Dapporto, L. | Search bioRxiv

bioRxiv2020

The Pleistocene species pump past its prime: evidence from European butterfly sister species

The Pleistocene glacial cycles had a profound impact on the ranges and genetic make-up of organisms. Whilst it is clear that the contact zones that have been described for many sister taxa are secondary and have formed during the last interglacial, it is unclear when the taxa involved began to diverge. Previous estimates based on small numbers of loci are unreliable given the stochasticity of genetic drift and the contrasting effects of incomplete lineage sorting and gene flow on gene divergence. Here we use genome-wide transcriptome data to estimate divergence for 18 sister species pairs of European butterflies showing either sympatric or contact zone distributions. We find that in most cases species divergence predates the mid-Pleistocene transition or even the entire Pleistocene period. We also show that although post divergence gene flow is restricted to contact zone pairs, they are not systematically younger than sympatric pairs. This suggests that contact zones are not limited to the embryonic stages of the speciation process, but can involve notably old taxa. Finally, we show that mitochondrial and nuclear divergence are only weakly correlated and mitochondrial divergence is higher for contact-zone pairs. This suggests a possible role of selective sweeps affecting mitochondrial variation in maintaining contact zones. Impact SummaryThe influence of the Pleistocene glacial cycles on structuring species and genetic diversity in temperate taxa has permeated biogeographic and phylogeographic thinking for decades. Although phylogeographic studies have repeatedly claimed that the Pleistocene acted as a species pump, systematic tests of this idea based on robust estimates of species divergence are lacking. Here we estimate divergence times for all sister species pairs of European butterfly using genome-wide transcriptome data. We find that most species pairs are substantially older than the onset of Pleistocene glacial cycling. We also show that post divergence gene flow is restricted to pairs that form contact-zones. However, in contrast to expectations under a null model of allopatric speciation contract zone pairs are not necessarily younger than sympatric pairs.

evolutionary biology↗

bioRxiv2020

Multiple colonisations and Wolbachia infections influence the fine-scale population structure of the widespread Common Blue butterfly (Polyomattus icarus) in the British Isles

The paradigm of differentiation in southern refugia during glacial periods followed by expansions during interglacials, producing limited genetic diversity and population sub-division in northern areas, dominates European phylogeography. However, the existence of complex structured populations in formerly glaciated areas, and on islands connected to mainland areas during glacial maxima, call for alternative explanations. Here, we reconstruct the mtDNA phylogeography of the widespread Polyommatus icarus butterfly over its native range, with an emphasis on the formerly glaciated and connected British Isles. We found distinct geographical structuring of CO1 mitotypes, with an ancient lineage restricted to the marginal European areas, including Northern Scotland and Outer Hebrides. We detected perfect mtDNA-Wolbachia associations in Northern Britain that support the possibility of at least two post-glacial Wolbachia-mediated sweeps, suggesting a series of sequential replacement of mtDNA in the British Isles and potentially in Europe. Population genomic analysis, using ddRADSeq genomic markers, also reveal unexpected genetic structuring within Britain. However, weak mito-nuclear concordance suggests the potential for independent histories of nuclear versus mitochondrial genomes. We found clustering of genomic SNPs of French samples, with respect to those in the British Isles, is not consistent with a scenario of a single recolonisation. Taken together our mtDNA and ddRADseq observations are consistent with a history of at least two distinct colonisations, a phylogeographic scenario previously put forth to explain diversity and structuring in other British flora and fauna. Additionally, we also present preliminary evidence that Wolbachia-induced feminization may be occurring in the isolated population in the Outer Hebrides.

evolutionary biology↗