Drivers and constraints of rapid adaptive diversification in Lake Poso ricefishes, an emerging model species flock

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Evolutionary radiations, that is the increase of species diversity in a clade, have given rise to the bulk of biodiversity. Yet the processes shaping radiations remain an area of active research and controversial discussion. Species flocks endemic to habitat islands allow studying the genomic background of adaptive divergence, including processes of reticulate evolution. Ecological opportunity is considered as the main driver of “adaptive radiation”, but appears insufficient to explain why lineages differ in their ability to adapt and diversify. Hybridization of related lineages can increase genetic and phenotypic variation, which in turn can facilitate population divergence in response to divergent selection. Here, we test hypotheses on the roles of ecological opportunity and hybridization in adaptive diversification in a single, small system closely related to one of the major vertebrate model organisms, the medaka (Oryzias latipes). The monophyletic species flock of three Oryzias ricefish species is endemic to ancient Lake Poso in Sulawesi, Indonesia. We test if ecological speciation can explain divergence of Lake Poso Oryzias, and hypothesize that competition with a sympatric but not closely related ricefish lineage constrained the spectrum of adaptation. We further test if hybridization of the lake fishes with stream populations affected lake fish evolution in interplay with ecological opportunity. Pilot data collected in the field suggest that the three Oryzias show largely distinct ecological niches, matching differences in their morphology, and in functional traits of trophic relevance. The data also suggest that their habitat niches are complementary to the sympatric ricefish lineage. We will further extend these observation data by implementing ROV-based transect methods, in order to characterize habitat use in in detail – based on sound sample sizes, and covering deep and offshore habitats. Fieldwork moreover revealed a potential hybrid population, which might account for signals of genomic introgression. Preliminary RADseq data distinguish the three lake species, and suggest relatedness of the potential hybrid fish with two of the lake species. The extended sampling and sequencing approach proposed, covering potential parental populations from habitats close by the lake, will enable appropriate testing of the hybridization hypothesis benefitting from the close relatedness of the Oryzias flock to the model organism medaka. The complementary in-depth approaches will deliver a comprehensive understanding of the evolutionary history, and the drivers of adaptive divergence in the small Lake Poso Oryzias species flock.