Biology subjects

Jackson, B. C.

Publications and source records attributed to Jackson, B. C..

4 recordsLinked to original sources

bioRxiv2021

Selective sweeps influence diversity over large regions of the mouse genome

To what extent do substitutions in protein-coding versus gene-regulatory regions contribute to fitness change over time? Answering this question requires estimates of the extent of selection acting on beneficial mutations in the two classes of sites. New mutations that have advantageous or deleterious fitness effects can induce selective sweeps and background selection, respectively, causing variation in the level of neutral genetic diversity along the genome. In this study, we analyse the profiles of genetic variability around protein-coding and regulatory elements in the genomes of wild mice to estimate the parameters of positive selection. We find patterns of diversity consistent with the effects of selection at linked sites, which are similar across mouse taxa, despite differences in effective population size and demographic history. By fitting a model that combines the effects of selective sweeps and background selection, we estimate the strength of positive selection and the frequency of advantageous mutations. We find that strong positive selection is required to explain variation in genetic diversity across the murid genome. In particular, we estimate that beneficial mutations in protein-coding regions have stronger effects on fitness than do mutations in gene-regulatory regions, but that mutations in gene-regulatory regions are more common. Overall though, our parameter estimates suggest that the cumulative fitness changes brought about by beneficial mutations in protein-coding may be greater than those in gene-regulatory elements.

evolutionary biology↗

bioRxiv2021

Evidence for a force favouring GC over AT at short intronic sites in Drosophila simulans and D. melanogaster

Population genetics studies often make use of a class of nucleotide site free from selective pressures in order to make inferences about population size changes or natural selection at other sites. If such neutral sites can be identified, they offer the opportunity to avoid any confounding effects of selection. Here we investigate evolution at putatively neutrally evolving short intronic sites in natural populations of Drosophila melanogaster and D. simulans, in order to understand the properties of spontaneous mutations and the extent of GC-biased gene conversion in these species. Use of data on the genetics of natural populations is advantageous because it integrates information from large numbers of individuals over long timescales. In agreement with direct evidence from observations of spontaneous mutations in Drosophila, we find a bias in the spectrum of mutations towards AT basepairs. In addition, we find that this bias is stronger in the D. melanogaster lineage than the D. simulans lineage. The evidence for GC-biased gene conversion in Drosophila has been equivocal. Here we provide evidence for a weak force favouring GC in both species, which is stronger in D. simulans. Some homologous short intronic sites have diverged in GC content between the two species, which may have been caused by lineage-specific changes in the extent to which different regions of the genome are subject to a GC (or AT)-favouring force.

genetics↗

bioRxiv2019

Patterns of genetic variability in genomic regions with low rates of recombination

Surveys of DNA sequence variation have shown that the level of genetic variability in a genomic region is often strongly positively correlated with its rate of crossing over (CO) [1-3]. This pattern is caused by selection acting on linked sites, which reduces genetic variability and can also cause the frequency distribution of segregating variants to contain more rare variants than expected without selection (skew). These effects of selection may involve the spread of beneficial mutations (selective sweeps, SSWs), the elimination of deleterious mutations (background selection, BGS) or both together, and are expected to be stronger with lower rates of crossing over [1-3]. However, in a recent study of human populations, the skew was reduced in the lowest CO regions compared with regions with somewhat higher CO rates [4]. A similar pattern is seen in the population genomic studies of Drosophila simulans described here. We propose an explanation for this paradoxical observation, and validate it using computer simulations. This explanation is based on the finding that partially recessive, linked deleterious mutations can increase rather than reduce neutral variability when the product of the effective population size (Ne) and the selection coefficient against homozygous carriers of mutations (s) is [≤] 1, i.e. there is associative overdominance (AOD) rather than BGS [5]. We show that AOD can operate in a genomic region with a low rate of CO, opening up a new perspective on how selection affects patterns of variability at linked sites.

evolutionary biology↗

bioRxiv2018

Black or red: A sex-linked colour polymorphism in a songbird is maintained by balancing selection

Colour polymorphisms play a key role in sexual selection and speciation, yet the mechanisms that generate and maintain them are not fully understood. We therefore used genomic and transcriptomic tools to identify the precise genetic architecture and evolutionary history of a sex-linked colour polymorphism in the Gouldian finch Erythrura gouldiae that is also accompanied by remarkable differences in behaviour and physiology. We found that differences in colour are associated with an ~72-kbp region of the Z chromosome in a putative regulatory region for follistatin, an antagonist of the TGF-{beta} superfamily genes. The region is highly differentiated between morphs, unlike the rest of the genome, yet we found no evidence that an inversion is involved in maintaining the distinct haplotypes. Coalescent simulations confirm that that there is elevated nucleotide diversity and an excess of intermediate frequency alleles at this locus. We conclude that this pleiotropic colour polymorphism is most probably maintained by balancing selection.

evolutionary biology↗