A pericentric inversion in Neurospora, with unstable duplication progeny.

URING a study of the arginine-1 locus in Neurospora crmsa, one mutant strain (H4250) was found to produce, on crossing to wild type, an unusual class of progeny. These progeny initially show very slow “inhibited” growth with a characteristic spidery morphology, and are darkly pigmented on complete medium; they are therefore called Dark Agars. After several days, they escape from the inhibition and grow like wild type; many of the resulting cultures behave as if both mating-type alleles are present. The present study shows that the Dark Agars are viable duplications produced by crossing over in a pericentric inversion. The initial inhibition appears to be due to heterozygosity for the mating-type locus, which is included in the duplication; the cultures formed after escape from inhibition are mixtures of somatic derivatives that are homozygous or hemizygous for mating type. Somatic segregation for other markers in the duplicated region is also found. The H4250 inversion provides favorable material for studying somatic segregation, because the duplilcation covers 50 map units in a well marked region, and because there is strong selection for the products of somatic segregation, which can be recovered and purified by plating. Short duplications in Neurospora have been obtained previously by ST. LAWRENCE (1959), BARRY (1960, 1961) and DE SERRES (1957), among the progeny of insertional translocations; markers in these duplications do not appear to segregate somatically. Recently, however, PERKINS (1966) has found a long insertional translocation in Neurospora which produces Dark Agar duplication progeny, which look very much like those produced by the H4250 inversion; his duplication strains also segregate somatically for mating type. Furthermore, BAINBRIDGE and ROPER ( 1966) have described a translocation in Aspergillus n i d u h s which produces unstable duplications, and a brief report by NGA and ROPER (1966) describes a different unstable duplication in the same organism. The present paper is primarily concerned with proof and characterization of the inversion, location of the break points, description of the duplication, and evidence suggesting that the initial Dark Agar phenotype is due to mating-type heterozygosity; preliminary results bearing on the mechanism of somatic segregation are also given. A preliminary account has been published (NEWMEYER 1965).