Tetrahymena are a genus of free-living ciliate protozoa found in freshwater ponds. Each is 1 of 7 possible sexes.
Tetrahymena have 2 nuclei: a regular soma nucleus, which has a gene for its own sex, and a 2nd germline nucleus for reproduction. The germline nucleus carries incomplete versions of all 7 sex genes.
During growth, the somatic nucleus handles all gene transcription while the germline nucleus stays silent.
When it comes time to mate, the germline nucleus is sorted out until a single mating sex is determined. To maximize genetic diversity, Tetrahymena mate with another of a different sex. Offspring may be any 1 of the 7 sexes.