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Sympatric speciation has been demonstrated in few empirical case studies,
despite intense searches, because of difficulties in testing the criteria
for this mode of speciation. Here, we report a possible case of sympatric
speciation in ricefishes of the genus Oryzias on Sulawesi, an island of
Wallacea. Three species of Oryzias are known to be endemic to Lake Poso,
an ancient tectonic lake in central Sulawesi. Phylogenetic analyses using
RAD-seq-derived single nucleotide polymorphisms (SNPs) revealed that these
species are monophyletic. We also found that the three species are
morphologically distinguishable and clearly separated by
population-structure analyses based on the SNPs, suggesting that they are
reproductively isolated from each other. A mitochondrial DNA chronogram
suggested that their speciation events occurred after formation of the
tectonic lake, and existence of a historical allopatric phase was not
supported by coalescent-based demographic inference. Demographic inference
also suggested introgressive hybridization from an outgroup population.
However, differential admixture among the sympatric species was not
supported by any statistical tests. These results all concur with criteria
necessary to demonstrate sympatric speciation. Ricefishes in this
Wallacean lake provide a promising new model system for the study of
sympatric speciation.
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