Experimental evidence for effective heterokaryon formation in the ciliate Euplotes crassus

Abstract Morphological mutant doublets of Euplotes crassus containing two nuclear complexes and expressing a definite mating type were conjugated with normal singlets of a complementary mating type. After conjugation a class of clones was obtained each of which consisted of doublets having both a capability to form intraclonal pairs and no restriction in the mating spectrum. Doublets were isolated from this class of clones and were subsequently allowed to split into their component halves (singlets). The two singlet sib‐lines deriving by vegetative growth of a split doublet were tested for mating type and were usually found to be different. Also, different allelic combinations were consistently shown to exist at the mt locus, i. e., the locus determining the mating type. Our data support the hypothesis that these doublets which gave rise to two different singlet sib‐lines should have contained nuclear complexes with different, active mt alleles. Thus these doublets capable of expressing two mating types at the same time can be considered to be effective heterokaryons.