SPITE · The adaptive nature of spiteful behaviour

The adaptive nature of spiteful behaviour

The project investigated how the degree of conflicting interests (originating from the degree of relatedness) affects spiteful behaviour between competing sexes. In the polyembryonic wasp Copidosoma floridanum, females commonly lay one male and one female egg in a lepidopteran host egg. Both sexes proliferate clonally within the growing host larva. Distinct larval castes develop from each wasp egg: the majority being ‘reproductives’ plus some ‘soldiers,’ which sacrifice reproduction and attack competitors. Maturing mixed sex broods are usually female biased, as expected when intra-brood mating is common. Pre-mating dispersal followed by outbreeding is expected to increase sexual conflict over brood sex ratios and result in greater soldier attack rates. Due to sexually asymmetric relatedness, intra-brood conflicts are expected to be resolved primarily via female soldier attack. We observed soldier behaviour in vitro to test whether lower intra-brood relatedness (manipulated by whether or not the father was from the maternal population) increased inter-sexual aggression by female as well as male soldiers. We kept two populations of wasps, one collected from the field in Sumter County, South Carolina, and kept in the lab over 20 years (‘lab’), and one collected from Tift County, Georgia, in the autumn of 2011 (‘field’). In Hymenoptera, males are haploid with all genes deriving from their mother, and females are biparentally diploid. We let virgin ‘field’ females from a brood lay male eggs, and mated other females from the brood with either ‘field’ males or ‘lab’ males, to produce sister broods that were more or less related to their haploid brothers. In each replicate, a female or a male soldier was placed in a culture well with opposite sex polymorula (‘reproductives’), with a soldier being either more or less related to the sibling polymorula. The number of soldiers that attacked the polymorulae was recorded. We also tested whether soldiers attacked opposite sex soldiers in addition to attacking the reproductives. As in prior studies, females were more aggressive than males but, contrary to expectations, soldiers of both sexes showed more aggression towards more closely related polymorulae. However, none of the soldiers attacked an opposite sex soldier. Ecological conditions might contribute to the value of the less related siblings as a mate in this naturally inbred species. In nature, there is variation in host density and availability. Thus, mating opportunities for single sex broods of Copidosoma floridanum can be scarce. We speculate that lower than average intra-brood relatedness indicates maternal outbreeding, i.e. parents of a brood originate from far apart. Thereby, potentially low chances of finding a mate after dispersal may enhance the value of opposite sex siblings in a brood as mates. This result is of interest to a wide sector of the academic audience, because it adds to the understanding of the ecological conditions and evolutionary processes leading to antisocial behaviour and to behavioural differences between sexes. It will thus extend the knowledge base of those studying social behaviour, social evolution, sexual conflict, sex ratio allocation, interspecific and intraspecific competition, the origins of pathogen virulence, and the evolution and maintenance of sexual reproduction.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз