(Alliance, FBgn0002709 )

Females homozygous for mei-218 exhibit reduced levels of meiotic exchange. The residual exchanges are distributed such that the probability of euchromatic exchange becomes more nearly proportional to the polytene-chromosome length (Baker and Carpenter, 1972; Carpenter and Sandler, 1974; Baker and Hall, 1976). This relaxation of the normal constraints on the distribution of euchromatic exchange is clearly demonstrated by the ability of mei-218 females to allow exchange between the normally achiasmate fourth chromosomes (Sandler and Szauter, 1978). mei-218 does not, however, permit exchanges to occur in heterochromatic intervals (Carpenter and Baker, 1982). Ultrastructural analysis of pachytene in mei-218 and mei-2186-7 females demonstrates a reduced number of late recombination nodules (to about 8% of normal), which are distributed in a fashion that parallels the residual exchange events (Carpenter, 1979, 1989). Many of the nodules which are observed are morphologically abnormal. There is also some evidence that early recombination nodules may be either fewer in number or more ephemeral in mei-218 females (Carpenter, 1989). Although mei-218 alleles reduce the frequency of reciprocal meiotic exchange, the absolute frequency of gene conversion at the rosy locus is two-fold elevated relative to wild-type controls (Carpenter, 1982, 1984). Moreover, co-conversion distances are shorter than those recovered from controls or from mei-9 females (Carpenter, 1984). Thus the function of this locus is required for the generation of reciprocal exchanges and not for gene conversion (Carpenter, 1984). The reduced levels of reciprocal exchange which are characteristic of mei-218 females allow for high frequencies of meiotic loss and nondisjunction (see Baker and Hall, 1976). Nondisjunction occurs at the first meiotic division and only nonexchange chromosomes nondisjoin (Carpenter and Sandler, 1974). All assays for an effect of mei-218 on mitotic chromosome behavior in males or on somatic chromosome behavior are negative (Baker and Carpenter, 1972; Baker et al., 1978; Lutken and Baker, 1979; Gatti, 1979).