A breeding analysis of a wild strain (Indian Lake 12) of variety 1, Tetrahymena pyriformis , discloses recessive lethal alleles at two independently assorting loci. Conclusions concerning the cytogenetic behavior of this species have been verified through the scoring of a large number of offspring segregating the lethals. In particular, cytogamy does not occur in these strains with a frequency higher than 2%. if indeed it occurs at all. The finding of lethal heterozygosity in a wild strain suggests that close inbreeding usually does not take place in nature and supports the conclusion that variety 1 of T. pyriformis is an outbreeding species.
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