Complementary sex determination, inbreeding depression and inbreeding avoidance in a gregarious sawfly

Abstract

Although most Hymenoptera reproduce via arrhenotokous haplodiploidy, the underlying genetic mechanisms vary. Of these, the most widespread mechanism appears to be single-locus complementary sex determination (sl-CSD), in which individuals that are diploid and heterozygous at a sex-determining locus are female, and individuals that are homozygous or hemizygous are male. Because inbreeding increases the probability of producing diploid males, which are often sterile or inviable, sl-CSD can generate substantial inbreeding depression. To counteract this, Hymenoptera with traits that promote inbreeding, such as gregariousness, may evolve one or more of the following: inbreeding avoidance, functional diploid males or alternative sex determination mechanisms. Here, we investigate sex determination, inbreeding depression and inbreeding avoidance in Neodiprion lecontei, a gregarious, pine-feeding sawfly in the family Diprionidae. First, via inbreeding experiments and flow cytometry, we demonstrate that this species has CSD. By modeling expected sex ratios under different conditions, we also show that our data are consistent with sl-CSD. Second, via tracking survival in inbred and outbred families, we demonstrate that inbred families have reduced larval survival and that this mortality is partly attributable to the death of diploid males. Third, using a no-choice mating assay, we demonstrate that females are less willing to mate with siblings than nonsiblings. Together, these results suggest that inbreeding depression stemming from CSD has shaped mating behavior in N. lecontei. These results also set the stage for future comparative work that will investigate the interplay between sex determination, ecology and behavior in additional diprionid species that vary in larval gregariousness.

Oviposition traits generate extrinsic postzygotic isolation between two pine sawfly species

“…We collected cocoons as they were spun and stored them in individual gelatin capsules until adult emergence. We maintained all larvae and cocoons at 22  ° C, 70% relative humidity, and an 18–6 h light-dark cycle [40, 46]. Upon emergence, live adults were stored at 4  ° C until needed for crosses, morphological measurements, or behavioral assays.…”

“…We first placed females in a clear 3.25-ounce deli cup with a single male until mating occurred. Neodiprion , like most hymenopterans, have arrhenotokous haplodiploidy, in which unfertilized eggs develop into haploid males [46, 52]. Although both mated and unmated females will oviposit readily ( personal observation ), we used mated females in our N. lecontei/N.…”

North-Western Palaearctic species of Pristiphora (Hymenoptera, Tenthredinidae)

“…not affecting protein sequence) and so far there is no evidence for paralogous NaK and TPI genes at least in sawflies, these polymorphic sites might indicate heterozygosity because of diploidy. This can happen if there is no heterozygosity at the complementary sex determination (CSD) locus, preventing female development from a diploid embryo (Naito and Suzuki 1991, Heimpel and de Boer 2008, Harper et al 2016. Survival of diploid males is known in sawflies (Naito and Suzuki 1991, Cook et al 2013, Harper et al 2016 and is common in nature at least in some species of Apocrita (Liebert et al 2005, Retamal et al 2016.…”

Gregariousness does not vary with geography, developmental stage, or group relatedness in feeding redheaded pine sawfly larvae

“…These observations suggest that, when it comes to forming and maintaining larval aggregations, larvae do not discriminate between kin and nonkin (Figure ). These findings cannot be explained by a lack of capacity for kin recognition in this species because previous work has shown that adults do discriminate between related and unrelated mates (Harper et al., ). One possible implication of our results is that individuals derive sufficient direct benefits from aggregating that kin selection need not be invoked to explain the evolution and maintenance of larval gregariousness.…”