As in all ciliates, T.
thermophila's sexual phase is conjugation, a temporary union of two cells
in which there is genetic exchange and macronuclear replacement. For
conjugation to occur, cells must be sexually mature and of complementary mating
type. Following macronuclear development at conjugation, cells are sexually
immature and incapable of mating. The immaturity period in inbred strains is
typically 40-60 fissions after conjugation, but sexual descendants of
wild-caught cells may have immaturity periods of >100 fissions. An average
of 23.2% (N=5476) of wild-caught T. thermophila cells were sexually
immature, indicating recent conjugation (such cells were identified as T.
thermophila as based on their immobilization by antisera directed at the
surface immobilization antigen). The finding of immature cells in natural
populations indicates that conjugation indeed occurs in nature and that it
occurs frequently.
Cells identified as immature eventually reach maturity, and in the vast majority of cases, matings typically yield >90% viable progeny. The high fertility of genomic exclusion crosses (a type of conjugation which results in genome-wide homozygosis) indicates the relative absence of recessive lethals. "Senescent" cells with reduced or zero fertility, while relatively common among inbred strains, are rarely observed among wild-caught cells.