A huge diversity of metopids (Ciliophora, Armophorea) in soil from the Murray River floodplain, Australia. III. Morphology, ontogenesis and conjugation of Metopus boletus nov. spec., with implications for the phylogeny of the SAL supercluster.

The morphology, ontogenesis, conjugation, and phylogenetic position of Metopus boletus nov. spec. were studied using live observation, various silver impregnation methods, scanning electron microscopy, morphometry, and the 18S rRNA gene sequence. The new species is outstanding in having a mushroom-like appearance; a globular to broadly ellipsoid macronucleus in anterior body half; 5-10 elongated caudal cilia; 4-6 dikinetids curved rightwards in the anterior portion of the first postoral kinety; and an adoral zone composed of an average of 28 small polykinetids. Ontogenesis of M. boletus follows the metopid mode and the species-specific vegetative morphology is obtained after division. Its conjugation is temporary, isogamic and the partners unite ventral-to-dorsal, forming strongly arched to almost rod-like pairs, which indicates a heteropolar arrangement. There are only two maturation divisions and a single synkaryon division in exconjugants. The conjugation data corroborate a sister group relationship of the classes Armophorea and Litostomatea within the SAL (Spirotrichea + Armophorea + Litostomatea) supercluster in that the partners unite ventral-to-dorsal and the main body axes are antiparallel. On the other hand, the last common ancestor of the spirotricheans very likely had a ventral-to-ventral and homopolar conjugation mode with the main body axes oriented in parallel.