Mitochondrial introgression by ancient admixture between two distant lacustrine fishes in Sulawesi Island

Sulawesi, an island located in a biogeographical transition zone between Indomalaya and Australasia, is famous for its high levels of endemism. Ricefishes (family Adrianichthyidae) are an example of taxa that have uniquely diversified on this island. It was demonstrated that habitat fragmentation due to the Pliocene juxtaposition among tectonic subdivisions of this island was the primary factor that promoted their divergence; however, it is also equally probable that habitat fusions and resultant admixtures between phylogenetically distant species may have frequently occurred. Previous studies revealed that some individuals of  Oryzias sarasinorum endemic to a tectonic lake in central Sulawesi have mitochondrial haplotypes that are similar to the haplotypes of  O. eversi , which is a phylogenetically related but geologically distant (ca. 190 km apart) adrianichthyid endemic to a small lake. In this study, we tested if this reflects ancient admixture of  O. eversi  and  O. sarasinorum .  Population genomic analyses of genome-wide single-nucleotide polymorphisms revealed that  O. eversi  and  O. sarasinorum  are substantially reproductively isolated from each other. Comparison of demographic models revealed that the models assuming ancient admixture from O. eversi  to  O. sarasinorum  was more supported than the models assuming no admixture; this supported the idea that the  O. eversi -like mitochondrial haplotype in  O. sarasinorum  was introgressed from  O. eversi . This study is the first to demonstrate ancient admixture of lacustrine organisms in Sulawesi beyond 100 km. The complex geological history of this island enabled such island-wide admixture of lacustrine organisms, which usually experience limited migration.