SYNOPSIS. Thirty‐one stocks of a marine ciliate, Euplotes minuta Yocom, collected from different localities, can be grouped in seven mating types. True pairs are formed only in mixtures of stocks belonging to different mating types. No selfing pairs or intraclonal conjugation have ever been observed. Synclonal inheritance of mating types is the rule, although about 10% of pairs show clonal inheritance. The latter can be explained by assuming syncaryon formation through cytogamy or through caryogamy of pronuclei derived from different products of meiosis. Mating type determination is due to 7 alleles at the single locus mt. There is complete dominance among the 7 alleles which can be orderly seriated, as shown in Table 3, according to their dominance relationship. The 5 stocks, and only these 5, of mating type VII have the autogamy trait. The mortality rate in F1 and F2 is very low–a maximum of 10%; however, the F2'S obtained by autogamy from F1 progenies in which mating type VII is involved have a very high mortality rate. The two facts (high mortality rate in F2 and strict association of autogamy trait with mating type VII in natural populations) have been considered as evidences of a probable isolation mechanism existing between mating type VII and the other 6 mating types. Thus, the 7 mating types have been assigned to the same syngen only tentatively.
Sexual reproduction i n Ciliates occurs through pair formation (conjugation) between cells of complementary mating types. In many species, after complementary cells are brought together, a waiting period ensues, which lasts up to the initial visible ciliary interaction leading to cell pairing. An exchange of information, mediated by soluble factors (gamones) takes place during the waiting period of Oxytricha bifaria. This hypotrich ciliate possesses a multiple mating type breeding system, and conjugation may thus occur between cells belonging to any two mating types. The excreted gamones being mating type specific, differ from each other. Any cell pretreated with any one of the complementary gamones starts pairing immediately after mixing. Thus cell interaction does not require heterologous cell to cell contact during the waiting period. A simple readily made apparatus to detect the exchange of information through soluble factors, is described.
SYNOPSIS. Mating‐type dependent cell pairing in Oxytricha hymenostoma was studied by both light autoradiography and transmission electron microscopy. The process results in the macronuclear bodies of the 2 partners fusing completely with each other 2 by 2. In each fusion the 2 chromatin masses, easily distinguishable from each other for the first 6 h of the pairing, progressively intermingled and underwent a sort of rearrangement so that they eventually acquired a uniform pattern. The possible mechanisms regulating macronuclear fusion and DNA amount in the daughter cells are discussed.
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