The often overlooked coming out of ciliates: biological and experimental benefits from accepting genetically identical conspecifics as sexual partners

Ciliates usually manifest sex in the form of conjugation, a unique phenomenon in which cells temporarily unite two by two in mating pairs to perform a mutual exchange of gamete-nuclei derived from meiotic products of their germinal micronucleus. The native view of conjugation as a spontaneous manifestation associated with environmental famine conditions was eventually denied by the milestone Sonneborn’s finding (PNAS, 1937) that the most popular ciliate, Paramecium, actually controls conjugation through a genetic mechanism of mating types. Being only two in the Paramecium species studied by Sonneborn, these mating types were functionally equated to ‘male’ and ‘female’ sexes. And, as a consequence of this equation, conjugation was since thought of as a phenomenon committed to involve, as a rule, genetically distinct cells representing two ‘complementary’ mating types. However, this is a wrong tenet adverse the evidence that many ciliates conjugate with no discrimination between sex identity and diversity. And from this no discrimination both the ciliate biology and the students of ciliate biology draw benefit. The ciliate biology, because the homo-sexual pairs (yet ineffective to reshuffle the species gene pool) multiply the opportunity for cells to practice conjugation which, in every case, determines the initiation of a new life cycle and the replacement of the cell ‘old’ transcriptionally active somatic (macronuclear) genome with a completely new one generated from the permanently ‘young’ transcriptionally inert germinal (micronuclear) genome. The students of ciliate biology, because homo-sexual pairs form without requiring physical interactions between sexually/genetically different cells. They form as well in cultures of cells of the same identity previously suspended with filtrates from cultures of conspecific cells of different identity. Which immediately identifies species that interact sexually via water-borne mating signals (pheromones), and greatly facilitates the isolation and function-structure characterization of these signals directly from cell-culture filtrates