Biology subjects

Mila, B.

Publications and source records attributed to Mila, B..

4 recordsLinked to original sources

bioRxiv2018

Change in sexual signaling traits outruns morphological divergence in a recent avian radiation across an ecological gradient

The relative roles of natural and sexual selection in promoting evolutionary lineage divergence remains controversial and difficult to assess in natural systems. Local adaptation through natural selection is known to play a central role in adaptive radiations, yet secondary sexual traits can vary widely among species in recent radiations, suggesting that sexual selection may also be important in the early stages of speciation. Here we compare rates of divergence in ecologically relevant traits (morphology) and sexually selected signaling traits (coloration) relative to neutral structure in genome-wide molecular markers, and examine patterns of variation in sexual dichromatism to understand the roles of natural and sexual selection in the diversification of the songbird genus Junco (Aves: Passerellidae). Juncos include divergent lineages in Central America and several dark-eyed junco (J. hyemalis) lineages that diversified recently as the group recolonized North America following the last glacial maximum (c.a. 18,000 years ago). We found an accelerated rate of divergence in sexually selected characters relative to ecologically relevant traits. Moreover, a synthetic index of sexual dichromatism comparable across lineages revealed a positive relationship between the degree of color divergence and the strength of sexual selection, especially when controlling for neutral genetic distance. We also found a positive correlation between dichromatism and latitude, which coincides with the latitudinal pattern of decreasing lineage age but also with a steep ecological gradient. Finally, we detected an association between outlier loci potentially under selection and both sexual dichromatism and latitude of breeding range. These results suggest that the joint effects of sexual and ecological selection have played a role in the junco radiation and can be important in the early stages of lineage formation.

evolutionary biology↗

bioRxiv2018

Local adaptation and sexual selection drive intra-island diversification in a songbird lineage: differential divergence in autosomes and sex chromosomes.

Recently diverged taxa showing marked phenotypic and ecological diversity are optimal systems to test the relative importance of two major evolutionary mechanisms, adaptation to local ecological conditions by natural selection, or mechanisms of reproductive isolation such as assortative mating mediated by sexually selected mating signals or post-zygotic incompatibilities. Whereas local adaptation is expected to affect many loci throughout the genome, traits acting as mating signals are expected to be located on sex chromosomes and have a simple genetic basis. We used genome-wide markers to test these predictions in Reunion Islands gray-white eye (Zosterops borbonicus), which has recently diversified into five distinct plumage forms. Two of them correspond to a polymorphic highland population that is separated by a steep ecological gradient from three distinct lowland forms that show narrow contact zones in plumage color traits, yet no association with environmental variables. An analysis of population structure using genome-wide SNP loci revealed two major clades corresponding to highland and lowland forms, respectively, with the latter separated further into three independent lineages corresponding to plumage forms. Coalescent tests of alternative demographic scenarios provided support for divergence of highland and lowland lineages with an intensification of gene flow in the last 60,000 years. Landscapes of genomic variation revealed that signatures of selection associated with elevation are found at multiple regions across the genome, whereas most loci associated with the lowland forms are located on the Z sex chromosome. A gene ontology analysis identified TYRP1, a Z-linked color gene, as a likely candidate locus underlying color variation among lowland forms. Our results are consistent with the role of natural selection in driving the divergence of locally adapted highland populations, and the role of sexual selection in differentiating lowland forms through reproductive isolation mechanisms, showing that both modes of lineage divergence can take place at very small geographic scales in birds.

evolutionary biology↗

bioRxiv2018

A role for differential gene regulation in the rapid diversification of melanic plumage coloration in the dark-eyed junco (Junco hyemalis)

Color plays a prominent role in reproductive isolation, therefore understanding the proximal basis of pigmentation can provide insight into speciation. Subspecies of the dark-eyed junco (Junco hyemalis) have evolved marked differences in plumage coloration since the Last Glacial Maximum, yet whether color differences are caused by mutations in coding regions of expressed genes or are instead the result of regulatory differences remains unknown. To address this question, we studied the pigment composition and the genetic basis of coloration in two divergent subspecies, the slate-colored and Oregon juncos. We used HPLC and light microscopy to investigate pigment composition and deposition in feathers from four body areas. We then used RNAseq to compare the relative roles of differential gene expression in developing feathers and sequence divergence in transcribed loci under common garden conditions. Junco feathers differed in eumelanin and pheomelanin content and distribution. Within subspecies, in lighter feathers melanin synthesis genes were downregulated (including PMEL, TYR, TYRP1, OCA2, MLANA), ASIP was upregulated. Feathers from different body regions also showed differential expression of HOX and Wnt genes. Feathers from the same body regions that differed in color between the two subspecies showed differential expression of ASIP and three other genes (MFSD12, KCNJ13, HAND2) associated with pigmentation in other taxa. Sequence variation in the expressed genes was not related to color differences. Our findings support the hypothesis that differential regulation of a few genes can account for marked differences in coloration, a mechanism that may underlie the rapid diversification of juncos.

evolutionary biology↗

bioRxiv2018

Genome-wide signals of drift and local adaptation during rapid lineage divergence in a songbird

The formation of independent evolutionary lineages involves neutral and selective factors, and understanding their relative roles in population divergence is a fundamental goal of speciation research. Correlations between allele frequencies and environmental variability can reveal the role of selection, yet the relative contribution of drift can be difficult to establish. Recently diversified systems such as that of the Oregon junco (Aves: Emberizidae) of western North America provide ideal scenarios to apply genetic-environment association analyses (GEA) while controlling for population structure. Genome-wide SNP loci analyses revealed marked genetic structure consisting of differentiated populations in isolated, dry southern mountain ranges, and more admixed recently expanded populations in humid northern latitudes. We used correlations between genomic and environmental variance to test for three specific modes of evolutionary divergence: (i) drift in geographic isolation, (ii) differentiation along continuous selective gradients, and (iii) isolation by adaptation. We found evidence of strong drift in southern mountains, but also signals of local adaptation in several populations, driven by temperature, precipitation, elevation and vegetation, especially when controlling for population history. We identified numerous variants under selection scattered across the genome, suggesting that local adaptation can promote rapid differentiation over short periods when acting over multiple independent loci.

evolutionary biology↗