Mating Types and Mating Type Inheritance in Euplotes minuta Yocom (Ciliata, Hypotrichida)

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.