CYTOGENETICS OF AN INTRACHROMOSOMAL TRANSPOSITION IN NEUROSPORA

Knowledge of intrachromosomal transpositions has until now been primar ily cytological and has been limited to Drosophila and to humans, in b oth of which segmental shifts can be recognized by altered banding pat terns. There has been little genetic information. In this study, we de scribe the genetic and cytogenetic properties of a transposition in Ne urospora crassa. In Tp(1R --> IL)T54M94, a 20 map unit segment of link age group I has been excised from its normal position and inserted nea r the centromere in the opposite arm, in inverted order. In crosses he terozygous for the transposition, about one-fifth of surviving progeny are duplications carrying the transposed segment in both positions. T hese result from crossing over in the interstitial region. There is no corresponding class of progeny duplicated for the interstitial segmen t. The duplication strains are barren in test crosses. A complementary deficiency class is represented by unpigmented, inviable ascospores. Extent of the duplication was determined by duplication-coverage tests . Orientation of the transposed was determined using Tp x Tp crosses h eterozygous for markers inside and outside the transposed segment, and position of the insertion relative to the centromere was established using quasi-ordered half-tetrads from crosses x Spore killer. Quelling was observed in the primary transformants that were used to introduce a critical marker into the transposed segment by repeat-induced point mutation (RIP).