Stepping‐stone expansion and habitat loss explain a peculiar genetic structure and distribution of a forest insect

Abstract

It is challenging to unravel the history of organisms with highly scattered populations. Such species may have fragmented distributions because extant populations are remnants of a previously more continuous range, or because the species has narrow habitat requirements in combination with good dispersal capacity (naturally or vector borne). The northern pine processionary moth Thaumetopoea pinivora has a scattered distribution with fragmented populations in two separate regions, northern and south-western Europe. The aims of this study were to explore the glacial and postglacial history of T. pinivora, and add to the understanding of its current distribution and level of contemporary gene flow. We surveyed published records of its occurrence and analysed individuals from a representative subset of populations across the range. A 633 bp long fragment of the mtDNA COI gene was sequenced and nine polymorphic microsatellite loci were genotyped. Only nine nucleotide sites were polymorphic in the COI gene and 90% of the individuals from across its whole range shared the same haplotype. The microsatellite diversity gradually declined towards the north, and unique alleles were found in only three of the northern and three of southern sites. Genetic structuring did not indicate complete isolation among regions, but an increase of genetic isolation by geographic distance. Approximate Bayesian model choice suggested recent divergence during the postglacial period, but glacial refugia remain unidentified. The progressive reduction of suitable habitats is suggested to explain the genetic structure of the populations and we suggest that T. pinivora is a cold-tolerant relict species, with situation-dependent dispersal.

A little further south: Host range and genetics of the Northern pine processionary moth, Thaumetopoea pinivora (Lepidoptera: Notodontidae) at the southern edge of its distribution

“…This record was exceptional for three reasons. Firstly, it was 375 km south of the southernmost record (Torre el Val, Teruel) listed by Cassel-Lundhagen et al (2013). Secondly, the larvae were feeding on P. nigra J.F.…”

“…Arnold, despite the fact that Sierra de Baza and the proximate Sierra Nevada maintain the two southernmost populations of P. sylvestris. According to the hypothesis of Cassel-Lundhagen et al (2013), P. sylvestris was the only host. Thirdly, the site was at a lower elevation (1,500 m) than expected for a southern location, as the previous fi ndings in central Spain were at 1,600-1,800 m (see Table 1 in Cassel-Lundhagen et al, 2013).…”

“…microsatellite loci, and a 633 base pair long fragment of the second part of the mitochondrial DNA gene, cytochrome oxidase I (COI) were analysed as described in Cassel-Lundhagen et al (2009), Ronnås et al (2011) and Cassel-Lundhagen et al (2013). Estimates of allelic richness (AR), unique allelic richness (UAR), and observed (Ho) and expected (He) heterozygosity were calculated as described in Cassel-Lundhagen et al (2013). Because there were indications of null alleles, we estimated pairwise F ST values using the so-called ENA method, as implemented in the FreeNA software (Chapuis & Estoup, 2007); levels of signifi cance were estimated by means of bootstrapping (1,000 replicates) using the same software.…”

“…The Northern pine processionary moth Thaumetopoea pinivora (Treitschke, 1834) (Lepidoptera: Notodontidae) is geographically widespread but locally rare (Cassel-Lundhagen et al, 2013). The latitudinal range of this species extends more than 5,000 km but is very fragmented.…”

“…This pattern is thought to be explained by ecological specialization, where species with wider distributions are predominantly habitat generalists with a broad environmental tolerance (Brown, 1984;Lawton, 2000). There is, however, limited empirical support for this theory (Hanski et al, 1993), and there are examples of organisms that challenge this apparent rule (Louy et al, 2007;Habel & Schmidtt, 2012;Cassel-Lundhagen et al, 2013).…”