Biology subjects

Greenberg, R.

Publications and source records attributed to Greenberg, R..

3 recordsLinked to original sources

bioRxiv2018

Low genomic divergence and high gene flow between locally adapted populations of the swamp sparrow

Populations that have recently diverged across sharp environmental gradients provide an opportunity to study the mechanisms by which natural selection drives adaptive divergence. Inland and coastal populations of the North American swamp sparrow have become an emerging model system for studies of natural selection because they are morphologically and behaviourally distinct despite a very recent divergence time (<15,000 years), yet common garden experiments have demonstrated a genetic basis for their phenotypic differences. We characterized genomic patterns of variation within and between inland and coastal swamp sparrows via reduced representation sequencing in order to reconstruct the contributions of demography, gene flow and selection to this case of recent adaptive divergence. Compared to inland swamp sparrows, coastal swamp sparrows exhibited fewer polymorphic sites and reduced nucleotide diversity at those sites, indicating that a bottleneck and/or recent selective sweeps occurred in that population during coastal colonization and local adaptation. Estimates of genome-wide differentiation (FST=0.02) and sequence divergence ({Phi}ST=0.05) between inland and coastal populations were very low, consistent with postglacial divergence. A small number of SNPs were strongly differentiated (max FST=0.8) suggesting selection at linked sites. Swamp sparrows sampled from breeding sites at the habitat transition between freshwater and brackish marshes exhibited high levels of genetic admixture. Such evidence of active contemporary gene flow makes the evolution and maintenance of local adaptation in these two populations even more notable. We summarize several features of the swamp sparrow system that may facilitate the maintenance of adaptive diversity despite gene flow, including the presence of a magic trait.

evolutionary biology↗

bioRxiv2018

Whole genome scan reveals the multigenic basis of recent tidal marsh adaptation in a sparrow

Natural selection acts on functional molecular variation to create local adaptation, the \"good fit\" we observe between an organisms phenotype and its environment. Genomic comparisons of lineages in the earliest stages of adaptive divergence have high power to reveal genes under natural selection because molecular signatures of selection on functional loci are maximally detectable when overall genomic divergence is low. We conducted a scan for local adaptation genes in the North American swamp sparrow (Melospiza georgiana), a species that includes geographically connected populations that are differentially adapted to freshwater vs. brackish tidal marshes. The brackish tidal marsh form has rapidly evolved tolerance for salinity, a deeper bill, and darker plumage since colonizing coastal habitats within the last 15,000 years. Despite their phenotypic differences, background genomic divergence between these populations is very low, rendering signatures of natural selection associated with this recent coastal adaptation highly detectable. We recovered a multigenic snapshot of ecological selection via a whole genome scan that revealed robust signatures of selection at 31 genes with functional connections to bill shape, plumage melanism and salt tolerance. As in Darwins finches, BMP signaling appears responsible for changes in bill depth, a putative magic trait for ecological speciation. A signal of selection at BNC2, a melanocyte transcription factor responsible for human skin color saturation, implicates a shared genetic mechanism for sparrow plumage color and human skin tone. Genes for salinity tolerance constituted the majority of adaptive candidates identified in this genome scan (23/31) and included vasoconstriction hormones that can flexibly modify osmotic balance in tune with the tidal cycle by influencing both drinking behavior and kidney physiology. Other salt tolerance genes had potential pleiotropic effects on bill depth and melanism (6/31), offering a mechanistic explanation for why these traits have evolved together in coastal swamp sparrows, and in other organisms that have converged on the same \"salt marsh syndrome\". As a set, these candidates capture the suite of physiological changes that coastal swamp sparrows have evolved in response to selection pressures exerted by a novel and challenging habitat.

evolutionary biology↗

bioRxiv2016

DNA damage in 3D constricted migration or after lamin-A depletion in 2D: shared mechanisms of repair factor mis-localization under nuclear stress

Cells that migrate through small, rigid pores and that have normal levels of the nuclear structure protein lamin-A exhibit an increase in DNA damage, which is also observed with lamin-A depletion in diseases such as cancer and with many lamin-A mutations. Here we show nuclear envelope rupture is a shared feature that increases in standard culture after lamin-A knockdown, which causes nuclear loss of multiple DNA repair factors and increased DNA damage. Some repair factors are merely mis-localized to cytoplasm whereas others are partially depleted unless rescued by lamin-A expression. Compared to standard cultures on rigid glass coverslips, the growth of lamin-A low cells on soft matrix relaxes cytoskeletal stress on the nucleus, suppresses the mis-localization of DNA repair factors, and minimizes DNA damage nearly to wildtype levels. Conversely, constricted migration of the lamin-A low cells causes abnormally high levels of DNA damage, consistent with sustained loss of repair factors. The findings add insight into why monogenic progeroid syndromes that often associate with increased DNA damage and predominantly impact cells in stiff tissues result from mutations only in lamin-A or DNA repair factors.

cell biology↗