Independent evolution for sex determination and differentiation in the family in animals.

Some family genes including arthropod , nematode , and vertebrate are involved in sex determination and/or differentiation in bilaterian animals. Although there have been some reports about evolutionary analyses of the family by using its phylogenetic trees, it is still undecided as to whether these three sex determination-related genes share orthologous relationships or not. To clarify this question, we analyzed evolutional relationships among the family members in various bilaterians by using not only phylogenetic tree analysis, but also synteny analysis. We found that only four genes, , , and were commonly present in invertebrate bilateria. The syntenies of - and - are conserved before and after two rounds of whole genome duplication in the ancestral vertebrate. Importantly, this indicates that must have appeared in the common vertebrate ancestor. In addition, , , or formed each different cluster at a distance in our phylogenetic tree. From these findings, we concluded that the three sex determination-related genes, , , and have no orthologous relationships, and suggested independent evolution for sex determination and differentiation in the gene family. Our results may supply clues about why sex-determining systems have diverged during animal evolution.