Sex determination and sex differentiation in Isospora (Toxoplasma) gondii

Although sexual differentiation in the life-cycle of Eimeriina (Coccidia) becomes morphologically evident during development of the gamonts, there are indications in the literature that pregametogonic schizonts might already be sexually differentiated. On the other hand development of both sexes from cloned postzygotic, presumably haploid stages has been reported. In the present experiments we cloned successive stages of the coccidian representative Isopora (Toxoplasma) gondii to determine the time and stage of onset of sexual differentiation in the life-cycle. Clones of a single proliferative form, of single cysts and of a single cystozoite were made under visual control using a de Fonbrune micromanipulator. Seronegative conventional (CV) and specific pathogen-free (SPF) cats were orally infected, each with brain tissue cysts from a single clone. Periods of oocyst excretion by CV cats were recorded; SPF cats were autopsied at day 5 or 6 after infection (p.i.) and their small intestines were examined histologically. Infectivity of shed oocysts, after sporulation, was determined by mouse inoculation. All clones produced infective oocysts with oocyst shedding for two to nine days between day 4 and 14 p.i., and clones of all three stages produced both types of gamonts and gametes. The present results demonstrate: (i) that all stages of I. (T.) gondii in the intermediate host (as well as the free-living stages) are bisexual, although presumably haploid; (ii) that sexual differentiation is not genetically determined, if the first sporulation fission is indeed meiotic, but is a phenotypic change induced by some final host factor(s). This is the first report of the development of both sexes from a single cyst and cystozoite of I. (T.) gondii.