Meiotic Nondisjunction and Aneuploids in Intersyngenic Hybrids of PARAMECIUM CAUDATUM.

Artificially induced intersyngenic crosses in Paramecium caudatum can produce viable and fertile hybrids. When F(1) hybrids of double E mating type (Mt(1)/Mt(3) or Mt(12)/Mt(3)) were crossed with mating type O (mt/mt), aberrant segregants of double E and single O type were produced. This segregation was not explained by ordinary equal or unequal crossing over. Breeding analyses of these segregants by using linkage between Mt and cnrA (a behavioral mutant) revealed that they were produced by meiotic nondisjunction of bivalent chromosomes carrying Mt genes, and thus the double E and single O segregants were aneuploids: trisomics ( Mt(1)/Mt(3)/mt or Mt( 12)/Mt(3)/mt) and monosomics (mt), respectively. An aberrant segregant was also obtained for another locus, tnd 2, independent of both Mt and cnrA, suggesting the occurrence of meiotic nondisjunction throughout hybrid genomes. These aneuploids will be useful for genetic study in this species. The occurrence of meiotic nondisjunction in the intersyngenic hybrids also suggests that syngens of P. caudatum have been reproductively isolated for long enough to develop chromosomal incompatibility in their meiotic process.