Parasite-offspring competition for female resources can explain male-biased parasitism in plants

Male-biased susceptibility to parasites is common in dioecous plants. However, why males have higher parasite loads than females is unclear. Unlike males, females have to subsidize post-fertilization costs of reproduction (e.g. seed and fruit development). As a result, females may have smaller pools of resources potentially available to parasites, thus leading to lower parasite loads. We tested this prediction in New Zealand’s largest native moth (Aenetus virescens: Lepidoptera), whose larvae parasitize Aristotelia serrata (Elaeocarpaceae), an endemic species of dioecous tree. We measured parasite loads in male and female trees, as well as annual seed set in females. We then derived a technique to equate the energetic cost of seed set in females to an equivalent number of parasitic larvae. Our results showed evidence for male-biased parasitism; male trees harboured more larval parasites than female trees. However, when parasite loads in males were compared to parasite loads in females, plus the energetic cost of seed production calculated as an equivalent number of parasitic larvae, differences in parasitism between the sexes disappeared. We conclude that male-biased parasitism in plants could arise from parasite-offspring competition for female resources.

雌雄异株植物中，雄性偏向的寄生虫易感性现象较为普遍。然而，为何雄性的寄生虫负荷量高于雌性，目前尚无定论。与雄性不同，雌性需承担受精后的繁殖成本（如种子与果实发育）。因此，雌性可用于寄生虫寄生的潜在资源储备池规模更小，进而寄生虫负荷量更低。我们以新西兰本土最大蛾类*Aenetus virescens*（鳞翅目Lepidoptera）为研究对象验证该预测，其幼虫寄生于新西兰特有雌雄异株树种*Aristotelia serrata*（杜英科Elaeocarpaceae）。我们测定了雌雄植株的寄生虫负荷量，并统计了雌性植株的年度结实量；随后开发了一种方法，可将雌性的种子发育能量成本换算为等价的寄生幼虫数量。研究结果证实了寄生现象的雄性偏向性：雄性植株的寄生幼虫数量显著多于雌性植株。然而，若将雄性的寄生虫负荷量，与雌性的寄生虫负荷量加上换算为寄生幼虫当量的种子生产能量成本进行对比，则两性间的寄生差异不复存在。综上，我们认为植物中出现雄性偏向性寄生的原因，可能是寄生虫与雌性后代竞争雌性植株的有限资源。