Protist symbionts of termites: diversity, distribution, and coevolution.

The symbiosis between termites and their hindgut protists is mutually obligate and vertically inherited. It was established by the late Jurassic in the cockroach ancestors of termites as they transitioned to wood feeding. Since then, protist symbionts have been transmitted from host generation to host generation by proctodeal trophallaxis (anal feeding). The protists belong to multiple lineages within the eukaryotic superphylum Metamonada. Most of these lineages have evolved large cells with complex morphology, unlike the non-termite-associated Metamonada. The species richness and taxonomic composition of symbiotic protist communities varies widely across termite lineages, especially within the deep-branching clade Teletisoptera. In general, closely related termites tend to harbour closely related protists, and deep-branching termites tend to harbour deep-branching protists, reflecting their broad-scale co-diversification. A closer view, however, reveals a complex distribution of protist lineages across hosts. Some protist taxa are common, some are rare, some are widespread, and some are restricted to a single host family or genus. Some protist taxa can be found in only a few, distantly related, host species. Thus, the long history of co-diversification in this symbiosis has been complicated by lineage-specific loss of symbionts, transfer of symbionts from one host lineage to another, and by independent diversification of the symbionts relative to their hosts. This review aims to introduce the biology of this important symbiosis and serve as a gateway to the diversity and systematics literature for both termites and protists. A searchable database with all termite-protist occurrence records and taxonomic references is provided as a supplementary file to encourage and facilitate new research in this field.