Adaptive phenotypic variation among clonal ant workers

Within-colony genetic diversity differentially affects foraging, nest maintenance, and aggression in two species of harvester ants

There is accumulating evidence that genetic diversity improves the behavioral performance and consequently the fitness in groups of social animals. We examined the behavioral performance of colonies of two co-occurring, congeneric harvester ant species (Messor arenarius and a non-described Messor sp.) in fitness-related behaviors, pertaining to foraging performance, nest maintenance, and aggression. We linked these behaviors to the colonial genetic diversity, by genotyping workers, using six and five microsatellite markers for M. arenarius and M. sp., respectively. Correlations of genetic diversity with colony performance and aggression level contrasted between the two species. In M. arenarius, genetic diversity was correlated with foraging performance and nest maintenance but not with the overall aggression level, while in M. sp., genetic diversity was correlated with the overall aggression level, but not with foraging performance or nest maintenance. The two species exhibited similar specific aggression levels, with higher aggression shown towards heterospecifics and lower towards non-nestmate conspecifics and nestmates. However, M. sp. workers displayed a tendency to interact for longer with heterospecifics than did M. arenarius. We speculate that the different foraging strategies, group vs. individual foraging, and possibly also the different mating systems, contribute to the differences found in behavior between the two species.

Effects of aphid parasitism on host plant fitness in an aphid-host relationship

Aphids are serious agricultural insect pests which exploit the phloem sap of host plants and thus transmit pathogens to their hosts. However, the degree to which aphid parsitism affects the fitness of the host plants is not well understood. The aphid, Macrosiphoniella yomogicola, parasitizes the mugwort Artemisia montana in Japan. During summer most mugworts carry aphids, but most aphid colonies die out after the budding of A. montana inflorescences in late summer. A few aphid colonies survive to late autumn, at which point sexuparae appear to later lay overwintering eggs after copulation. The death of the aphid colonies seems to be caused by biochemical changes in the phloem sap in the host plant coincident with the budding of inflorescences. The surviving aphid colonies may suppress the budding of inflorescences to allow persistence of their genetic line into the following year. Our investigations demonstrate that aphid parasitism did not affect host plant growth, but that it did significantly decrease the number of inflorescences and the average weight of floral buds. Our results indicate that aphid parasitism has a strong negative effect on the fitness of host plants. The manner in which the aphids suppress floral budding in their hosts is worth examining from the perspective of the evolution of aphid-plant interactions.

Adaptive phenotypic variation among clonal ant workers

Phenotypic variations are observed in most organisms, but their significance is not always known. The phenotypic variations observed in social insects are exceptions. Genetically based response threshold variances have been identified among workers and are thought to play several important adaptive roles in social life, e.g. allocating tasks among workers according to demand, promoting the sustainability of the colony and forming the basis of rationality in collective decision-making. Several parthenogenetic ants produce clonal workers and new queens by asexual reproduction. It is not clearly known whether such genetically equivalent workers show phenotypic variations. Here, we demonstrate that clonal workers of the parthenogenetic ant Strumigenys membranifera show large threshold variances among clonal workers. A multi-locus genetic marker confirmed that colony members are genetic clones, but they showed variations in their sucrose response thresholds. We examined the changing pattern of the thresholds over time generating hypotheses regarding the mechanism underlying the observed phenotypic variations. The results support the hypothesis that epigenetic modifications that occur after eclosion into the adult form are the cause of the phenotypic variations in this asexual species.