Reproductive constraint in the social aphid Ceratovacuna japonica: Sterility regulation in the soldier caste of a viviparous insect

Differentiation of the non-reproductive caste is a unique feature of eusocial insects. Apoptosis in oocytes plays a major role in constraining the reproductivity of the eusocial insects including bees, ants, and termites. However, the regulation of reproductive constraint in non-reproductives of primitively eusocial insects other than hymenopterans and blattodeans is almost unknown. Here, we investigated the soldier sterility in a hemipteran insect, the social aphid Ceratovacuna japonica. We compared the gonads of soldiers, that are completely sterile, with those of reproductives in their viviparous development. We found that soldiers possess a pair of ovaries and the same number of germaria as reproductives, but soldiers' ovarioles were small and lacking gastrulating embryos. Unlike in most model social insects, the staining of cleaved Caspase-3 showed apoptosis in the maternal nutritive cells, rather in the oocyte, of soldier ovaries. In addition, the ubiquitous C. japonica vasa1 and piwi2a expression indicates the developmental failure of embryos in soldier ovaries. The absence of posterior nos1, an insect posterior determinant, indicates deficient posterior patterning in soldier ovarioles. Our findings suggest a different mode of reproductive constraint, which regulates both oogenesis and embryogenesis in a viviparous insect ovary. This is the first report of the reproductive constraint in a viviparous social insect at the molecular level.

Mating status and the evolution of eusociality: Oogenesis is independent of mating status in the solitary bee Osmia bicornis

The fundamental trait underlying eusociality is the reproductive division of labour. In honeybees (Apis mellifera), queens lay eggs while workers forage, defend and care for brood. The division of labour is maintained by pheromones including queen mandibular pheromone (QMP) produced by the queen. QMP constrains reproduction in adult honeybee workers, but in the absence of their queen workers can activate their ovaries and, although they cannot mate, they lay haploid male eggs. The reproductive ground plan hypothesis suggests that reproductive constraint may have evolved by co-opting mechanisms of reproductive control in solitary ancestors. In many insects mating is required to activate or accelerate oogenesis. Here, we use the solitary bee Osmia bicornis (Megachilidae) to test whether reproductive constraint evolved from ancestral control of reproduction by mating status. We present a structural study of the O. bicornis ovary, and compare key stages of oogenesis with honeybee workers. Importantly, we show that mating does not affect any aspect of the reproductive physiology of O. bicornis. We therefore conclude that mechanisms governing reproductive constraint in honeybees were unlikely to have been co-opted from mechanisms pertaining to mating status.

Ancestral hymenopteran queen pheromones do not share the broad phylogenetic repressive effects of honeybee queen mandibular pheromone

Queen pheromones effect the reproductive division of labour, a defining feature of eusociality. Reproductive division of labour ensures that one, or a small number of, females are responsible for the majority of reproduction within a colony. Much work on the evolution and function of these pheromones has focussed on Queen Mandibular Pheromone (QMP) which is produced by the Western or European honeybee (Apis mellifera). QMP has phylogenetically broad effects, repressing reproduction in a variety of arthropods, including those distantly related to the honeybee such as the fruit fly Drosophila melanogaster. QMP is highly derived and has little chemical similarity to the majority of hymenopteran queen pheromones which are derived from cuticular hydrocarbons. This raises the question of whether the phylogenetically widespread repression of reproduction by QMP also occurs with more basal saturated hydrocarbon-based queen-pheromones. Using D. melanogaster we show that saturated hydrocarbons are incapable of repressing reproduction, unlike QMP. We also show no interaction between the four saturated hydrocarbons tested or between the saturated hydrocarbons and QMP, implying that there is no conservation in the mechanism of detection or action between these compounds. We propose that the phylogenetically broad reproductive repression seen in response to QMP is not a feature of all queen pheromones, but unique to QMP itself, which has implications for our understanding of how queen pheromones act and evolve.