Population Genetic Structure and Demography of the Critically Endangered Chequered Blue Butterfly (Scolitantides orion) in a Highly Isolated Part of Its Distribution Range

SIMPLE SUMMARY: The disappearance of many butterfly species is currently observed in Europe, as most of them display strict habitat preferences and/or food specializations. A good example of such a species is the chequered blue butterfly (Scolitantides orion), whose caterpillars feed only on a few species of sedum and are guarded by ants. In Poland, this butterfly has survived only in one region and it is critically endangered. It is important to examine the genetic condition and demography of its extant populations, to help to create an effective conservation plan for the species. We studied the demography and genetic structure of the two largest populations inhabiting opposite banks of the Vistula River. Captured individuals were marked and released to estimate population sizes. Both populations were small, and they fluctuated in numbers, but adults were twice as numerous on the western riverbank. Analyses of various genetic markers have shown that the genetic variation is low in each population. Likewise, the genetic diversity between the two populations is low, which indicates that the gene flow exists, despite the river acting as a geographical barrier. The occasional migration occurs more frequently from the west to the east, which is consistent with the dominant regional wind direction. ABSTRACT: Scolitantides orion is a butterfly species threatened in many European countries. In Poland, it survived in a single highly isolated area (Vistula River valley), which is an example of the dramatic decline in the population number. We studied the two largest remaining populations inhabiting opposite banks of the river. Mark-release-recapture studies showed that both populations were small, and they fluctuated in numbers, but adult individuals were twice as numerous on the western site. Genetic analyses were carried out using a mitochondrial (COI, ND5) and nuclear markers (Wgl, EF-1α, and microsatellite loci). We found out that genetic variation was low at both sites but higher in the smaller eastern population. This pattern is likely to be better explained by past distribution, when the butterfly, as a continental species used to be much more widespread in the east. However, the genetic differentiation between populations was low. This could suggest that the existing gene flow is facilitated by dominant regional wind direction, which may also contribute to a better genetic condition of the western population. Finally, a comparison of the obtained COI sequences with others available enabled us to reveal the phylogeographic pattern of the S. orion from different localities within its range.