The free-living ciliates of the order Metopida Jankowski, 1980 are pivotal players in the microbial food web of the sulfuretum, acting as hosts to prokaryotic endo- and ectosymbionts. They are also of interest in the study of the function and evolution of their mitochondrion-related organelle, the hydrogenosome. The taxonomy and phylogeny of this group remains confused, due, in large part, to the fact that most of its taxa have not been characterized by modern methods including molecular sequencing. In this report we provide morphologic and molecular characterization of seven taxa from the poorly-known resurrected genus Urostomides obtained in the course of broad geographic sampling. Foissner (2016) established the family Apometopidae to include Apometopus (a junior synonym of Urostomides) and Cirranter Jankowski, 1964. These two genera differ from all other metopid genera in having a four-rowed perizonal ciliary stripe, the only currently recognizable morphologic synapomorphy for the family. The members of Urostomides show remarkable morphologic diversity. The genus has a broad geographic distribution, occurring on six continents. Urostomides species form a strongly supported clade in phylogenetic analyses. Relationships within the genus itself are less clearly resolved. The diagnoses of Apometopidae and Urostomides are emended.

Boise State University Department of Biological Sciences Boise ID 83725 1515 USA

Department of Zoology Faculty of Science Charles University Prague Czech Republic

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