Species divergence and repeated ancient hybridization in a Sulawesian lake system

Ghost introgression in ricefishes of the genus Adrianichthys in an ancient Wallacean lake

Because speciation might have been promoted by ancient introgression from an extinct lineage, it is important to detect the existence of 'ghost introgression' in focal taxa and examine its contribution to their diversification. In this study, we examined possible ghost introgression and its contributions to the diversification of ricefishes of the genus Adrianichthys in Lake Poso, an ancient lake on Sulawesi Island, in which some extinctions are known to have occurred. Population-genomic analysis revealed that two extant Adrianichthys species, A. oophorus and A. poptae are reproductively isolated from each other. Comparisons of demographic models demonstrated that introgression from a ghost population, which diverged from the common ancestor of A. oophorus and A. poptae, is essential for reconstructing the demographic history of Adrianichthys. The best model estimated that the divergence of the ghost population greatly predated the divergence between A. oophorus and A. poptae, and that the ghost population secondarily contacted the two extant species within Lake Poso more recently. Genome scans and simulations detected a greatly divergent locus, which cannot be explained without ghost introgression. This locus was also completely segregated between A. oophorus and A. poptae. These findings suggest that variants that came from a ghost population have contributed to the divergence between A. oophorus and A. poptae, but the large time-lag between their divergence and ghost introgression indicates that the contribution of introgression may be restricted.

Species Delimitation, Phylogenomics, and Biogeography of Sulawesi Flying Lizards: A Diversification History Complicated by Ancient Hybridization, Cryptic Species, and Arrested Speciation

The biota of Sulawesi is noted for its high degree of endemism and for its substantial levels of in situ biological diversification. While the island's long period of isolation and dynamic tectonic history have been implicated as drivers of the regional diversification, this has rarely been tested in the context of an explicit geological framework. Here, we provide a tectonically informed biogeographical framework that we use to explore the diversification history of Sulawesi flying lizards (the Draco lineatus Group), a radiation that is endemic to Sulawesi and its surrounding islands. We employ a framework for inferring cryptic speciation that involves phylogeographic and genetic clustering analyses as a means of identifying potential species followed by population demographic assessment of divergence-timing and rates of bi-directional migration as means of confirming lineage independence (and thus species status). Using this approach, phylogenetic and population genetic analyses of mitochondrial sequence data obtained for 613 samples, a 50-SNP data set for 370 samples, and a 1249-locus exon-capture data set for 106 samples indicate that the current taxonomy substantially understates the true number of Sulawesi Draco species, that both cryptic and arrested speciations have taken place, and that ancient hybridization confounds phylogenetic analyses that do not explicitly account for reticulation. The Draco lineatus Group appears to comprise 15 species-9 on Sulawesi proper and 6 on peripheral islands. The common ancestor of this group colonized Sulawesi ~11 Ma when proto-Sulawesi was likely composed of two ancestral islands, and began to radiate ~6 Ma as new islands formed and were colonized via overwater dispersal. The enlargement and amalgamation of many of these proto-islands into modern Sulawesi, especially during the past 3 Ma, set in motion dynamic species interactions as once-isolated lineages came into secondary contact, some of which resulted in lineage merger, and others surviving to the present. [Genomics; Indonesia; introgression; mitochondria; phylogenetics; phylogeography; population genetics; reptiles.].

Potential Contribution of Ancient Introgression to the Evolution of a Derived Reproductive Strategy in Ricefishes

Transitions from no parental care to extensive care are costly and involve major changes in life history, behavior, and morphology. Nevertheless, in Sulawesi ricefishes, pelvic brooding evolved from transfer brooding in two distantly related lineages within the genera Adrianichthys and Oryzias, respectively. Females of pelvic brooding species carry their eggs attached to their belly until the fry hatches. Despite their phylogenetic distance, both pelvic brooding lineages share a set of external morphological traits. A recent study found no direct gene flow between pelvic brooding lineages, suggesting independent evolution of the derived reproductive strategy. Convergent evolution can, however, also rely on repeated sorting of preexisting variation of an admixed ancestral population, especially when subjected to similar external selection pressures. We thus used a multispecies coalescent model and D-statistics to identify gene-tree-species-tree incongruencies, to evaluate the evolution of pelvic brooding with respect to interspecific gene flow not only between pelvic brooding lineages but also between pelvic brooding lineages and other Sulawesi ricefish lineages. We found a general network-like evolution in Sulawesi ricefishes, and as previously reported, we detected no gene flow between the pelvic brooding lineages. Instead, we found hybridization between the ancestor of pelvic brooding Oryzias and the common ancestor of the Oryzias species from the Lake Poso area. We further detected signs of introgression within the confidence interval of a quantitative trait locus associated with pelvic brooding in O. eversi. Our results hint toward a contribution of ancient standing genetic variation to the evolution of pelvic brooding in Oryzias.

Multiple colonizations and hybridization of a freshwater fish group on a satellite island of Sulawesi

Repeated colonizations and resultant hybridization may increase lineage diversity on an island if introgression occurs only in a portion of the indigenous island lineage. Therefore, to precisely understand how island biodiversity was shaped, it is essential to reconstruct the history of secondary colonization and resultant hybridization both in time and space. In this study, we reconstructed the history of multiple colonizations of the Oryzias woworae species group, a freshwater fish group of the family Adrianichthyidae, from Sulawesi Island to its southeast satellite island, Muna Island. Phylogenetic and species tree analyses using genome-wide single-nucleotide polymorphisms revealed that all local populations on Muna Island were monophyletic, but that there were several genetically distinct lineages within the island. Population structure and phylogenetic network analyses demonstrated that colonization of this island occurred more than once, and that secondary colonization and resultant introgressive hybridization occurred only in one local population on the island. The spatially heterogeneous introgression induced by the multiple colonizations were also supported by differential admixture analyses. In addition, the differential admixture analyses detected reverse colonization from Muna Island to the Sulawesi mainland. Coalescence-based demographic inference estimated that these mutual colonizations occurred during the middle to late Quaternary period, during which sea level repeatedly declined; this indicates that the colonizations occurred via land bridges. We conclude that these mutual colonizations between Muna Island and the Sulawesi mainland, and the resultant spatially heterogeneous introgression shaped the current biodiversity of this species group in this area.

Genetic basis for the evolution of pelvic-fin brooding, a new mode of reproduction, in a Sulawesian fish

Modes of reproduction in animals are diverse, with different modes having evolved independently in multiple lineages across a variety of taxa. However, an understanding of the genomic change driving the transition between different modes of reproduction is limited. Several ricefishes (Adrianichthyidae) on the island of Sulawesi have a unique mode of reproduction called "pelvic-fin brooding," wherein females carry externally fertilized eggs until hatching using their pelvic fins. Phylogenomic analysis demonstrated pelvic-fin brooders to have evolved at least twice in two distant clades of the Adrianichthyidae. We investigated the genetic architecture of the evolution of this unique mode of reproduction. Morphological analyses and laboratory observations revealed that females of pelvic-fin brooders have longer pelvic fins and a deeper abdominal concavity, and that they can carry an egg clutch for longer than non-brooding adrianichthyids, suggesting that these traits play important roles in this reproductive mode. Quantitative trait locus mapping using a cross between a pelvic-fin brooder Oryzias eversi and a non-brooding O. dopingdopingensis reveals different traits involved in pelvic-fin brooding to be controlled by different loci on different chromosomes. Genomic analyses of admixture detected no signatures of introgression between two lineages with pelvic-fin brooders, indicating that introgression is unlikely to be responsible for repeated evolution of pelvic-fin brooding. These findings suggest that multiple independent mutations may have contributed to the convergent evolution of this novel mode of reproduction.