Mechanisms that influence sex ratio variation in the invasive hymenopteran Sirex noctilio in South Africa

Abstract

Sirex noctilio is an economically important invasive pest of commercial pine forestry in the Southern Hemisphere. Newly established invasive populations of this woodwasp are characterized by highly male‐biased sex ratios that subsequently revert to those seen in the native range. This trend was not observed in the population of S. noctilio from the summer rainfall regions in South Africa, which remained highly male‐biased for almost a decade. The aim of this study was to determine the cause of this persistent male bias. As an explanation for this pattern, we test hypotheses related to mating success, female investment in male versus female offspring, and genetic diversity affecting diploid male production due to complementary sex determination. We found that 61% of females in a newly established S. noctilio population were mated. Microsatellite data analysis showed that populations of S. noctilio from the summer rainfall regions in South Africa are far less genetically diverse than those from the winter rainfall region, with mean Nei's unbiased gene diversity indexes of 0.056 and 0.273, respectively. These data also identified diploid males at low frequencies in both the winter (5%) and summer (2%) rainfall regions. The results suggest the presence of a complementary sex determination mechanism in S. noctilio , but imply that reduced genetic diversity is not the main driver of the male bias observed in the summer rainfall region. Among all the factors considered, selective investment in sons appears to have the most significant influence on male bias in S. noctilio populations. Why this investment remains different in frontier or early invasive populations is not clear but could be influenced by females laying unfertilized eggs to avoid diploid male production in populations with a high genetic relatedness.

Biotic and abiotic effects on density, body size, sex ratio, and survival in immature stages of the European woodwasp, Sirex noctilio

“…This stands in contrast to other xylophagous hymenopterans (sawflies) that adjust the placement of male versus female eggs in response to host quality (Cárcamo et al., 2005; Craig et al., 1992; Mopper & Whitham, 1992; Morrill et al., 2000). This finding also complements recent work showing that genetic drivers of male bias (specifically low genetic diversity together with a complementary sex determination system) are overshadowed by ecological factors (Queffelec et al., 2019). Higher female survival in bottom logs could reflect higher resource quality which may be more important for females who must invest in egg production as well as growth.…”

“…Poor mating success leading to an overabundance of unmated females (which can only produce males) is also a possibility, as is the overproduction of diploid males in populations with low genetic diversity where sex is determined by a complementary sex determination system (Boer et al, 2012;Cook & Crozier, 1995). Queffelec et al (2019) recently explored several proposed mechanisms of male bias in South African populations of S. noctilio using field data and simulation modeling.…”

“…Queffelec et al. (2019) recently explored several proposed mechanisms of male bias in South African populations of S. noctilio using field data and simulation modeling. While this study found a relatively high proportion of unmated females (39%), the authors ultimately concluded that female investment in males (among mated females) is likely a stronger driver of male bias at the egg stage.…”

“…Based on our results, the impressive size variation and male bias in South African S. noctilio populations arise in no small part from variation within and among trees (primarily the former, at least within P. patula). In comparison to mating limitation and other genetic mechanisms with the potential to influence sex ratio (Queffelec et al, 2019), ecological effects linked to microsite variation within trees appears strong.…”

“…Based on our results, the impressive size variation and male bias in South African S. noctilio populations arise in no small part from variation within and among trees (primarily the former, at least within P. patula ). In comparison to mating limitation and other genetic mechanisms with the potential to influence sex ratio (Queffelec et al., 2019), ecological effects linked to microsite variation within trees appears strong. The observed male bias in South Africa could emerge as a consequence of introduction into evolutionarily “unfamiliar” conditions, and the tendency for male proportion to decrease over time may therefore reflect adaptation to the local resource environment, either in reproductive investment by females, in larval development, or both.…”