The guts of lower termites are inhabited by host-specific consortia of cellulose-digesting flagellate protists. In this first investigation of the symbionts of the family Serritermitidae, we found that Glossotermes oculatus and Serritermes serrifer each harbor similar parabasalid morphotypes: large Pseudotrichonympha-like cells, medium-sized Leptospironympha-like cells with spiraled bands of flagella, and small Hexamastix-like cells; oxymonadid flagellates were absent. Despite their morphological resemblance to Pseudotrichonympha and Leptospironympha, a SSU rRNA-based phylogenetic analysis identified the two larger, trichonymphid flagellates as deep-branching sister groups of Teranymphidae, with Leptospironympha sp. (the only spirotrichosomid with sequence data) in a moderately supported basal position. Only the Hexamastix-like flagellates are closely related to trichomonadid flagellates from Rhinotermitidae. The presence of two deep-branching lineages of trichonymphid flagellates in Serritermitidae and the absence of all taxa characteristic of the ancestral rhinotermitids underscores that the flagellate assemblages in the hindguts of lower termites were shaped not only by a progressive loss of flagellates during vertical inheritance but also by occasional transfaunation events, where flagellates were transferred horizontally between members of different termite families. In addition to the molecular phylogenetic analyses, we present a detailed morphological characterization of the new spirotrichosomid genus Heliconympha using light and electron microscopy.

Evolutionary Biology Institute for Biology Zoology Freie Universität Berlin Berlin 14195 Germany

Faculty of Forestry and Wood Sciences Czech University of Life Sciences Prague Prague 6 Suchdol 16500 Czech Republic

Insect Gut Microbiology and Symbiosis Group Max Planck Institute for Terrestrial Microbiology Marburg 35043 Germany

Insect Gut Microbiology and Symbiosis Group Max Planck Institute for Terrestrial Microbiology Marburg 35043 Germany Department of Organismal Biology Uppsala University Uppsala 75236 Sweden

Laboratory of Experimental and Comparative Ethology EA 4443 Université Paris 13 Sorbonne Paris Cité Villetaneuse 93430 France Institute of Research for Development U 242 Sorbonne Universités Institute of Ecology and Environmental Sciences of Paris Bondy 93143 France

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