Oviposition Traits Generate Extrinsic Postzygotic Isolation Between Two Pine Sawfly Species

Background: Although empirical data indicate that ecological speciation is prevalent in nature, the relative importance of different forms of reproductive isolation and the traits generating reproductive isolation remain unclear. To address these questions, we examined a pair of ecologically divergent pine-sawfly species: while Neodiprion pinetum specializes on a thin-needled pine (Pinus strobus), N. lecontei utilizes thicker-needled pines. We hypothesized that extrinsic postzygotic isolation is generated by oviposition traits. To test this hypothesis, we assayed ovipositor morphology, oviposition behavior, and host-dependent oviposition success in both species and in F1 and backcross females. Results: Compared to N. lecontei, N. pinetum females preferred P. strobus more strongly, had smaller ovipositors, and laid fewer eggs per needle. Additionally, we observed host- and trait-dependent reductions in oviposition success in F1 and backcross females. Hybrid females that had pinetum-like host preference (P. strobus) and lecontei-like oviposition traits (morphology and egg pattern) fared especially poorly. Conclusions: Together, these data indicate that maladaptive combinations of oviposition traits in hybrids contribute to extrinsic postzygotic isolation between N. lecontei and N. pinetum, suggesting that oviposition traits may be an important driver of divergence in phytophagous insects

Variation in DNA Methylation Is Not Consistently Reflected by Sociality in Hymenoptera

Changes in gene regulation that underlie phenotypic evolution can be encoded directly in the DNA sequence or mediated by chromatin modifications such as DNA methylation. It has been hypothesized that the evolution of eusocial division of labor is associated with enhanced gene regulatory potential, which may include expansions in DNA methylation in the genomes of Hymenoptera (bees, ants, wasps, and sawflies). Recently, this hypothesis garnered support from analyses of a commonly used metric to estimate DNA methylation in silico, CpG content. Here, we test this hypothesis using direct, nucleotide-level measures of DNA methylation across nine species of Hymenoptera. In doing so, we generated new DNA methylomes for three species of interest, including one solitary and one facultatively eusocial halictid bee and a sawfly. We demonstrate that the strength of correlation between CpG content and DNA methylation varies widely among hymenopteran taxa, highlighting shortcomings in the utility of CpG content as a proxy for DNA methylation in comparative studies of taxa with sparse DNA methylomes. We observed strikingly high levels of DNA methylation in the sawfly relative to other investigated hymenopterans. Analyses of molecular evolution suggest the relatively distinct sawfly DNA methylome may be associated with positive selection on functional DNMT3 domains. Sawflies are an outgroup to all ants, bees, and wasps, and no sawfly species are eusocial. We find no evidence that either global expansions or variation within individual ortholog groups in DNA methylation are consistently associated with the evolution of social behavior

Host_ Needle_Width

This folder contains the data from the "Host needle width" section of the manuscript. It includes both mature and seedling measurement

BC_Oviposition_success

This folder contains the data for the "Impact of oviposition traits on BCL oviposition success" section of the manuscript. It includes ovipositor landmark coordinates, ovipositor size measurements, oviposition pattern, and associated oviposition success for N. lecontei backcross females

Interspecific_cross

This folder contains the data for the "cross oviposition behavior and success" section of the manuscript. It includes oviposition willingness, oviposition preference, and oviposition success for the interspecific crosses

Parental_Ovipositor_Morphology

This folder contains the data for the "Ovipositor Morphology" section of the manuscript. It includes the landmark coordinates and ovipositor size measurements for N. lecontei and N. pinetu

Parental_Oviposition_Behavior

This folder contains the data for the "oviposition behavior" section of the manuscript. It includes oviposition willingness, oviposition preference, Average number of eggs per needle, preslit presence, and egg spacing for N. lecontei and N. pinetu