Biology subjects

Simon Henry Martin

Publications and source records attributed to Simon Henry Martin.

3 recordsLinked to original sources

bioRxiv2016

Natural selection and genetic diversity in the butterfly Heliconius melpomene

A combination of selective and neutral evolutionary forces shape patterns of genetic diversity in nature. Among the insects, most previous analyses of the roles of drift and selection in shaping variation across the genome have focused on the genus Drosophila. A more complete understanding of these forces will come from analysing other taxa that differ in population demography and other aspects of biology. We have analysed diversity and signatures of selection in the neotropical Heliconius butterflies using resequenced genomes from 58 wild-caught individuals of H. melpomene, and another 21 resequenced genomes representing 11 related species. By comparing intra-specific diversity and inter-specific divergence, we estimate that 31% of amino acid substitutions between Heliconius species are adaptive. Diversity at putatively neutral sites is negatively correlated with the local density of coding sites as well as non-synonymous substitutions, and positively correlated with recombination rate, indicating widespread linked selection. This process also manifests in significantly reduced diversity on longer chromosomes, consistent with lower recombination rates. Although hitchhiking around beneficial non-synonymous mutations has significantly shaped genetic variation in H. melpomene, evidence for strong selective sweeps is limited overall. We did however identify two regions where distinct haplotypes have swept in different populations, leading to increased population differentiation. On the whole, our study suggests that positive selection is less pervasive in these butterflies as compared to fruit flies; a fact that curiously results in very similar levels of neutral diversity in these very different insects.

Evolutionary Biology↗

bioRxiv2015

Efficient strategies for calculating blockwise likelihoods under the coalescent

The inference of demographic history from genome data is hindered by a lack of efficient computational approaches. In particular, it has proven difficult to exploit the information contained in the distribution of genealogies across the genome. We have previously shown that the generating function (GF) of genealogies can be used to analytically compute likelihoods of demographic models from configurations of mutations in short sequence blocks (Lohse et al., 2011). Although the GF has a simple, recursive form, the size of such likelihood computations explodes quickly with the number of individuals and applications of this framework have so far been limited to small samples (pairs and triplets) for which the GF can be written down by hand. Here we investigate several strategies for exploiting the inherent symmetries of the coalescent. In particular, we show that the GF of genealogies can be decomposed into a set of equivalence classes which allows likelihood calculations from non-trivial samples. Using this strategy, we used Mathematica to automate block-wise likelihood calculations based on the GF for a very general set of demographic scenarios that may involve population size changes, continuous migration, discrete divergence and admixture between multiple populations. To give a concrete example, we calculate the likelihood for a model of isolation with migration (IM), assuming two diploid samples without phase and outgroup information, and compare the power of our approach to that of minimal pairwise samples. We demonstrate the new inference scheme with an analysis of two individual butterfly genomes from the sister species Heliconius melpomene rosina and Heliconius cyndo.

Genetics↗

bioRxiv2015

Speciation in Heliconius Butterflies: Minimal Contact Followed by Millions of Generations of Hybridisation

Documenting the full extent of gene flow during speciation poses a challenge, as species ranges change over time and current rates of hybridisation might not reflect historical trends. Theoretical work has emphasized the potential for speciation in the face of ongoing hybridisation, and the genetic mechanisms that might facilitate this process. However, elucidating how the rate of gene flow between species may have changed over time has proved difficult. Here we use Approximate Bayesian Computation (ABC) to fit a model of speciation between the Neotropical butterflies Heliconius melpomene and Heliconius cydno. These species are ecologically divergent, rarely hybridize and display female hybrid sterility. Nevertheless, previous genomic studies suggests pervasive gene flow between them, extending deep into their past, and potentially throughout the speciation process. By modelling the rates of gene flow during early and later stages of speciation, we find that these species have been hybridising for hundreds of thousands of years, but have not done so continuously since their initial divergence. Instead, it appears that gene flow was rare or absent for as long as a million years in the early stages of speciation. Therefore, by dissecting the timing of gene flow between these species, we are able to reject a scenario of purely sympatric speciation in the face of continuous gene flow. We suggest that the period of minimal contact early in speciation may have allowed for the accumulation of genomic changes that later enabled these species to remain distinct despite a dramatic increase in the rate of hybridisation.

Evolutionary Biology↗