damp wood termite from an early diverging lineage, whose symbionts were first investigated in the 1990s. In 1937, the protist genera Pyrsonympha and Dinenympha were only known to inhabit Reticulitermes . But this was a mystery because the closest relatives of Reticulitermes do not harbour any protists related to Pyrsonympha or Dinenympha . Their discovery in Hodotermopsis , along with several other protists that are otherwise unique to Reticulitermes , solved this mystery. It seems that the ancestor of Reticulitermes underwent a complete faunal replacement, losing the characteristic protists of its rhinotermitid relatives and replacing them with a fauna very similar to that of Hodotermopsis . How exactly this happened is a matter of speculation, but termites occasionally lose their protists as a result of a cold shock or other acute stressors (note that Reticulitermes inhabit colder areas than other termite lineages). If the ancestors of Reticulitermes experienced this and then had a skirmish with a Hodotermopsis colony, the protists could have been transferred during the necrophagy that typically accompanies termite warfare. Regardless of the specific mechanism, the distribution of protists is very difficult to explain except by symbiont transfer. This case adds nuance to the idea of host specificity by demonstrating that while symbionts are largely maintained within a host lineage (e.g. for ~60 million years of Reticulitermes evolution), they can also be transferred from one termite lineage to another. But the question of host specificity is perhaps more appropriately addressed on a finer taxonomic scale, i.e. can a single protist species be found in multiple host species? Here the paradigm seems to have shifted the most as a result of our molecular perspective. Whereas many, if not most, protist morphospecies were previously reported to inhabit multiple host species, gene sequences have revealed significant evolutionary distances between protist species from distinct hosts. Trichonympha agilis , Coronympha octonaria , and Staurojoenina assimilis are perfect examples of this: they were each reported to inhabit multiple host species, but molecular data have demonstrated distinct phylotypes for these species in each of their hosts. Consistent with this view,
reported in the late 19th century. This captured the attention of protozoologists who began to study termite microbiota in more detail and build a classification system for the protists. These early researchers relied solely on light microscopy of live or fixed, stained specimens to study the protists. As more termite species were investigated and the number of described protist species increased, the distribution of protist species across termite species emerged as an intriguing puzzle. One question articulated in 1937 by Harold Kirby of the University of California at Berkeley, a major figure in termite protist research, still endures today: what is the degree of host–symbiont specificity? In Kirby’s day, as today, the short answer was “it varies”. It is clear that some protist lineages are restricted to certain termite lineages while others have broader distributions across multiple termite lineages. For example, it was known by 1937, and still holds today, that the protist genus Pseudotrichonympha can only be found in members of the termite family Rhinotermitidae, while the protist genus Trichonympha is present (albeit patchily) in most termite families and Cryptocercus . But our understanding of other key distributions has changed dramatically with the application of molecular approaches. For example, the protist genus Spirotrichonympha has been reported to inhabit several distinct lineages of termites, on the basis of morphology. But after gene sequences were determined, it emerged that Spirotrichonympha actually comprises multiple distinct protist lineages that happen to share a similar morphotype. Moreover, these ‘ Spirotrichonympha ’ lineages were each confined to a distinct host lineage. Spirotrichonympha from Reticulitermes hosts will keep the name because they have taxonomic priority, but ‘ Spirotrichonympha ’ species from Coptotermes and Heterotermes are now Cononympha , and the ‘ Spirotrichonympha ’ from Paraneotermes is now Cuppa . This case highlights the difficulty of determining relatedness among termite-associated protists by morphology alone, but more importantly, it increases the degree of host specificity that we can observe in this symbiosis. Another discovery that impacted our view of host specificity was the protist fauna of Hodotermopsis sjostedti, a
93 Microbiology Today October 2022 | microbiologysociety.org
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