A Paramecium tetraurelia mutant that has long autogamy immaturity period and short clonal life span.

We have isolated an unprecedented mutant of Paramecium tetraurelia that has a long immaturity period until autogamy. This mutant stock, d4-RK, screened for 0% autogamy at the age of 27 fissions, began to undergo autogamy around the age of 50 fissions in some clones and underwent autogamy scarcely even after the age of 100 fissions in others. d4-RK expressed its mutant phenotype at 25 degrees C, but resembled the wild-type phenotype at 32 degrees C. Genetic analyses indicated that a single recessive gene, named rie (remote immaturity exit), was responsible for the mutant phenotype. This is the first report to show a gene that elongates the time to sexual maturation in unicellular organisms. The clonal life span was shorter and fission rate was lower in the rie mutant than in the wild-type, both at 25 degrees C and 32 degrees C. Even in the fourth autogamous generation following the third backcross to the wild-type, the progeny with the elongated autogamy immaturity period still had a short clonal life span and low fission rate, while those with the wild-type phenotype in autogamy immaturity period showed the wild-type phenotypes in clonal life span and fission rate, too.