Recombination and disjunction in female germ cells of Drosophila depend onthe germline activity of the gene Sex-lethal

Gametogenesis in males and females differs in many ways. An important diffe rence in Drosophila is that recombination between homologous chromosomes oc curs only in female meiosis, Here, me report that this process relies on th e correct functioning of Sex-lethal (Sxl) which is primarily known as the m aster gene in somatic sex determination. Certain alleles of this gene (Sxl( fs)) disrupt the germline, but not the somatic function of Sri and cause an arrest of germ cell development during cystocyte proliferation. Using domi nant suppressor mutations that relieve this early block in Sxl(fs) mutant f emales, we discovered additional requirements of Sxl for normal meiotic dif ferentiation of the oocyte, Females mutant for Sxl(fs) and carrying a suppr essor become fertile, but pairing of homologous chromosomes and formation o f chiasmata is severely perturbed, resulting in an almost complete lack of recombinants and a high incidence of non-disjunction events. Similar result s were obtained when germline expression of mild-type Sxl was compromised b y mutations in virilizer (vir), a positive regulator of Sxl. Ectopic expres sion of a Sri transgene in premeiotic stages of male germline development, on the other hand, is not sufficient to allow recombination to take place, which suggests that Sxl does not have a discriminatory role in this female- specific process. We propose that Sxl performs at least two tasks in oogene sis: an 'early' function in formation of the egg chamber, and a 'late' func tion in progression of the meiotic cell cycle, suggesting that both events are coordinated by a common mechanism.