Temporal Population Genetics of Time Travelling Insects: A Long Term Study in a Seed-Specialized Wasp

Abstract

Many animal species experiencing spatial or interannual fluctuations of their environment are capable of prolonged diapause, a kind of dormancy that extends over more than one year. Such a prolonged diapause is commonly perceived as a temporal demographic refuge in stochastic environments, but empirical evidence is still lacking of its consequences on temporal population genetic structures. In this long-term study, we investigated how a particular pattern of prolonged diapause may influence the temporal population genetics of the invasive seed-specialized wasp Megastigmus schimitscheki (Hymenoptera: Torymidae) in southeastern France. We characterized the diapause strategy of M. schimitscheki using records of emergence from diapause in 97 larval cohorts, and we conducted a temporal population genetic study on a natural invasive wasp population sampled during ten consecutive years (1999–2008) using polymorphic microsatellite markers. We found that M. schimitscheki can undergo a prolonged diapause of up to five years and displays two main adult emergence peaks after two and four years of diapause. Such a bimodal and atypical pattern did not disrupt temporal gene flow between cohorts produced in even and in odd years during the period of the study. Unexpectedly, we found that this wasp population consisted of two distinct genetic sub-populations that strongly diverged in their diapause strategies, with very few admixed individuals. One of the sub-populations displayed both short and prolonged diapause (2 and 4 years respectively) in equal proportions, whereas the other sub-population displayed mainly short diapause. This study provided empirical evidence that prolonged diapause phenotypes can substantially contribute to reproduction and impact temporal genetic structures. Prolonged diapause is likely to act as both demographic and genetic refuges for insect populations living in fluctuating environments.

Linking niche theory to ecological impacts of successful invaders: insights from resource fluctuation‐specialist herbivore interactions

“…In the more recent cohorts (2008–2010), the proportions of emerging adults of M. schimitscheki and M. pinsapinis were extrapolated from well‐known and consistent patterns of diapause in each wasp species (Suez et al . ). A summary of the different types of data and their uses for modelling purposes is provided in Table .…”

“…The remaining parameters, which corresponded to the diapause rates and sex ratios at emergence, were known from previous laboratory experiments and are highly consistent among years and sites (Suez et al . ; Boivin et al . ).…”

“…; Suez et al . ; Boivin et al . ), and from which one might expect divergences in resource use favouring invasion success and coexistence.…”

The ecology of predispersal insect herbivory on tree reproductive structures in natural forest ecosystems

“…For example, the resource budget model is almost always of the storage type although this is not the only mechanism by which resources can act on seed productions (Pearse et al ., ). Similarly, the dynamics of the predator's emergence from prolonged diapause is always considered monotonous (most often geometric) although other patterns are observed in natural populations (Suez et al ., ).…”

“…When resource availability displays low predictability and severe spatial or interannual fluctuations, insects may prolong their life cycles by increasing diapause duration beyond years of low resource availability (Hanski, ). Such prolonged diapause can be concentrated on a single extra year of emergence in a fraction of a larval cohort, but emergences are mostly spread over several years with decreasing frequencies through time (Annila, ; Hanski, ; Pélisson et al ., ), or follow more rarely a bimodal pattern (Suez et al ., ). Evolution toward prolonged life cycles in RSI may primarily occur in the trophic specialists as trophic generalists are expected to find alternative resources on other plant structures in the absence of fruits or seed cones.…”