Structure and meiotic behaviour of B chromosomes in Sphaerium corneum/S. nucleus complex (Bivalvia: Sphaeriidae)
Abstract
Karyotypes of eight populations of Sphaerium corneum and two populations of S. nucleus (Bivalvia: Sphaeriidae) were compared. The basic chromosome set is formed by 30 biarmed autosomes and exhibits only slight interpopulational variation in morphology.
One pair of nucleolar organiser regions was detected by silver staining. The first meiotic division in both species is highly modified, including a diffuse stage.
The diffuse stage has not been reported in bivalves so far. Bivalents of the following stages are achiasmatic.
The two species are further peculiar for occurrence of B chromosomes. The small metacentric B chromosomes exhibit intra- and interindividual variability in number, they show irregular meiotic pairing and segregation (formation of bivalents or univalents), and possess larger proportional amount of constitutive heterochromatin than the A chromosomes.
Interestingly, the B chromosomes also undergo decondensation during the diffuse stage like A chromosomes.