Does the common sexually transmitted parasite Trichomonas vaginalis have sex?

Introduction The parasite Trichomonas vaginalis is a haploid, flagellated, eukaryotic microbe that adheres to the human urogenital tract and causes the most common sexually transmitted parasitic infection, trichomoniasis ("trich"), with around 250 million new cases reported annually worldwide and about 7 million cases in the United States [1]. Previously considered a female “nuisance disease,” T. vaginalis has now been associated with adverse pregnancy outcomes such as preterm delivery, low birth weight, increased risk of HIV infection, and cervical and prostate cancers [2]. In contrast to other parasitic protists that can encyst (e.g., Giardia), T. vaginalis appears to have only one morphological form, a sexually transmitted, mitotically dividing trophozoite. Besides its important role as the causative agent of trich, the parasite is of interest due to its unusually large genome size (around 160 Mb), of which 65% is made up of families of transposable elements (TEs), members of which are highly similar to each other [3]. The nature and size of the T. vaginalis genome raises questions about its evolution and, in particular, how this supposedly asexually reproducing organism survives the deleterious effects of so many active TE families. Indeed, several lines of evidence described below suggest that T. vaginalis may engage in genetic exchange or has done so in its recent evolutionary past. Elucidating a possible sexual cycle in T. vaginalis is crucial not only for learning about its biology and parasitism (e.g., virulence and spread of drug resistance), but also to provide generalizable models for the evolution of sex in other parasites.