Potential increase in mating frequency of queens in feral colonies of Bombus terrestris introduced into Japan

Polyandry may mitigate the negative impact of reproductive interference among bumblebees in Japan

In social hymenopterans, monandry of the queen is an ancestral trait, and polyandry is a derived trait. Polyandry of the queen is the norm in a limited number of lineages, such as honeybees, leaf-cutting ants, Pogonomyrmex ants, and Vespula wasps, which presumably provide fitness advantages for the whole colony. The queen of the introduced bumblebee, Bombus terrestris, is polyandrous in Japan, whereas it is monandrous in native regions. We hypothesize that polyandry can evolve in a process that avoids the negative impacts of reproductive interference caused by interspecific mating and conducted genetic studies of the invasive species B. terrestris and two native subspecies, Bombus hypocrita sapporoensis and Bombus hypocrita hypocrita, in Japan. Our results revealed that although the native queens of B. hypocrita hypocrita allopatric with B. terrestris were strictly monandrous, the native queens of B. hypocrita sapporoensis sympatric with B. terrestris were polyandrous. These results suggested that the queens of native B. hypocrita sapporoensis do not experience negative impacts on interspecific mating from the invasive B. terrestris. We discuss the possibility that reproductive interference is a driving force in selection for multiple mating through an arms race between sympatric species.

Unexpected worker mating and colony-founding in a superorganism

The emergence of caste-differentiated colonies, which have been defined as 'superorganisms', in ants, bees, and wasps represents a major transition in evolution. Lifetime mating commitment by queens, pre-imaginal caste determination and lifetime unmatedness of workers are key features of these animal societies. Workers in superorganismal species like honey bees and many ants have consequently lost, or retain only vestigial spermathecal structures. However, bumble bee workers retain complete spermathecae despite 25-40 million years since their origin of superorganismality, which remains an evolutionary mystery. Here, we show (i) that bumble bee workers retain queen-like reproductive traits, being able to mate and produce colonies, underlain by queen-like gene expression, (ii) the social conditions required for worker mating, and (iii) that these abilities may be selected for by early queen-loss in these annual species. These results challenge the idea of lifetime worker unmatedness in superorganisms, and provide an exciting new tool for the conservation of endangered bumble bee species.

Temporal Analysis of Effective Population Size and Mating System in a Social Wasp

Highly social species are successful because they cooperate in obligately integrated societies. We examined temporal genetic variation in the eusocial wasp Vespula maculifrons to gain a greater understanding of evolution in highly social taxa. First, we wished to test if effective population sizes of eusocial species were relatively low due to the reproductive division of labor that characterizes eusocial taxa. We thus estimated the effective population size of V. maculifrons by examining temporal changes in population allele frequencies. We sampled the genetic composition of a V. maculifrons population at 3 separate timepoints spanning a 13-year period. We found that effective population size ranged in the hundreds of individuals, which is similar to estimates in other, non-eusocial taxa. Second, we estimated levels of polyandry in V. maculifrons in different years to determine if queen mating system varied over time. We found no significant change in the number or skew of males mated to queens. In addition, mating skew was not significant within V. maculifrons colonies. Therefore, our data suggest that queen mate number may be subject to stabilizing selection in this taxon. Overall, our study provides novel insight into the selective processes operating in eusocial species by analyzing temporal genetic changes within populations.

Reproductive Interference in an Introduced Bumblebee: Polyandry may Mitigate Negative Reproductive Impact

As a signature of reproductive interference (RI), we reviewed hybrid production in eusocial bumblebees in Japan, by comparing introduced Bombus terrestris with native B. ignitus in Honshu (main island of Japan) and with native B. hypocrita sapporoensis in Hokkaido (northern island of Japan). In this review, we present additional new data showing hybrid production between introduced B. terrestris and native B. ignitus in Honshu. Interspecific mating with introduced B. terrestris disrupts the reproduction of native B. h. sapporoensis and B. ignitus, which belong to the same subgenus of Bombus, through inviable egg production. This interference appears to facilitate species replacement on Hokkaido. Simultaneously, the mating frequencies for queens of B. terrestris have increased, suggesting that polyandry might evolve in response to the extent of RI between B. terrestris and B. h. sapporoensis. To suppress the population size of B. terrestris in Hokkaido, two methods have been proposed: the mass release of B. h. sapporoensis males to induce RI between the two species and the spraying of insecticides against foraging workers so that the workers will carry the insecticides back to their colonies, killing the immature bees within the colonies. A candidate insecticide type is insect growth regulator, which may disrupt larval development without any apparent effect on foraging workers.