Sex-ratio drive in Drosophila simulans: variation in segregation ratio of X chromosomes from a natural population.
The sex-ratio trait that exists in a dozen Drosophila species is a case of naturally occurring X chromosome drive that causes males to produce female-biased progeny. Autosomal and Y polymorphism for suppressors are known to cause variation in drive expression, but the X chromosome polymorphism has never been thoroughly investigated. We characterized 41 X chromosomes from a natural population of Drosophila simulans that had been transferred to a suppressor-free genetic background. We found two clear-cut groups of chromosomes, sex-ratio and standard. The sex-ratio X chromosomes differed in their segregation ratio (81-96% females in the progeny), the less powerful drivers being less stable in their expression. A sib analysis, using a moderate driver, indicated that within-X variation in drive expression depended on genetic (autosomal) or epigenetic factors and that the age of the males also affected the trait. The other X chromosomes produced equal or roughly equal sex ratios, but again with significant variation. The continuous pattern of variation observed within both groups suggested that, in addition to a major sex-ratio gene, many X-linked loci of small effect modify the segregation ratio of this chromosome and are maintained in a polymorphic state. This was also supported by the frequency distribution of sex ratios produced by recombinant X chromosomes.
Montchamp-Moreau
Catherine
C
Populations, Génétique et Evolution, CNRS, 91198 Gif sur Yvette Cedex, France. montchamp@pge.cnrs-gif.fr
Cazemajor
Michel
M
D000704:Analysis of Variance; D000818:Animals; D020090:Chromosome Segregation / Q000235:genetics* / Q000502:physiology; D003433:Crosses, Genetic; D004330:Drosophila / Q000235:genetics* / Q000502:physiology; D005260:Female; D008297:Male; D012744:Sex Ratio; D014960:X Chromosome