Linkage of genes responsible for sex determination and expression of a sperm-specific isozyme of glucose-6-phosphate isomerase in the polychaete Polydora brevipalpa

Abstract

In Polydora brevipalpa (Polychaeta, Spionidae), an unusual electrophoretic pattern of glucose-6-phosphate isomerase (GPI) which is determined by a single GPI gene locus and represented by two isozymes (GPI-1 and GPI-2) has been revealed. The anode isozyme (GPI-1) was detected in both males and females, whereas the cathode enzyme GPI-2 appears in males and results from alternative splicing of the pre-mRNA transcripts. In males, GPI-2 was shown to express only in sperm. The results obtained suggest that the genes responsible for sex determination and expression of the alternative isozyme GPI-2 are tightly linked. However, rare recombination events between these genes produce reciprocal deviant GPI phenotypes in males (GPI-2 is absent) and females (GPI-2 is present). In the sample of 376 individuals (202 males and 174 females) six recombinant males and six recombinant females were found, which ratio is in good agreement with the expected 1:1 ratio. Recombinant males of different genotypes at the GPI locus were also found. This indicates the absence of negative selection against all genotypes of the locus, except those carrying the unique allele that controls the cathode allozyme GPI-1. The results obtained suggest genetic sex determination in P. brevipalpa. Based on the fact that genetic sex determination is also characteristic of other species of the genus Polydora, which diverged independently during several million years, and taking into account that only six recombination events occurred in the history of 376 P. brevipalpa genomes examined, the conclusion is made that the gene responsible for sex determination and that responsible for the expression of the alternative GPI-2 isozyme are tightly linked.