Elimination of Y chromosome-bearing spermatids during spermiogenesis in an autosomal <i>sex-ratio</i> mutant of <i>Drosophila simulans</i>

Abstract

Sex ratio distortion, which is commonly abbreviated as sex-ratio, has been studied in many Drosophila species, but the mechanism remains largely unknown. Here, we report on the sex-ratio mutant of D. simulans named excess of females (exf). The third chromosomal recessive mutation results in a sex ratio of approximately 0.2 or less (males/total). Cytological observation demonstrated that meiosis appeared to be completed normally, but that most Y chromosome-bearing nuclei failed to elongate during spermiogenesis, as revealed by fluorescence in situ hybridization using sex chromosome-specific probes. These aberrant nuclei contained membranous inclusions as revealed by electron microscopic analysis. Most of the aberrant exf spermatids failed to individualize and mature, suggesting that a later stage of spermiogenesis is involved in prevention of production of sperm with abnormal morphology. On the one hand, in exf seminal vesicles, sperm nuclei with a length of 5-8.5 μm were occasionally observed, in addition to those with wild-type sperm dimensions, that is, a length of approximately 10 μm. Thus, spermatids with less severe nuclear defects can escape elimination and be released into the seminal vesicles as mature sperm. Furthermore, we constructed His2AvD-GFP and ProtamineB-eGFP transgenic lines in D. simulans, and examined the processes involved in replacement of chromatin proteins over a time course, according to nuclear morphology. We found that both normal and abnormal sperm heads demonstrated equal chromatin replacement during late spermiogenesis. Our results suggest that exf belongs to a unique class of meiotic drive systems in that (1) intranuclear membranous inclusions cause failure of nuclear shaping of Y-bearing spermatids without affecting the histone-protamine transition, and (2) a portion of the aberrant spermatids differentiate into mature sperm; these are transferred to and stored by females.

Essential and recurrent roles for hairpin RNAs in silencing de novo sex chromosome conflict in Drosophila simulans

“…Such defects remain during individualization, where control spermatid nuclei are needle shaped and stain uniformly with DAPI, but roughly half of nmy mutant nuclei are abnormally shaped and lack uniform chromatin condensation. DNA FISH experiments labeling the sex chromosomes indicate that the affected nmy nuclei correspond to Y-bearing sperm ( S2 Fig ), consistent with previous reports [ 25 ]. By contrast, tmy mutants exhibit severe defects in meiotic nuclei.…”

“…Previous studies of nmy mutant alleles used heterogeneous genetic backgrounds [ 16 , 18 , 25 ]. Moreover, these nmy alleles are unmarked, rendering genetic crosses technically challenging, especially as this species lacks balancer chromosomes.…”

The hpRNA/RNAi Pathway Is Essential to Resolve Intragenomic Conflict in the Drosophila Male Germline

“…We examined three nmy alleles SR12-2-7, nmy[5-2] and exf (Tao et al, 2007b; Yasuno et al, 2013), and validated that homozygous males are fertile, but sire extremely female-biased broods at 18°C (Figure 1A–B). Thus, loss of this individual locus is sufficient to lead to near population extinction, and these stocks require constant attention for maintenance since balancer chromosomes are not available in D. simulans .…”

Sex Chromosome Drive

“…Dox and MDox appear to be transcribed as RNAs that are noncoding or that have a very limited coding potential. Dox does not seem to cause meiotic defects, but rather postmeiotic failure of nuclear condensation in the Y-bearing sperm (Tao et al 2007b;Yasuno et al 2013). Remarkably, Dox gave rise through retrotransposition to an autosomal suppressor, Not much yang (Nmy).…”