Why and how does sociality change the fecundity/longevity trade-off in termites?

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Ageing and the fecundity/longevity trade-off are directly linked as an increase in fecundity is associated with lifespan reduction. However, social insects illustrate this is not a universal biological rule. Queens of termites, ants or honey bees have extraordinary reproductive rates and are amongst the most long-living insects. Even more striking, reproduction actually increases longevity. The repeated, convergent evolution of this apparent reversal of the fecundity/longevity trade-off in all social insects suggests that sociality causally reshapes this fundamental life-history trade-off. Yet, why and how it does, remains unclear. One major finding from the modelling project of the first funding phase was that the reproductive potential of workers (i.e., the probability that a worker produces offspring) is a main determinant of caste-specific ageing rates. Declining workers’ reproductive potential – which commonly correlates with social complexity - is predicted to increase the lifespan of queens relative to that of workers. These predictions correspond to what we found empirically for two termite species. In a species with sterile workers (Macrotermes bellicosus), no evidence of ageing was detected at the gene expression level in reproductives, while workers showed strong signs of ageing. By contrast, we found exactly the reverse for a species (Cryptotermes secundus) in which reproductives develop from totipotent workers.During the second funding phase, we want to test these findings for generality by comparatively studying termite species in which the worker’s reproductive potential differs. As outgroup we use a representative of their sister taxon, the woodroaches. By covering the whole termite phylogeny and considering confounding factors, such as colony size or species- and caste-specific ecology, we will (i) analyse how molecular mechanisms that underlie the fecundity/longevity trade-off change with increasing sociality and / or worker’s reproductive potential by undertaking transcriptome studies, and (ii) specifically model the evolution of the fecundity/longevity trade-off with varying sociality, taking into account idiosyncrasies of termites. Therefore, our project links results from the proximate framework of the first funding phase with those of the ultimate perspective. Additionally, it extends its scale from two species at opposite ends of termites’ sociality spectrum to a representative range of species covering all social transitions. By doing this, our project will contribute fundamental insights into how and why sociality changes the fecundity/longevity trade-off - and thus ageing – by using the oldest social insect lineage, the termites, as a test case.