Ecology of Sex Ratios in Mountain Pine Beetles, Dendroctonus ponderosae

Abstract

Biased sex ratios pose a fundamental question in evolutionary ecology. Populations are expected to have equal numbers of males and females when each offspring is produced from one mother and one father with equal genetic contributions. In mountain pine beetles (MPB), Dendroctonus ponderosae Hopkins, consistent reports of female-biased adult sex ratios (2:1 F:M) demand explanation. I investigated whether adult sex ratios of MPB reflected initial investment or became biased through adaptive or non-adaptive processes. Differential mortality is a potentially non-adaptive process that could bias sex ratios at later life stages, an idea often invoked to explain female-biased adult sex ratios in this system. However, Fisher noted that if mortality occurs during parental investment, selection will act to bias the primary sex ratio towards the sex with higher mortality, achieving an evolutionary stable state where expenditure to each sex is equal. Individual allocation strategies may differ depending on maternal condition. In insects, daughters were expected to benefit more from increased investment because of the strong relationship between body size and fecundity. Through genotyping of eggs from individual broods reared in the laboratory, I found that primary sex ratios of MPB were male-biased (~1:0.86). Male-biased primary sex ratios were counter to expectations if the sex-ratio distorter Wolbachia was acting in the system; Wolbachia was not detected in a screen of ten populations. Female-biased adult sex ratios arose from subsequent male-biased mortality, observed in both laboratory-reared broods and in experimentally manipulated overwintering field populations. Moreover, condition-dependent allocation occurred, but was counter to expectations, with larger females producing more males than did smaller females. Larger mothers produced larger eggs that experienced less male mortality. In this case, larger mothers gain more from investing in the rare sex than investing in daughters. Given male-biased developmental mortality, a male-biased primary sex ratio is consistent with Fisher’s theory of equal investment favouring the sex with greater mortality during parental investment. Further, differential mortality of the sexes is a strong selective force to which condition-dependent sex allocation may respond more strongly than increases in body size.