MATING SYSTEM OF A HOMOTHALLIC FISSION YEAST

Exposed to iodine vapors, colonies of a homothallic strain of Schizosaccharomyces pombe were of two classes: P, with many black streaks, and d, with scarcely any. Contiguous P and d colonies, but not contiguous P colonies nor contiguous d colonies, gave the iodine junction reaction, a black line along the common boundary of two colonies. Neither class could be purified. On replating, a P colony gave rise to a P plate, which contained mostly P but also d colonies; a d colony gave rise to a d plate, which contained mostly d but also P colonies. The P/d colony ratio of a fresh isolate (if isolated as a P colony) was very high or (if isolated as a d conoly) very low. It fell, if initially high, or rose, if initially low, on subsequent replatings of the same isolate. Maintained for many generations, an isolate attained a fairly constant P/d colony ratio that was less than unity. Tetrad analysis showed 2:2 segregation of the classes. We conclude that a homothallic clone is a mixture of two types of cells: P, which gives rise to a P colony, and d, to a d colony. The two types are sexually complementary and interconvertible. The rate of intercoversion of P to d exceeds that of d to P by a factor of about 2. © 1979 Springer-Verlag.