Resource partitioning is not coupled with assortative mating in sympatrically divergent ricefish in a Wallacean ancient lake.

Sympatric speciation is considered to be difficult without the coupling between ecological traits that allow resource partitioning and reproductive traits that allow assortative mating. Such "magic traits" are known to be involved in most of the compelling examples of sympatric speciation. In this study, we report a possible case of sympatric speciation without magic traits. Three species of ricefish (genus Oryzias) are suggested to have diverged sympatrically within Lake Poso, an ancient lake in Sulawesi. An analysis of genome-wide single-nucleotide polymorphisms showed that these three species are reproductively isolated from each other throughout the lake. Stable isotope analyses revealed that the three species use different food resources, which reflect differences in their feeding morphologies (gill rakers and digestive tracts) and feeding sites. Field and laboratory observations showed that O. nebulosus and O. orthognathus share a mating habitat of cobbles, where they scatter fertilized eggs, whereas this site is never used by O. nigrimas, indicating that assortative mating is partly achieved by spatial isolation. The small, less-adhesive eggs of O. nebulosus and O. orthognathus probably reflect their adaptation to spawning on cobble beaches. Laboratory mating experiments showed strong prezygotic isolation between O. nebulosus and O. orthognathus, which is achieved by strong species recognition presumably by both sexes based on species-specific mating dances and nuptial coloration. In summary, the assortative mating of O. nebulosus and O. orthognathus is probably not coupled to resource partitioning. We discussed how sympatric speciation among these species might have been achieved even without magic traits.