Research from North Carolina State University in the US explores the origins of the termite/gut symbiont relationship.
Gut symbionts are integral to the success of termites as a group, enabling them to be one of the few groups of animals capable of successfully digesting cellulose. Ongoing studies have unravelled some of the mysteries of this symbiotic relationship and its role in the evolution of termites.
Evolution in lower termites
Lower termites are more ‘primitive’ termites and include Mastotermes as well as the dampwood and drywood termites. The Rhinotermitidae are also classed as lower termites, but they are a more recent evolution, exhibiting some higher termite behaviours.
The gut symbionts in termites die prior to the termite moulting. So how are these gut symbionts retained within a termite population and how did such a relationship evolve? Christine Nalepa from North Carolina State University in the US has proposed a new hypothesis.1
Lower termites and their closest living cockroach ancestor, the sub-social wood-feeding cockroach Cryptocercus, both contain flagellate eukaryotic protists as their gut symbionts to aid with the digestion of wood. In Cryptocercus these symbionts form cysts prior to the cockroach moulting. These cysts are viewed as an evolutionary artefact, with the flagellates in Cryptocercus having evolved from parasitic flagellates of gregarious cockroaches, which form cysts to be passed in faeces and subsequently ingested by other cockroaches.
However, in Cryptocercus these cysts play no demonstrable role in the transmission of flagellates between nestmates and between generations. Instead the gut symbionts are passed directly from parents to offspring through feeding on hindgut secretions (proctodeal trophallaxis).
The hypothesis proposes that it was the change in behaviour in ancient social cockroaches, in moving from coprophagy to proctodeal feeding, that was critical in the development of the termite lineage. Not only did this accelerate parental care and increased social behaviour, but it also drove the evolution of the flagellates from a parasitic relationship to a mutualistic one.
Evolution of higher termites
Higher termites, the Termitidae, consist of the most recent species from an evolutionary point of view, including Microcerotermes and Nasutitermes.
Whilst lower termites rely on symbiotic protists to digest wood, higher termites (Termitidae) lack these flagellates altogether. With 70% of termite species being in the Termitidae family, the loss of protists is seen as a major evolutionary step in the development of termites. Instead of protists, higher termites have a range of bacteria and other gut symbionts to aid with digestion. Researchers believe that this change in gut symbionts drove this speciation through variation in diet, behaviour and morphology, allowing higher termites to utilise a wider range of niches.2
Although the mechanisms that drove this evolution remain speculative, a review paper on the subject provides two drivers for the evolution of the higher termites. Although lower termites feed primarily on wood, 85% of higher termites are classed as soil feeders, feeding on rotten wood and decomposed vegetable matter in the soil. By switching to a soil diet it would have starved the specialist protist symbionts of their source of cellulose and therefore eliminated them from the gut.
However, within the higher termites there are two groups of wood feeders – the Macrotermitinae and Sphaerotermitinae – which have also lost their cellulose digesting protist. They manage to ‘digest’ the wood outside of their bodies using fungi (Termitomyces) in the case of Macrotermitinae (main picture, above) or bacterial combs in the case of Sphaerotermitinae.
It’s not known which of these developments occurred first or whether they occurred independently, but they certainly provided the platform for the diversification and success of the higher termites.
More termite information
1 Nalepa, C.A., 2020. Origin of Mutualism Between Termites and Flagellated Gut Protists: Transition From Horizontal to Vertical Transmission. FRONTIERS IN ECOLOGY AND EVOLUTION 8. https://doi.org/10.3389/fevo.2020.00014
2 Chouvenc, T., Sobotnik, J., Engel, M.S., Bourguignon, T., 2021. Termite evolution: mutualistic associations, key innovations, and the rise of Termitidae. CELLULAR AND MOLECULAR LIFE SCIENCES 78, 2749–2769. https://doi.org/10.1007/s00018-020-03728-z
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