title: Empirical evidence for son-killing X chromosomes and the operation of SA-zygotic drive.
abstract: BACKGROUND
  Diploid organisms have two copies of all genes, but only one is carried by each haploid gamete and diploid offspring. This causes a fundamental genetic conflict over transmission rate between alternative alleles. Single genes, or gene clusters, only rarely code for the complex phenotypes needed to give them a transmission advantage (drive phenotype). However, all genes on a male's X and Y chromosomes co-segregate, allowing different sex-linked genes to code for different parts of the drive phenotype. Correspondingly, the well-characterized phenomenon of male gametic drive, occurring during haploid gametogenesis, is especially common on sex chromosomes. The new theory of sexually antagonistic zygotic drive of the sex chromosomes (SA-zygotic drive) extends the logic of gametic drive into the diploid phase of the lifecycle, whenever there is competition among siblings or harmful sib-sib mating. The X and Y are predicted to gain a transmission advantage by harming offspring of the sex that does not carry them.
  
  
  RESULTS
  Here we analyzed a mutant X-chromosome in Drosophila simulans that produced an excess of daughters when transmitted from males. We developed a series of tests to differentiate between gametic and SA-zygotic drive, and provide multiple lines of evidence that SA-zygotic drive is responsible for the sex ratio bias. Driving sires produce about 50% more surviving daughters than sons.
  
  
  CONCLUSIONS
  Sex-ratio distortion due to genetic conflict has evolved via gametic drive and maternally transmitted endosymbionts. Our data indicate that sex chromosomes can also drive by harming the non-carrier sex of offspring.
authors.affiliation: Department of Ecology, Evolution and Marine Biology, University of California Santa Barbara, Santa Barbara, California, United States of America. Urban.Friberg@ebc.uu.se
authors.forename: Urban
authors.initials: U
authors.lastname: Friberg
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Stewart
authors.forename: William R
authors.initials: WR
authors.lastname: Rice
mesh_terms: D000465:Algorithms; D000818:Animals; D004171:Diploidy; D005260:Female; D005718:Gametogenesis / Q000235:genetics; D017344:Genes, Insect / Q000235:genetics; D006238:Haploidy; D040582:Inheritance Patterns; D008297:Male; D008957:Models, Genetic; D012641:Selection, Genetic; D012737:Sex Factors; D012744:Sex Ratio; D014960:X Chromosome / Q000235:genetics*; D014998:Y Chromosome / Q000235:genetics*; D015053:Zygote / Q000378:metabolism*