Sex-linked expression of a sexually selected trait in the stalk-eyed fly, Cyrtodiopsis dalmanni.
Recent theoretical and empirical work has suggested that the X chromosome may play a special role in the evolution of sexually dimorphic traits. We tested this idea by quantifying sex chromosome influence on male relative eyespan, a dramatically sexually selected trait in the stalk-eyed fly, Cyrtodiopsis dalmanni. After 31 generations of artificial sexual selection on eyespan:body length ratio, we reciprocally crossed high- with low-line flies and found no evidence for maternal effects; the relative eyespan of F1 females from high- and low-line dams did not differ. However, F1 male progeny from high-line dams had longer relative eyespan than male progeny from low-line dams, indicating X-linkage. Comparison of progeny from a backcross involving reciprocal F1 males and control line females confirmed X-linked inheritance and indicated no effect of the Y chromosome on relative eyespan. We estimated that the X chromosome accounts for 25% (SE = 6%) of the change in selected lines, using the average difference between reciprocal F1 males divided by the difference between parental males, or 34%, using estimates of the number of effective factors obtained from reciprocal crosses between a high and low line. These estimates exceed the relative size of the X in the diploid genome of a male, 11.9% (SE = 0.3%), as measured from mitotic chromosome lengths. However, they match expectations if X-linked genes in males exhibit dosage compensation by twofold hyperactivation, as has been observed in other flies. Therefore, sex-linked expression of relative eyespan is likely to be commensurate with the size of the X chromosome in this dramatically dimorphic species.
Department of Biology, University of Maryland, College Park 20742, USA. lw137@umail.umd.edu
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Wolfenbarger
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Wilkinson
D000818:Animals; D004175:Diptera / Q000033:anatomy & histology* / Q000235:genetics*; D005123:Eye / Q000033:anatomy & histology*; D005260:Female; D016678:Genome; D008297:Male; D012641:Selection, Genetic; D012726:Sexual Behavior, Animal; D014960:X Chromosome / Q000235:genetics*; D014998:Y Chromosome / Q000235:genetics*