Adding the heterochromatic YL arm to an X chromosome reduces reproductive fitnesses in Drosophila melanogaster: implications for the evolution of rDNA, heterochromatin, and reproductive isolation.
For X-Y exchange to be of importance in the coevolution of X and Y rDNA, there must be a mechanism to maintain cytologically normal X chromosomes in the face of continual infusions of X.YL chromosomes produced by X-Y exchanges. Replicated populations were founded with different frequencies of isogenic X and X.YL chromosomes. The X.YL chromosome declined in frequency over time in all lines. Relative fitnesses, estimated from chromosome frequency trajectories, were 0.40, 1.01, and 1.0 for X.YL/X.YL, X.YL/X, and X/X females and 0.75 and 1.0 for X.YL/Y and X/Y males, respectively. The equilibrium frequency for the X.YL chromosome due to the balance between X-Y exchange and selection was predicted to be 4-16 x 10(-4). The results strengthen the evidence for the involvement of X-Y exchange in the coevolution of X and Y rDNA arrays. Conditions for the evolution of reproductive isolation by sex-chromosome translocation are much less probable than previously supposed since the X.YL translocation chromosome is at a selective disadvantage to cytologically normal X chromosomes. Additional heterochromatin was not neutral but was only deleterious beyond a threshold, as one dose of the heterochromatic XL arm did not reduce female reproductive fitness, but two doses did.
Frankham
R
R
School of Biological Sciences, Macquarie University, Sydney, New South Wales, Australia.
D000818:Animals; D005075:Biological Evolution; D004275:DNA, Ribosomal / Q000235:genetics*; D004276:DNA, Satellite / Q000235:genetics; D004331:Drosophila melanogaster / Q000235:genetics*; D005260:Female; D005787:Gene Frequency; D006570:Heterochromatin; D006579:Heterozygote; D006720:Homozygote; D008297:Male; D012098:Reproduction / Q000235:genetics; D014178:Translocation, Genetic; D014960:X Chromosome; D014998:Y Chromosome
D004275:DNA, Ribosomal; D004276:DNA, Satellite; D006570:Heterochromatin