Abstract
Strongly female-biased sex ratios are typical for the fungalfeeding haplodiploid Xyleborini (Scolytinae, Coleoptera), and are a result of inbreeding and local mate competition (LMC). These ambrosia beetles are hardly ever found outside of trees, and thus male frequency and behavior have not been addressed in any empirical studies to date. In fact, for most species the males remain undescribed. Data on sex ratios and male behavior could, however, provide important insights into the Xyleborini’s mating system and the evolution of inbreeding and LMC in general.In this study, I used in vitro rearing methods to obtain the first observational data on sex ratio, male production, male and female dispersal, and mating behavior in a xyleborine ambrosia beetle. Females of Xyleborinus saxesenii Ratzeburg produced between 0 and 3 sons per brood, and the absence of males was relatively independent of the number of daughters to be fertilized and the maternal brood sex ratio. Both conformed to a strict LMC strategy with a relatively precise and constant number of males. If males were present they eclosed just before the first females dispersed, and stayed in the gallery until all female offspring had matured. They constantly wandered through the gallery system, presumably in search of unfertilized females, and attempted to mate with larvae, other males, and females of all ages. Copulations, however, only occurred with immature females. From galleries with males, nearly all females dispersed fertilized. Only a few left the natal gallery without being fertilized, and subsequently went on to produce large and solely male broods. If broods were male-less, dispersing females always failed to found new galleries.
Highlights
In panmictic species, natural selection usually favors balanced sex ratios (Fisher 1930)
Natural selection should favor the production of that number of males that maximizes the mean number of inseminated females dispersing from a brood
My data suggest that males in Xyleborinus saxesenii Ratzeburg are extremely successful in locating and fertilizing all their sisters in the natal gallery
Summary
In panmictic (randomly mating) species, natural selection usually favors balanced sex ratios (Fisher 1930). In a nonrandomly mating population or species with strictly local offspring reproduction, a mother will gain highest fitness by producing as many daughters as possible and just enough sons to ensure fertilization of all sisters in the brood (Hamilton 1967). This extreme economy in the production of males is common in small arthropods with regular brother-sister mating and local mate competition (LMC) between brothers (Hamilton 1967; Norris 1993). Arrhenotoky potentially gives a mother precise control over the sex of each offspring, as she may choose to produce diploid daughters by fertilizing an egg, or haploid sons by leaving it unfertilized (Hamilton 1967; Charnov 1982). Hamilton (1978) suggested that the ancestral habitat for the aforementioned biofacies is situated under the bark of dead trees, even if the support for this quotation is weak and claims on the original habitat of ancient lineages are highly speculative (Normark et al 1999)
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