Sexual selection and male body size plasticity in mites

Developmental plasticity in life history traits such as age and size at maturity allows organisms to adaptively match trait expression to environmental gradients. Most arthropods incl. mites are sexually dimorphic with females being larger than males. It is commonly acknowledged that sexual size dimorphism and sex-specific differences in size plasticity are due to differential but interrelated selective forces. Female body size is primarily fecundity selected while male body size is primarily sexually selected. We recently examined sex-specific body size plasticity in two predatory mite species, Phytoseiulus persimilis and Neoseiulus californicus. Our study revealed that both species are similarly plastic in female body size but male P. persimilis are much less plastic in size than male N. californicus. We hypothesize that this difference is mainly due to different mating systems and associated male mating opportunities. Males of both species are similarly polygynous. However, P. persimilis females are low level polyandrous - they re-mate occasionally but need only a single mating to achieve maximum egg production - while N. californicus are high level polyandrous - they mate repeatedly throughout life and need multiple matings for maximum egg production. Consequently, P. persimilis females have a lower lifetime mating frequency than N. californicus females, which reduces the number of mating opportunities for males and makes missed matings more costly for male P. persimilis than male N. californicus. Based on this hypothesis we propose to scrutinize and compare the fitness consequences of male body size plasticity in P. persimilis and N. californicus. In cooperation with a colleague from Kyoto University, Japan, who is a specialist in molecular ecology of mites, we plan to conduct four experimental series on (1) lifetime mating frequency and reproductive success of small and large males, (2) paternity success and male mating sequence, (3) female choice/male competition and male body size plasticity, and (4) paternity success of small and large males when females mate multiply. The outcome of our proposal should advance our understanding of the selective forces shaping life history plasticity, lead to the development of novel molecular markers for mites, and in a broad sense contribute to improve the use of P. persimilis and N. californicus as biocontrol agents of herbivorous mites and insects.

Male body size plays a major role in many animals, particularly in mating-related processes, events and phenomena. Small males are commonly at a disadvantage in reproductive success and competition for receptive females, compared to standard-sized males. Moreover, small males often have a lower mating success in species with low than high female mating rates (polyandry). Thus, in the course of evolution possible deviations from male standard size should become smaller in species with low than high female mating rates. Strong selection pressure on male body size should result in reduced plasticity of male body size. Exactly these differences in the plasticity of male body size and polyandry are documented in the predatory mite species Phytoseiulus persimilis (low male body size plasticity; low level polyandrous) and Neoseiulus californicus (high male body size plasticity; medium level polyandrous). We therefore tested the hypothesis that deviations from the standard male body size cause higher costs in lifetime reproductive success, male-male competition for females, and the paternity success in multiply mated females for P. persimilis males than N. californicus males. To obtain the baseline data on the evolutionary fitness implications of polyandry in these two predatory mite species, we conducted mating experiments, quantified reproductive traits and performed microsatellite genotyping. The medium level polyandrous N. californicus received clear direct and indirect benefits: multiply mated females produced more offspring, over longer times, having higher survival chances than singly mated females. In contrast, singly and multiply mated females of the low level polyandrous P. persimilis produced similar offspring numbers having similar survival chances. In both species, multiple mating resulted in mixed offspring paternities, opening the chance for indirect fitness benefits. However, the female re-mating likelihood and the paternity chance of non-first male mates were lower in P. persimilis than N. californicus. In both species, the first mating duration and female re-mating likelihood were negatively correlated. Fertilization assurance and enhancement of genetic compatibility, complementarity and/or variability are likely the ultimate drivers of polyandry in P. persimilis. Competition for females and lifetime reproductive success were not affected by male body size in N. californicus, while small P. persimilis males were inferior competitors and sired fewer offspring than standard-sized P. persimilis males. In double matings of P. persimilis and N. californicus females with first a small male and then a standard-sized male and in reverse order, respectively, in P. persimilis the paternity success of the first male mate was lower if this male was small. By contrast, in N. californicus the paternity success was not influenced by male body size. In summary, our study supports the hypothesis that small male body size incurs higher costs regarding mating and reproduction in males of P. persimilis than N. californicus. Therefore, the lower body size plasticity of P. persimilis males compared to N. californicus males is the result of stronger past sexual selection pressure on this trait.