Coevolution between Parabasalia protist symbionts and Neoisoptera hosts.

Protist-dependent termites participate in an obligate symbiosis with the community of protists in their hindgut. The termite host and protist symbionts are interdependent, with the host relying on their symbionts to digest the cellulose in their wood-based diet. Cospeciation is commonly observed between hosts and their symbiotic protists due to the strict vertical inheritance of symbionts via a process called proctodeal trophallaxis (anus-to-mouth feeding). While codiversification is an expected pattern, not all protist lineages follow this trend and must be investigated on a case-by-case basis. Protist hindgut communities were characterized from termite hosts of the major lineage Neoisoptera (excluding Termitidae, which lack symbiotic protists) in order to observe the differential pattern of cospeciation between protist lineages and host. 18S sequencing was performed on isolated protist cells from the phylum Parabasalia, specifically the genera: Pseudotrichonympha, Holomastigotoides, Cononympha, and Cthulhu and used to infer their phylogenies, evaluate their presence/absence across Neoisoptera hosts, and assess their congruency with the host tree. Pseudotrichonympha is observed to be present in almost all investigated termites and displays a strong pattern of codiversification with hosts, all having a single, unique species present (except one host with two). Holomastigotoides and Cononympha are missing in many host species and subfamilies and are generally found to have 2 species per host (sometimes 3-4 Holomastigotoides species). Cthulhu was present at the lowest frequency which may suggest it is not host specific. This investigation highlights variability of termite and protist coevolution, even when examining symbionts from the same host lineage.