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14 Dosage Compensation and the Sex Chromosomes Calvin Bridges and Thomas Hunt Morgan discovered the existence of dosage differences on the X chromosome of fruit flies when comparing the allele of white eyes called eosin to that of the allele of white eyes calledapricot.Theycalledthephenomenonbicolorism,butdidnotmake a generalization about it. In a stock of eosin flies, females had a darker eye color than males. In a stock of apricot flies, the eye color of the male and the female was the same. Eosin arose in a bottle of white-eyed flies asasolitarymalefly.Itwasinterpretedasapartialreversemutationfrom white to eosin. About a decade later, in 1926, Curt Stern (1902–1981) discovered a mutation called bobbed bristles. It was the first genetic character found on the Y chromosome in fruit flies that was not associated with fertility.1 As it turns out, the shorter and slightly elevated bristles are associated with a gene on both the X and the Y chromosome. This made the gene behave like an autosomal recessive. Normal males and females had two doses, but XO males had a single dose and XXY fertile females had a triple dose. Stern noted that as the number of bobbed alleles increases, there is a normalizing effect. At about the same time, H. J. Muller was in Austin, Texas following up the radiation work he did in 1927 with studies of gene action. He was able to use x-rays as a tool to produce what he called “deleted-X chromosomes ”: chromosomes that were trimmed to contain the small region that included the white alleles. By doing this, Muller could contrast an allele like apricot with the eosin allele.2 He found that in eosin the doses aresimplyadditive:afemaleheterozygousforeosinandadeletionforthe white-eyed region is paler than a diploid female with two eosin alleles. A B w+ w+ w+ Normal Bicolorism Dosage Compensated we we we C wa wa wa Figure 14.1. Dosage compensation equalizes the doses of the genes on female and male X chromosomes. In A, the red-eyed male and the red-eyed female have the same red eye color,butthefemalehastwodosesofw+andthemalehasone.InB,themutanteosininthe female has two doses of we and the male has one. Thus eosin is not dosage-compensated in the male, which has a lighter eye color than the female. This was originally called bicolorism .InC,themutantapricotbehaveslikethenormalgenesontheX.Thesex-chromosome difference is dosage compensated and the male’s one dose gives the same apricot eye color as the two doses of the female. Muller attributed the different responses to dosagecompensating genes, some of which he identified as being on the X chromosome. 98 In turn, a female with two eosin alleles and a deleted X bearing the eosin allele is darker than a diploid female with two eosin genes. A male with the eosin allele showed the same eye color as a female heterozygous for eosinandadeletionforthewhite-eyedregion.ThisdifferedfromBridges andMorgan’sresultswhenlookingattheeyecolorinhemizygousapricot males and homozygous apricot females, where the two doses in the XX female somehow got equalized to the one dose in the XY or XO male (Figure 14.1). Muller worked with Bessie League and Carlos Offermann to producethisresearch;theypublishedanabstractoftheirworkin1931. The next year, Muller presented a lengthy paper on what he described as “the nature and causes of gene mutations.”3 He presented it to the International Congress of Genetics, which was held at Cornell, under very trying circumstances. A few months earlier, Muller had become depressed, attempted suicide by taking an overdose of barbiturates , and was found somewhat dazed by a posse of faculty and graduate students who had gone looking for him after he disappeared from his home, classes, and laboratory. Many of those attending the meetings found it difficult to follow his paper, because the rumors about his state of mind were circulating among those in attendance. Muller renamed the phenomenon that Bridges had found, “dosage compensation,” and argued that it was a fundamental property of gene function associated with the X chromosomes. Over the next twenty years, he explored possible ways to identify genes associated with the dosage compensation process. He also used dosage compensation as evidence of the “precision of genetic adaptation” in evolution.4 Over a long time period, there had to be a mechanism that equalized the doses of genes on the X chromosomes, because many of those genes were associated with vital organs and processes. Something like bicolorism or simple additivity of doses would have made it difficult, if not impossible, for populations to survive. The Genetic Basis of Dosage Compensation in Drosophila...


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