The symbiotic intestinal ciliates and the evolution of their hosts

The evolution of sophisticated differentiations of the gastro-intestinal tract enabled herbivorous mammals to digest dietarycellulose and hemicellulose with the aid of a complex anaerobic microbiota. Distinctive symbiotic ciliates, which are uniqueto this habitat, are the largest representatives of this microbial community. Analyses of a total of 484 different 18S rRNAgenes show that extremely complex, but related ciliate communities can occur in the rumen of cattle, sheep, goats and red deer(301 sequences). The communities in the hindgut of equids (Equus caballus, Equus quagga), and elephants (Elephas maximus,Loxodonta africanus; 162 sequences), which are clearly distinct from the ruminant ciliate biota, exhibit a much higher diversitythan anticipated on the basis of their morphology. All these ciliates from the gastro-intestinal tract constitute a monophyleticgroup, which consists of two major taxa, i.e. Vestibuliferida and Entodiniomorphida. The ciliates from the evolutionarily olderhindgut fermenters exhibit a clustering that is specific for higher taxa of their hosts, as extant species of horse and zebra onthe one hand, and Africa and Indian elephant on the other hand, share related ciliates. The evolutionary younger ruminantsaltogether share the various entodiniomorphs and the vestibuliferids from ruminants.