Inheritance patterns of photoperiodic diapause induction in Leptinotarsa decemlineata

Photoperiod is a reliable indicator of season and an important cue that many insects use for phenological synchronization. Undergoing range expansion insects can face a change in the local photoperiod to which they need to resynchronize. Rapid range expansion can be associated with rapid photoperiodic adaptation, which can be associated with intense selection on strongly heritable polygenic traits. Alternatively, it is proposed that, in insects with an XO sex-determination system, genes with large effect residing on the sex chromosome could drive photoperiodic adaptation because the gene or genes are exposed to selection in the sex carrying only a single X-chromosome. The present study seeks to understand which of these alternatives more likely explains the rapid photoperiodic adaptation in European Colorado potato beetles Leptinotarsa decemlineata Say. Diapause induction is assessed in beetles from a northern and a southern population, as well as from reciprocal hybrid crosses between the northern and southern population, when reared at an intermediate length photoperiod. The crosses within population display the expected responses, with the northern and southern populations showing high and low diapause propensity, respectively. The hybrids show intermediate responses in all studied traits. No clear difference in the responses in hybrids depending on the latitudinal origin of their father or mother is detected, even though partial paternal line dominance is seen in the responses of male beetles in one hybrid cross. These results therefore indicate that, in L. decemlineata, photoperiodic diapause induction is strongly heritable, and has an additive polygenic autosomal background.