Abstract
The evolution of intrinsic barriers to gene flow is a crucial step in the process of speciation. Chromosomal changes caused by fusion and fission events are one such barrier and are common in several groups of Lepidoptera. However, it remains unclear if and how chromosomal changes have contributed to speciation in this group. I tested for a phylogenetic signal of varying chromosome numbers in Erebia butterflies by combining existing sequence data with karyological information. I also compared different models of trait evolution in order to infer the underlying evolutionary mechanisms. Overall, I found significant phylogenetic signals that are consistent with non-neutral trait evolution only when parts of the mitochondrial genome were included, suggesting cytonuclear discordances. The adaptive evolutionary model tested in this study consistently outperformed the neutral model of trait evolution. Taken together, these results suggest that, unlike other Lepidoptera groups, changes in chromosome numbers may have played a role in the diversification of Erebia butterflies.
Highlights
The process of speciation can be seen as a continuum, ranging from initially undifferentiated populations to genetically distinct and reproductively isolated species [1,2,3]
These results suggest that, unlike other Lepidoptera groups, changes in chromosome numbers may have played a role in the diversification of Erebia butterflies
I compared the fit between the number of chromosomes with the phylogeny using three different evolutionary models implemented in the package MV MORPH [45]: (i) Brownian Motion (BM), which is based on a random walk process [32]. (ii) Ornstein-Uhlenbeck (OU), which fits a random walk with a central tendency towards a particular range of phenotypes representing an adaptive optimum [34]. (iii) Early Burst (EB), which assumes initially rapid evolution that is followed by relative stasis [33]
Summary
The process of speciation can be seen as a continuum, ranging from initially undifferentiated populations to genetically distinct and reproductively isolated species [1,2,3] Stages along this continuum are often ephemeral, as potential maladaptive gene flow is mainly prevented by extrinsic factors, and interspecific gene flow, e.g., upon secondary contact, is still possible [2,3,4]. Changes in chromosome numbers through fusion and fission may, in particular, result in reproductive isolation, and promote speciation. This is because complex and unstable meiotic chains can form in hybrids between species with different chromosome numbers, leading to meiotic nondisjunction and sterility [14]. Diversification in this group [17,22,25,30,31]
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