CHROMOSOME NUMBER, STRUCTURE AND AUTOSOMAL POLYMORPHISM IN THE MARINE MUSSELS MYTILUS EDULIS AND MYTILUS CALIFORNIANUS

¹ College of Fisheries, University of Washington, Seattle, Washington 98105

The bay mussel Mytilus edulis (Linnaeus) and the California sea mussel Mytilus californianus (Conrad) have a common diploid chromosome number 28 (n = 14) that is, there were 14 bivalents during meiosis and 28 chromosomes during mitosis. A total of 35 specimens of the two species were examined from several intertidal locations in Puget Sound and the Northwest Pacific coast of the United States. The number of meiotic bivalents in stripped or spawned eggs was always constant but aneuploid counts of mitotic chromosomes were made in cleaving eggs and embryos. Heteromorphic bivalents in meiotic plates and heteromorphic homologues in mitotic karyotypes were observed. The number of metacentric, submetacentric and acrocentric chromosomes was variable. There were 2 to 6 pairs of acrocentrics in different complements. This polymorphism indicates the existence of pericentric inversions or centric-shifts in both meiotic and mitotic chromosomes.