Project A.12

Termites belong to the oldest group of social insects and their sex is determined by sex chromosomes, with most species known to possess an XY sex determination system. To date, studies of termites are rare, with few focusing on the sex chromosomes.

For my project, I aim to characterize sex chromosomes of termites. Furthermore, I aim to investigate the connection between sex and caste determination pathways and determine how sex-biased gene expression changes during development.

Termites are an understudied group of insects that is of vast interest due their sociality, as well as their ecological and economic impact. Biologically, termites (Isoptera) are a diverse and species-rich clade. They are eusocial insects that form complex societies with division of labour between non-reproducing worker and soldier castes and reproductively active individuals. Termites have evolved from non-social cockroaches approximately 150 million years ago. Eusociality evolved at the base of all termites, presumably only once. Within the termites, there are different levels of social complexity: the lower termites retain developmental complexity, and their worker caste is made up of immatures that can still develop into reproductives or soldiers. Higher termites, on the other hand, have defined castes with a true worker caste whose members are sterile for life.

In contrast to eusocial hymenopterans males hatch from fertilised eggs like females. They are also present in all castes and do not just serve for reproduction. While in hymenopterans the presence or absence of fertilisation determines the sex of the embryo, termites have chromosomal sex determination. Thus far, all studied termite species have shown some form of XY sex determination. Their closely related sister group (roaches) have X0 sex chromosomes that are large and heterochromatic. Interestingly, even though termites are ecologically and economically of great importance, their sex determination (SD) and reproduction has received very little attention.

The study of SD in this group of eusocial insects is interesting because sex and caste determination are not entirely separate processes. As individuals are separated into sterile (worker and soldier) castes and reproductives, sexual differentiation does differ between castes. Males and females are present among both, the reproductive and sterile castes, but only reproductives have fully developed and functioning gonads. Many pathways shown to be involved in SD in other arthropods such as vitellogenin and juvenile hormone are also important for caste determination. It has also been observed in many termite species, that there can be a skewed sex ratio in specific castes. For example, in the termite Nasutitermes takasagoensis, most soldiers are male. This could also be explained by a size threshold during development with males being larger in this species and thus having a higher chance to pass the threshold and develop into a soldier. A few recent studies indicated that there is probably a genetic and/or hormonal linkage between SD and genes that are involved in caste determination at least in some termite species. This makes termites an interesting group to study SD and explore in more detail the interplay between sex chromosomes, hormones, and the evolution of eusociality and social complexity.