ABSTRACT
In annelids, sex is determined by genes harbored on one or more chromosomes. Karyotypic heterogametism is described for five polychaete species alone. Selective breeding has led to the discovery of heterogametism in Dinophilus gyrociliatus (XX-XO) and Capitella capitata (ZW-ZZ). For the first time, a directory is assembled for chromosome numbers in annelids. By selective fertilization of large eggs by X-carrying sperm, D. gyrociliatus females have nullified the chromosomal mechanism of sex determination. In C. capitata, the expression of W gene(s) is relatively more stable and not amenable to environmental factors like density. But the gene(s) in Z chromosome are amenable. Consequently, phenotypic ZZ females and ZZ hermaphrodites are generated. The gametic compatibility between Hediste spp and high level of incompatibility between two morphs of Spirobranchus polycerus and that for the distantly located populations in Galeolaria caespitosa can be explained by the role played by bindin in the acrosome as an adhesive to attach the sperm to vitelline layers of eggs.
