Foster, W. A. | Search bioRxiv

bioRxiv2025

The kin-selected context of duelling in horned aphids: cooperation or conflict?

In one-on-one (dyadic) contests in group-living animals, relatedness is of potentially critical importance in determining the relative value of the resources for which the individuals are fighting. We investigated the function of dyadic contests over access to a food resource (plant phloem) in the horned aphid Astegopteryx bambusae, by observing competing aphids in the field and genotyping them with microsatellite markers. Each aphid colony consists of a small number of clones, creating levels of relatedness that can be both high and variable. The contests involve only low costs in terms of time or injury. Our results indicate that there is no kin-discrimination in the duelling pairs of aphids. Genetic relatedness between duelling pairs did not differ significantly from that of randomly selected pairs and showed no association with contest duration or outcome. The mean relatedness between a duelling pair was 0.79 {+/-} 0.12 (mean {+/-} SD, N = 75), with 56% (42/75) of duels occurring between clonal pairs. We suggest that the duels in these Astegopteryx aphids are not an aggressive fight for resources between different genotypes, but rather a low-cost method by which the aphids assess each others reproductive value. 83% (50/60) of the contests between aphids of different ages were won by the aphid that was older, and therefore of greater reproductive value, providing an indirect fitness benefit for the losing younger individual. Younger nymphs, when attacked by older aphids, yield feeding sites altruistically and gain inclusive fitness benefits: this provides a compelling selective context for the evolution of the young, rather than old, altruistic soldiers that are observed in the open colonies of many cerataphidine species.

ecology↗

bioRxiv2022

Eusocial evolution without a nest: kin structure of social aphids forming open colonies on bamboo

Living in nests is an almost universal feature of eusocial animals. In some aphids, however, sterile soldier castes have evolved in open colonies without a nest. To clarify the factors promoting the evolution of eusociality in these colonies, we used newly developed microsatellite markers to compare the kin structure of the open colonies of two aphid species on bamboo: the non-eusocial colonies of Astegopteryx bambucifoliae and the eusocial colonies of Pseudoregma alexanderi on Dendrocalamus latiflorus. Our samples, from over 1,000 hectares, contained 99 clones of A. bambucifoliae and 19 of P. alexanderi. Clonal mixing occurred in both species: average pairwise relatedness within a colony was 0.54 in A. bambucifoliae and 0.71 in P. alexanderi. Each clone of A. bambucifoliae occurred in a unique location, whereas those of P. alexanderi occurred in multiple locations and more than 90% of individuals came from just four clones. There was significant genetic variation among different colonies in the same clump (stem-cluster) in A. bambucifoliae but not in P. alexanderi, indicating that P. alexanderi colonies in a single clump are genetically homogenized, functioning as a large colony. In P. alexanderi, the proportion of sterile soldiers to normal first-instar nymphs was significantly different across the four clones. Our results indicate that the lack of input of migrants from the primary host and feeding on a large, stable host plant are important ecological factors that might favour the evolution of eusociality, enabling the production of genetically homogenised, large, and long-lived colonies. After eusociality evolves on the secondary host, the optimal strategy of soldier production might vary between different clones. Significance StatementNest-living has often been considered to be a necessary condition for the evolution of eusociality. In a small number of aphid species, however, sterile soldier castes have evolved in open colonies without a nest. To understand why these aphids are unique, we examined the kin structure and genetic relatedness of individuals within eusocial and non-eusocial open colonies of two aphid species on bamboo. We found that clonal mixing occurred in both species, but the eusocial colonies are more genetically homogenized, functioning as a large colony. Our results suggest that ecological conditions that promote genetically homogenized, large and long-lived colonies are important for the evolution of eusociality in these aphids. We propose that the open colonies of social aphids provide an ideal model system in which to study the evolution of altruism.

evolutionary biology↗