Orientation disruptor (ord): a recombination defective and disjunction defective meiotic mutant in Drosophila melanogaster

Abstract

The effects of a semidominant autosomal meiotic mutant, orientation disruptor (symbol: ord), located at 2-103.5 on the genetic map and in region 59B-D of the salivary map, have been examined genetically and cytologically. The results are as follows. Crossing over in homozygous females is reduced to about 7% of controls on all chromosomes, with the reduction greatest in distal regions. Crossing over on different chromosomes is independent. Reductional nondisjunction of any given chromosome is increased to about 30% of gametes from homozygous females. The probability of such nondisjunction is the same among exchange and nonexchange tetrads with the exception that a very proximal exchange tends to regularize segregation. Equational nondisjunction of each chromosome is increased to about 10% of gametes in homozygous females; this nondisjunction is independent of exchange. The distributive pairing system is operative in homozygous females. In homozygous males, reductional nondisjunction of each chromosome is increased to about 10%, and equational nondisjunction to about 20%, of all gametes. Cytologically, 2 distinct meiotic divisions occur in spermatocytes of homozygous males. The first division looks normal although occasional univalents are present at prophase I and a few lagging chromosomes are seen at anaphase I. However, sister chromatids of most chromosomes have precociously separated by metaphase II. Possible functions of the ord+ gene are considered.