appreciated. Another is lineage-specific loss of symbionts, which is certainly common and has a clear mechanism related to the establishment of new termite colonies. This is the cause of Trichonympha ’s patchy distribution across host lineages. Yet another is speciation of protists within a host lineage, which has been detected in Holomastigotoides symbionts of Rhinotermitidae and may also be more common than currently appreciated. In this way, coevolution in the termite–protist symbiosis resembles the coevolution of genes and organisms: their fundamentally parallel phylogenies are made incongruent by the same processes of gene transfer, lineage-specific loss, and independent diversification (gene duplication). Further reading Brugerolle G, Radek R. Symbiotic Protozoa of Termites. In: König H, Varma A (eds). Intestinal Microorganisms of Soil Invertebrates (Vol. 6). Berlin, Heidelberg: Springer-Verlag;2006. pp. 243–269.
every newly investigated termite species seems to yield new sequence types for its protists (the many examples range across multiple termite-associated protist lineages). However, ‘one protist, one termite’ may not be the final word. Two species of Zootermopsis each harbour the same nine protist species, as suggested by morphological observations and confirmed by molecular data. Nevertheless, the level of host specificity in termite-associated protists is higher than early researchers imagined. This further suggests that the number of termite-associated protist species has been drastically underestimated. Today, the longer answer to Kirby’s question is that the degree of host specificity is very high, owing to vertical inheritance and co-diversification, but this fact is obscured by other evolutionary processes. One of these, as we have seen between Hodotermopsis and Reticulitermes , is symbiont transfer. This is probably rare, given the distribution of protists across hosts, but it may be more common than is currently
94 Microbiology Today October 2022 | microbiologysociety.org
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