A survey for sex determination systems in East African Lake Malawi cichlids.

摘要

African cichlids are at an early stage in the evolution of sex chromosomes. Their sex chromosomes have not become distinct from the rest of their autosomal chromosomes, and no morphologically distinct sex chromosomes have been identified (Kornfield, 1984). These fishes therefore provide a unique model for understanding the early stages of sex chromosome evolution. It is further speculated that evolution of sex chromosomes may have an effect on cichlid speciation as well.; Lake Malawi formed in East African ∼700,000 years ago and contains a diverse flock of endemic cichlid fish (Danley at al., 2001). Within this flock the genus Metriaclima contains ∼70 species. Genotyping of families from seven species of Metriaclima identified a male heterogametic (XX: XY) sex determiner on linkage group 7. The genotypes of three other Metriaclima species identified a female heterogametic (WZ: ZZ) sex determiner on linkage group 5. One species, Metriaclima pyrsonotus , was segregating both the male (XY) and the female (WX) heterogametic sex determination systems. Two outgroup species, Aulonocara baenschi and Pseudotropheus 'dakarti', segregate for the male heterogametic system on linkage group 7. Thus at least two distinct genetic sex determination systems are present within the Lake Malawi species flock. The male heterogametic (XX: XY) sex determiner localized to a 10 cM region on linkage group 7. Within this region, comparative mapping identified four candidate genes for sex determination. The female heterogametic (WZ: ZZ) sex determiner localized to a 7.6 cM region on linkage group 5. These sex-determining loci in haplochromine cichlids are on different chromosomes than the sex-determining loci identified in tilapia, which are on chromosomes one and three.