Biology subjects

Rohwer, S.

Publications and source records attributed to Rohwer, S..

3 recordsLinked to original sources

bioRxiv2020

Signatures of mito-nuclear climate adaptation in a warbler species complex

Mitochondrial (mtDNA) and nuclear (nDNA) genes interact to govern metabolic pathways of mitochondria. When differentiated populations interbreed at secondary contact, incompatibilities between mtDNA of one population and nDNA of the other could result in low fitness of hybrids. In northwestern North America, two hybridizing species of warblers, Setophaga occidentalis (abbreviated as SOCC) and S. townsendi (STOW), provide an excellent system to investigate the potential co-adaptation of mitochondrial and nuclear DNA. The inland population of STOW (inland STOW) harbors mtDNA haplotype that is half a million years divergent from the SOCC mtDNA, and these populations also differ strongly in a few nDNA regions. Coastal populations of STOW (coastal STOW) have mixed ancestry, consistent with ancient hybridization of SOCC and inland STOW-like population. Of the few highly-differentiated nDNA regions between inland STOW and SOCC, one of these regions (on chromosome 5) is also differentiated between coastal STOW and inland STOW, and covaries with mtDNA among coastal STOW populations. Genes in this 1.2Mb region of chromosome 5 are associated with fatty acid oxidation and energy-related signaling transduction, both of which are closely associated with mitochondrial function. This chromosome 5 region is correlated with mtDNA haplotypes both within and across sampling sites, a pattern consistent with mitonuclear co-adaptation. We show that such mitonuclear coevolution might be maintained by climate-related selection, because mitonuclear ancestry is correlated with climatic conditions among sampling sites. Together, our observation suggests climatic-associated adaptation shaping mitonuclear differentiation and introgression in this species complex.

evolutionary biology↗

bioRxiv2019

Selection on a pleiotropic color gene block underpins early differentiation between two warbler species

When one species gradually splits into two, divergent selection on specific traits can cause peaks of differentiation in the genomic regions encoding those traits. Whether speciation is initiated by strong selection on a few genomic regions with large effects or by more diffused selection on many regions with small effects remains controversial. Differentiated phenotypes between differentiating lineages are commonly involved in reproductive isolation, thus their genetic underpinnings are key to the genomics architecture of speciation. When two species hybridize, recombination over multiple generations can help reveal the genetic regions responsible for the differentiated phenotypes against a genomic background that has been homogenized via backcrossing and introgression. We used admixture mapping to investigate genomic differentiation and the genetic basis of differentiated plumage features (relative melanin and carotenoid pigment) between hybridizing sister species in the early stage of speciation: Townsends (Setophaga townsendi) and Hermit warblers (S. occidentalis). We found a few narrow and dispersed divergent regions between allopatric parental populations, consistent with the divergence with gene flow model of speciation. One of the divergent peaks involves three genes known to affect pigmentation: ASIP, EIF2S2, and RALY (the ASIP-RALY gene block). After controlling for population substructure, we found that a single nucleotide polymorphism (SNP) inside the intron of RALY displays a strong pleiotropic association with cheek, crown, and breast coloration. In addition, we detect selection on the ASIP-RALY gene block, as the geographic cline of the RALY marker of this gene block has remained narrower than the plumage cline, which remained narrower than expected under neutral diffusion over two decades. Despite extensive gene flow between these species across much of the genome, the selection on ASIP-RALY gene block maintains stable genotypic and plumage difference between species allowing further differentiation to accumulate via linkage to its flanking genetic region or linkage-disequilibrium genome-wide.

evolutionary biology↗

bioRxiv2018

Cryptic and extensive hybridization between ancient lineages of American crows

Most species and therefore most hybrid zones have historically been described using phenotypic characters. However, both speciation and hybridization can occur with negligible morphological differentiation. The Northwestern Crow (Corvus caurinus) and American Crow (Corvus brachyrhynchos) are sister taxonomic species with a continuous distribution that lack reliable traditional characters for identification. In this first population genomic study of Northwestern and American crows, we use genomic SNPs (nuDNA) and mtDNA to investigate whether these crows are genetically differentiated and the extent to which they may hybridize. We found that American and Northwestern crows have distinct evolutionary histories, supported by two nuDNA ancestry clusters and two 1.1%-divergent mtDNA clades dating to the late Pleistocene, when glacial advances may have isolated crow populations in separate refugia. We document extensive hybridization, with geographic overlap of mtDNA clades and admixture of nuDNA across >1,400 km of western Washington and western British Columbia. This broad hybrid zone consists of late-generation hybrids and backcrosses, not recent (e.g., F1) hybrids. Nuclear DNA and mtDNA clines were both centered in southwestern British Columbia, farther north than previously postulated. The mtDNA cline was narrower than the nuDNA cline, consistent with Haldanes rule but not sex-biased dispersal. Overall, our results suggest a history of reticulate evolution in American and Northwestern crows, consistent with potentially recurring neutral expansion(s) from Pleistocene glacial refugia followed by lineage fusion(s). However, we do not rule out a contributing role for more recent potential drivers of hybridization, such as expansion into human-modified habitats.

evolutionary biology↗