SUMMARY: Wild-type 3(2), and many mutant and recombinant derivatives of it, are of the (Initial Fertility) type. At an early step in the production of recombinant strains from some of the first derivatives of 3 (2), a variant fertility type arose (: Normal Fertility), and subsequently and segregated within the pedigree of stock cultures. x crosses are about 100-fold less fertile than x or x crosses, but the clearest distinction between and is achieved by crossing with a strain of the previously described (Ultra-Fertile) type, when the difference in fertility approaches 1000-fold.

The strains give rise to strains with a high spontaneous frequency, and the frequency is increased by ultraviolet or X-irradiation but not appreciably by -methyl-'-nitro--nitrosoguanidine. strains do not give rise to variants with a high frequency.

The difference between and is determined by a chromosomal locus near the 9 o'clock position on the linkage map. There is no evidence for the infectious conversion of one type of strain to the other in a mixed culture.

In crosses with an strain, both and strains contribute the whole chromosome to the effective merozygotes, and the strain contributes the fragment, which obligatorily includes the 9 o'clock region. However, whereas in f x crosses there is obligate inheritance of the 9 o'clock region of the genome by all sexually produced progeny, this is not true of x crosses.


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