Abstract
The Palaearctic butterfly Melitaea didyma stands out as one of the most striking cases of intraspecific genetic differentiation detected in Lepidoptera: 11 partially sympatric mitochondrial lineages have been reported, displaying levels of divergence of up to 7.4%. To better understand the evolutionary processes underlying the diversity observed in mtDNA, we compared mtDNA and genome-wide SNP data using double-digest restriction site-associated DNA sequencing (ddRADseq) results from 93 specimens of M. didyma ranging from Morocco to eastern Kazakhstan. We found that, between ddRADseq and mtDNA results, there is a match only in populations that probably remained allopatric for long periods of time. Other mtDNA lineages may have resulted from introgression events and were probably affected by Wolbachia infection. The five main ddRADseq clades supported by STRUCTURE were parapatric or allopatric and showed high pairwise FST values, but some were also estimated to display various levels of gene flow. Melitaea didyma represents one of the first cases of deep mtDNA splits among European butterflies assessed by a genome-wide DNA analysis and reveals that the interpretation of patterns remains challenging even when a high amount of genomic data is available. These findings actualize the ongoing debate of species delimitation in allopatry, an issue probably of relevance to a significant proportion of global biodiversity.
Highlights
The study of global biodiversity is one of the fundamental commissions of biologists, but this task is one of the most challenging due to the diversity of life on Earth and the resources needed to document it accurately
The Palaearctic butterfly Melitaea didyma stands out as one of the most striking cases of intraspecific genetic differentiation detected in Lepidoptera: 11 partially sympatric mitochondrial lineages have been reported, displaying levels of divergence of up to 7.4%
To better understand the evolutionary processes underlying the diversity observed in mitochondrial DNA (mtDNA), we compared mtDNA and genome-wide SNP data using double-digest restriction site-associated DNA sequencing results from 93 specimens of M. didyma ranging from Morocco to eastern Kazakhstan
Summary
The study of global biodiversity is one of the fundamental commissions of biologists, but this task is one of the most challenging due to the diversity of life on Earth and the resources needed to document it accurately. A recent study focused on Europe found that, while the majority of species displayed relatively low intraspecific divergence, 27.7% of 299 species DNA barcoded showed multiple evolutionarily significant units (ESU) Such studies help to set the standard for what divergence within and between species empirically is, and how generally this corresponds well to species boundaries, against which the outliers can be seen. For a highly diverse group such as butterflies, to our knowledge, only two recent studies addressed the issue using a genomic approach [13,14] This underlines the need for further study to assess the significance of deep intraspecific DNA barcode splits and their implications for the global study of biodiversity. Using M. didyma as model, our goals were (1) to compare the evolutionary histories of mitochondrial and nuclear DNA and (2) to better understand evolutionary processes underlying deep mtDNA intraspecific splits and their potential to highlight cryptic diversity
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