Sex, genomics and epigenetics (#41)
Although sex determining pathways are highly conserved between vertebrates, the first step shows remarkable variation. Mammals, including humans, have a conserved X chromosome, and a degenerate Y that contains the male-dominant SRY gene that evolved from the X-borne SOX3. The Y is hard to sequence because it is full of repetitive sequence. Birds present the reverse situation, a highly conserved Z chromosome common to both sexes and a female-specific, degenerate W. Sex appears to be determined by differential dosage of a Z-borne gene DMRT1. Reptiles and fish show a huge variety of sex determining mechanisms. Some are genetic, including XY systems (male heterogamety) and ZW systems (female heterogamety), some highly differentiated like the human XY and chicken ZW, and some cytologically homomorphic. Sequencing projects reveal that these systems use a variety of different genetic triggers, but, remarkably, the same genes (including SOX3 and DMRT1) have independently evolved sex determining functions in several lineages. Many reptiles lack sex chromosomes, determining sex via incubation temperature. Some species, like the dragon lizard and a flatfish, do both, having chromosomal sex determination at moderate temperature and a sex-switch override at extremes. These systems may prove to be the most informative, yielding novel sex determining genes or, more remarkably, orthologues of the same genes, and providin1 g insights into how they interact with the environment via epigenetic pathways.
- Graves, J.A.M., 2014. Chromosome Research 22: 45-57 Graves, J.A.M. 2014. Nature Genetics 46, 215–217 402. Graves, J.A.M. 2013. Developmental Dynamics 242: 354-359