The remarkable success of the insect order Hymenoptera – which includes bees, wasps and ants – is probably a consequence of their immense range of behavioural adaptation, often evolving complex social systems that are found in few other animal groups. Two new studies published in Genome Biology have used comparative genomics and gene function annotation to reveal significant insights into the genetic mechanisms underlying polymorphic social behaviour in insects: Da-Wei Huang from the Institute of Zoology, Chinese Academy of Sciences, China, and colleagues, investigate the fig wasp Ceratosolen solmsi, while Douglas Yu from the Kunming Institute of Zoology, China, and colleagues, look to the sweat bee Lasioglossum albipes.
The fig and its pollinating wasp form one of the most striking examples of an obligatory insect-plant mutualism – an interaction in which either species cannot live without the other. Each species of fig tree has a specific pollinating wasp, and both are completely dependent on each other for their survival. The fig wasp is polymorphic: males and females are anatomically and behaviorally very different. In C. solmsi, the wingless, eyeless males spend their entire lives inside figs, while the females have a very brief excursion to the great outdoors in order to lay eggs in another fig tree, sometimes flying up to 160 km away to do so.
Huang and his team addressed whether a life spent largely inside a fig has led to an expected genome reduction, or if female migration prevents this from happening. They discovered that while the genome size of C. solmsi is typical of insects, gene families associated with environmental sensing (such as taste and smell), detoxification and environmental protection have markedly reduced. It may be that the fig wasp’s success in finding a new host is streamlined by the removal of genes that relate to sensing distracting aspects of the environment. The reduction in detoxification genes may be explained by a life almost entirely confined to a benign host, safe from the external environment and many antagonists.
Furthermore, C. solmsi transcriptome analyses demonstrate that males have reduced gene expression as compared to females, results that are consistent with the hypothesis that the males, spending their entire lives inside the fig fruit, use a much reduced gene repertoire. This sexual divergence of gene expression may result in the extreme female/male morphological dimorphism seen in fig wasps.
The sweat bee L. albipes exhibits a very different but equally fascinating form of social dimorphism within a species. Individuals inhabiting inland France and Germany are solitary, living independently. However, in warmer southwestern regions of France these bees operate under a ‘division of labour’ system referred to as eusociality. Yu and colleagues found several unique and expanded gene families compared to other Hymenoptera genomes, a result that contrasts with the reduced gene families found in C. solmsi by Huang.
The expanded gene families in L. albipes were associated with metabolism and nucleotide binding, suggesting that the sweat bee lineage has undergone selection pressure and rapid evolution in these areas. Comparison of data from solitary and social L. albipes identified six genes that appear to be rapidly diverging, including a putative odour receptor and a cuticular protein, a result that may indicate differences in chemical signalling and pheromone production between the two social forms.
The addition of L. albipes and C. solmsi to existing published genomes of the Hymenoptera order establishes a framework for further phylogenetic comparisons that could facilitate a deeper understanding of the evolution of complex social behaviour in insects.