SEGREGATION-DISTORTION AND REGULARLY NONFUNCTIONAL PRODUCTS OF SPERMATOGENESIS IN DROSOPHILA MELANOGASTER.

Abstract

HE meiotic process generally ensures that in any diploid heterozygote the two a1terna:ive homologous elements will be recovered in functional gametes in equal frequencies. Exceptions have been described in a number of organisms and where the inequality is shown to be a consequence of the meiotic divisions, meiotic drive may be said to be operative ( SANDLER and NOVITSKI 1957). Segregation-distortion is a striking case of meiotic drive in spermatogenesis of Drosophila melunogaster ( SANDLER, HIRAIZUMI and I. SANDLER 1959). In males heterozygous for Segregation-Distorter (SD) , a locus proximally located .on the right arm of chromosome 2, the SD-bearing chromosome is recovered considerably more frequently than its homologue. Many stocks now consistently yield a k value of 0 99, k being the proportion of SD-bearing flies in the total recovered progeny. SANDLER et al. showed that the failure of recovery of the SDf chromosome was not associated with zygote lethality. It was established that synapsis was a requirement for SD action and that the phenomenon was not evident in oogenesis; subsequently, a number of other properties have been described (cf. SANDLER and ROSENFELD 1962). Hypotheses of meiotic loss and extra replication (the latter being analogous to that reported by STURTEVANT and DOBZHANSKY (1936) for “sex-ratio” in Drosophila pseudoobscura), were negated by experimental analyses. An explanation in terms of meiotic drive was clearly demanded, but despite the magnitude of the effect of the locus its mode of action has not been understood. SANDLER et al. (1959) presented a formal cytogenetic model and offered some preliminary cytological observations in support of the hypothesis that the SD locus caused a break or misreplication in its synapsed homologue and that ensuing sister chromatid reunion generated a dicentric chromatid and an acentric fragment. The ultimate result would be death of the SDf products of the second meiotic division. This paper is concerned with the description of experiments and observations designed to elucidate the nature of the SD phenomenon. It has been shown that although the genetic consequences of SD are clear and unambiguous, there is no counterpart in meiosis which could account for these effects. in fact, meiosis apThis investigation was supported by grants from the Public Health Seri-ice I iTl GhI 373-05) and the Satronzil