OCCURRENCE OF REPEAT INDUCED POINT MUTATION IN LONG SEGMENTAL DUPLICATIONS OF NEUROSPORA

Previous studies of repeat induced point mutation (RIP) have typically involved gene-size duplications resulting from insertion of transform ing DNA at ectopic chromosomal positions. To ascertain whether genes i n larger duplications are subject to RIP, progeny were examined from c rosses heterozygous for long segmental duplications obtained using ins ertional or quasiterminal translocations. Of 17 distinct mutations fro m crossing 11 different duplications, 13 mapped within the segment tha t was duplicated in the parent, one was closely linked, and three were unlinked. Half of the mutations in duplicated segments were at previo usly unknown loci. The mutations were recessive and were expressed bot h in haploid and in duplication progeny from Duplication x Normal, sug gesting that both copies of the wildtype gene had undergone RIP. Seven transition mutations characteristic of RIP were found in 395 base pai rs (bp) examined in one ro-11 allele from these crosses and three were found in similar to 750 bp of another. A single chain-terminating C t o T mutation was found in 800 bp of arg-6. RIP is thus responsible. Th ese results are consistent with the idea that the impaired fertility t hat is characteristic of segmental duplications is due to inactivation by RIP of genes needed for progression through the sexual cycle.