A population genetic study of the egg parasitoid Baryscapus servadeii reveals large scale automictic parthenogenesis and almost fixed homozygosity

Abstract

The pine processionary moth Thaumetopoea pityocampa – T. wilkinsoni is one of the main defoliators of pine and cedar forests in the Mediterranean area. Its distribution is currently expanding northward and to higher altitudes due to climate warming. This species is associated with a rich community of predators and parasitoids. The specialist egg parasitoid Baryscapus servadeii is among the most abundant, and is thought to significantly affect the population dynamics of its host. To determine if the parasitoid tracks the range expansion of its host and shows similar population genetic patterns, we developed eight de novo microsatellite markers using SSR enrichment and 454 pyrosequencing. Despite a large-scale sampling design, ranging from Northern Africa to Western Europe, Crete and Cyprus, genotyping revealed an unexpectedly high level of homozygosity and reduced genetic diversity, with 90% of studied individuals being characterized by only 2 multi-locus genotypes. We then tested if B. servadeii is associated with parthenogenesis-inducing endobacteria, and found that most individuals were associated with a Rickettsia symbiont, not Wolbachia. Interestingly, genetic diversity and an absence of symbionts were documented in some individuals of the congeneric hyperparasitoid B. transversalis, which were found in the same samples. Altogether, the results suggest that microsatellite nuclear markers will not be useful for deciphering the demography and range expansion routes of this parasitoid. However, B. servadeii can be considered an interesting and novel example of Rickettsia inducing thelytoky in chalcid parasitoids.