Nonhomologous chromosome pairing in female Drosophila. Before or after exchange?1

Reanalysis of certain data published earlier showed that the meiotic loss of the Y chromosome in the female meiosis of Drosophila melanogaster was slightly but significantly lower in those meioses where the nonhomologous pairing partner of the Y chromosome had undertaken an exchange, thus preventing nonrandom heterologous segregation, than in those meioses where exchange had not taken place, and nonrandom heterologous segregation of Y and an autosome occurred. This observation is inconsistent with the distributive pairing hypothesis, i.e. the view that heterologous pairing occurs as a de novo phenomenon after exchange. The result supports the alternative hypothesis, i.e. that heterologous pairing is initiated before exchange but can involve the segregation of noncrossover chromosomes only. Thus, heterologous pairing becomes segregationally effective after exchange.