10. The Discovery of Sex Chromosomes

From: The 7 Sexes

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10 The Discovery of Sex Chromosomes TheideaofthecontinuityofchromosomesaroseamongGermancytologistsinthe1880s .Atthattime,manyAmericanbiologistswouldgettheir PhDs (especially at Johns Hopkins University) and take a trip to Europe to visit the laboratories of German, Belgian, Dutch, French or Italian biologists where much of the work on meiosis, mitosis, and reproductive biology was taking place. They would then return to the United States to begin their own cytological studies. In 1891, the German biologist Herman Henking (1858–1942) studied the fire wasp, Pyrrhochoris,1 which is not actually a wasp, or Hymenopteran, but a true bug, or Hemipteran. He noted that, during the spermatogenesis of the fire wasp, there is an unusual chromosome: a nucleolar object that takes on a very dark stain in the first meiotic division. In the second division, this unit did not divide, and it appeared to remain in only one of the two cells produced. Because it was unusual in its staining, its morphology, and its behavior, Henking called it an X element, using X as a mathematical symbol for an unknown to be solved. The next year, when Henking was given an opportunity to take on an important and more financially rewarding position in German fisheries, he dropped cytology, focusing on fisheries for the rest of his career. Henking made no association between his X element and sex determination. Henking’s X element came up again in 1898, when Clarence Erwin McClung (1870–1946) at the University of Kansas studied spermatogenesis in the grasshopper Xiphidium fasciatum at the suggestion of entomologist William Morton Wheeler (1865–1937), who had studied oogenesis in this species.2 McClung found something similar to what 64 The 7 Sexes Henking found, but he was convinced it was a chromosome and not a nucleolar body. He dropped the term X element, instead calling the mysterious nucleolar body an accessory chromosome. A year later, he realized that males had an odd number of chromosomes in their somatic cells at the start of spermatogenesis. As a result, he correctly saw the accessory chromosome as sex determining, but he erred in also believing it was male determining. His interpretation led to a dispute with Thomas H. Montgomery Jr. (1873–1912) at the Wistar Institute in Philadelphia . Montgomery worked with Hemipteran bugs. He found that chromosomes paired during the first meiotic division and called these pairs homologous chromosomes. He noted that the accessory chromosome in some of his bugs was unpaired but, because of its appearance, he shared Henking’s skepticism that this was a chromosome. He also doubted that Henking’s X element was equivalent to McClung’s accessory chromosome.3 A major advance in an understanding of sex chromosomes came from the work of Nettie Stevens (1869–1912). She worked with Thomas HuntMorganwhenhewasatBrynMawrandthenwenttoStanfordUniversity for her PhD. Then she returned to Bryn Mawr where she began a lengthy study of insect cytology, using both Diptera and Hemipteran bugs.4 One such bug, Tenebrio, the mealworm, is assigned its popular name for its larval stage. In 1905, Stevens noted the existence of what she called heterochromosomes to distinguish them from Montgomery’s autosomes,orhomologouspairsofchromosomes.Thereisadistinctheterochromosome found in both males and females. In females, it appears as a pair of homologues during the first meiotic division. During male spermatogenesis, it appears as a single chromosome, but in some of the species she looked at it is accompanied by a smaller heterochromosome with which it pairs and separates during the first meiotic division. While Stevens was working with her heterochromosomes, Edmund Beecher Wilson (1856–1939) was working with Hemipteran bugs. He noted that in some species there is a solitary chromosome in the male somatic tissues or in the spermatogonia, but in the females of the same species the chromosome number has an even number that is one more than the chromosome number of the males.5 He found different species in which the male has a much smaller chromosome that pairs with the The Discovery of Sex Chromosomes 65 larger chromosome that is found twice in the oogonia of the females. Here the males and the females of the species have an identical chromosome number. He called these two non-homologous chromosomes idiochromosomes.Laterhechangedthenametomakesenseofaconfusing multiplicity of findings and conflicting terminology. He called the idiochromosomefoundinmalesandfemalesanXchromosome,andthe idiochromosome only found in males a Y chromosome.6 Assigning Hereditary Attributes to Sex Chromosomes Wilson not only recognized that the X and Y chromosomes were involved in sex determination, he also realized that these chromosomes were qualitatively different from the autosomes, and that the first assignable hereditary...