean arm ratio similarly showed little interpopulation variation in the case of the 6 metacentric pairs. Some differences in mean arm ratio were apparent, however, amongst the 8 submetacentric pairs. This was largely concentrated on submetacentric pair 8, which is characterized by the presence of a large, pale-staining, heterochromatic (C-band positive), terminal region on the long arm. The number of subtelocentric pairs varied from cell to cell and also between individuals. The maximum number of subtelocentrics was 5 but the usual figure was 2 or 3, with no evidence of significant population differences. The results of this study do not support the existence of a major chromosomal difference between M. edulis and M. galloprovincialis.