B chromosomes affect sex determination in cichlid fishes

B chromosomes have a functional effect on sex determination in a species of cichlid fishes from Lake Victoria, according to a study by Japanese researchers to be published in open-access journal PLoS Genetics on August 18th, 2011. The researchers found sex-ratio distortions caused by B chromosomes in the breeding line of the cichlids, as well as several protein-coding genes in the B chromosomes. The resultant ratio was female biased, suggesting a role for B chromosomes in female sex determination.

The B chromosome was first identified by E. B. Wilson in 1907. B chromosomes (or accessory chromosomes) are additional to the standard set of chromosomes (autosomes and sex chromosomes) in an organism. They have been found in many species (more than 1,000) over the last hundred years. Although they are widely distributed, the effect of B chromosomes on host organisms has not been detected. Indeed, B chromosomes have been thought to be a selfish genetic element, lacking genes and having no effect on the organism. It has previously been proposed that the unique features shared by B chromosomes and sex chromosomes imply an evolutionary relation between them. However, research into B chromosomes has been limited to a few species and the question has not been resolved.

In the present study, the researchers performed an extensive analysis of B chromosomes in cichlid fishes from Lake Victoria to investigate their effect on cichlid evolution. Karyotype analysis of Lake Victoria cichlids suggested that, in one species, Lithochromis rubripinnis, there are female-specific B chromosomes. Crossbreeding experiments suggested that an effector of female sex determination in this species was the B chromosome. Furthermore, analyses of large-scale sequences of B chromosomes in Lake Victoria cichlids revealed multiple protein-coding genes in B chromosomes.

The sex determination gene was not found in this research. However, the researchers provide evidence that B chromosomes have a functional effect on sex determination and that they demonstrate a capacity to evolve into sex chromosomes.