Characterising and quantifying the genome size expansion in wood-white (Leptidea) butterflies. This study includes Genome sequencing of Leptidea sinapis and resequencing of Leptidea reali and Leptidea juvernica.

Characterizing and quantifying genome size variation among organisms has been a long standing goal in evolutionary biology and of particular interest is to understand if genome size evolves as a consequence of adaptive or stochastic processes. Here we investigate genome size variation and association with transposable elements (TEs) across lepidopteran lineages using a novel genome assembly of the common wood-white (Leptidea sinapis) and population re-sequencing data from both L. sinapis and the closely related L. reali and L. juvernica together with 10 previously available lepidopteran genome assemblies. A phylogenetic analysis confirms established relationships among species, but identifies previously unknown intraspecific structure within Leptidea lineages. The genome assembly of L. sinapis is the largest of any lepidopteran taxon so far (643 Mb) and genome size is correlated with abundance of TEs, both in Lepidoptera in general and within Leptidea where L. juvernica from Kazakhstan has considerably larger genome size than any other Leptidea population. Specific TE subclasses have been active in different Lepidoptera lineages with a pronounced expansion of predominantly LINEs, DNA-elements and LTRs in the Leptidea lineage after the split from other Pieridae. The rate of genome expansion in Leptidea in general has been in the range of 4 Mb / Million years (My), with an increase in L. juvernica from Kazakhstan to 150 Mb / My. The differences in accumulation rates of specific TE classes in different lineages indicate that TE activity and insertion bias rather than adaptive processes drive genome size evolution in butterflies and moths.