Origins of two hemiclonal hybrids among three species (Teleostei: Hexagrammidae): genetic diversification through host switching.

Two natural, hemiclonal hybrid strains were discovered in three species. The natural hybrids, all of which were females that produced haploid eggs containing only the genome (maternal ancestor; hereafter ), generated F hybrid-type offspring by fertilization with haploid sperm of or (paternal species; and , respectively). This study was performed to clarify the extent of diversification between the two hybrids and the maternal ancestor. Genealogical analysis using mtDNA revealed that all 38 / hybrids formed a branch (Branch I) with 18 of the 33 / hybrids. No haplotype sharing was observed with the maternal ancestor. Further, microsatellite DNA analysis suggested that the members of Branch I shared the same hemiclonal genome set. The results suggested that / hybrids originated by anomalous hybridization, or "host switching," between / and , and not from interspecific hybridization between and . The remaining 9 of 11 / haplotypes and all of the 27 haplotypes were mixed within the genealogical tree, as if they had originated from multiple mutations. However, / could also mate with . Although offspring from this host switch (Backcross-) have the same genome as normal , a part of their genome retains genetic factors capable of producing hemiclones. Consequently, when a descendant of a BC- hybrid mates with males, a new hemiclone lineage will arise. Multiple haplotype revival through host switching from a single mutation in hybrids is another possible hypothesis for the observed mixing of / haplotypes within the mtDNA genealogical tree.