9. Ploidy Levels and Sex Determination

From: The 7 Sexes

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9 Ploidy Levels and Sex Determination We tend to think of all animals as being diploid, represented as 2N, and their gametes as haploid, represented as N. Exceptions are rare: this is not true for only about 18 of the many thousands of taxonomic groups of animals. In plants like mosses and liverworts, an alternation of haploid and diploid states is common and among a category of insects called the Hymenoptera,whichincludesbees,ants,andwaspsthepresenceofboth haploid and diploid organisms is virtually the rule. These insects share a common mode of sex determination and also a social structure, called eusociality, in which there is usually one egg-laying queen, a huge number of sterile female workers or helpers, and a small number of drones that inseminate the queen (in some species just a single male for just a single encounter serves that role). Bees have immense commercial value, fertilizing one third of the plants we consume and having been usedforhoneyproductionsinceantiquity.Theycannotbedomesticated, however, and experiments to mate specific males with specific females failed until the 1940s, when people started using artificial insemination to initiate bee genetics. Nevertheless, bees played an important role in the history of the study of sex determination. In 1845, Johann Dzierzon (1811–1906) proposed a theory that male bees arose from unfertilized eggs and workers, all of them sterile females, arose from fertilized eggs. His words sound startlingly modern, “I maintain that a) the queen must be fertilized by a drone if she is to be functional, and b) mating takes place in the air, but thatc)droneeggsrequirenosuchfertilization,whiledroneparticipation is absolutely necessary in the production of working bees.”1 Dzierzon, a Silesian (as Polish as he was German) scientist who became a priest, is Ploidy Levels and Sex Determination 59 considered the founder of beekeeping. He used movable shelves in beehives he constructed that allowed him to study the behavior of bees. He claimed that the queens differed from the workers through nutritional differences established by the workers. Bees make a “nuptial flight,” in which each virgin queen-to-be flies and is mated in the air by a drone. The airborne queens then return to either battle for primacy of the hive they left or individually establish a new hive. In 1854, he identified and named royal jelly as the substance involved in queen bee development. In 1851, Carl F. E. von Siebold (1804–1885) put Dzierzon’s theory to a test.2 He used a microscope to study bees and their eggs and published his findings in 1856. He discovered that the unfertilized eggs of bees form drones, while both workers and queens were products of eggs fertilized by sperm. In 1913, Hans Nachtsheim (1890–1979) showed that thechromosomenumberofdroneswas16andthechromosomenumber of worker bees was 32.3 This system is called a haplodiploid mechanism of sex determination. Nachtsheim later translated The Physical Basis of Heredity, by Thomas Hunt Morgan (1866–1945), into German and becameheavilyinvolvedintheracehygienemovement .DuringtheSecond World War, he was accused of having participated in experiments on Dachau inmates (mostly adolescent boys and young men), studying the deprivation of oxygen in chambers to simulate high altitude conditions. But he was not prosecuted and spent his postwar years studying human genetics. Polyploidy is not a problem for bee sex determination. It has occurred in at least five different species of bees. While an understanding ofbeegeneticswouldrequirethedevelopmentofartificialinsemination, which did not occur until almost a century after the work of Dzierzon, the genetics of wasps could be studied much earlier because wasp matings can be arranged without difficulty. The study of wasp genetics was led by the American geneticist Phineas W. Whiting (1887–1978).4 Whiting used the parasitic wasp Habrobracon biennis (later renamed Bracon hebetor),whichhasasmallerchromosomenumber(N=10)thanbeesand can be cultivated in a laboratory. In 1918, Whiting reported that he had identified haplodiploid sex determination in wasps. Because he could mate specific wasps, he was able to isolate mutations and begin mapping the chromosomes of Habrobracon . He also induced mutations with x-rays in 1928. X-ray-induced 60 The 7 Sexes mutations were first done by Hermann Joseph Muller (1890–1967), who induced mutations in fruit flies in 1927. Whiting found abundant mutations because the haploid male wasps immediately expressed any visible mutationsinanyofthetenchromosomes.5Whitingintroducedatheory of genetic sex determination based on complementary sex-determining (CSD)allelesthatmadefemalesheterozygousandmaleshemizygousfor either of the two alleles. The homozygous form of either gene was sterile or aborted and eaten (See Table 9.1). Sometimes a mutation in the CSD gene would lead to altered sex ratios or...