The 7 Sexes: Biology of Sex Determination

12. The Discovery of Sex in Microorganisms

12 The Discovery of Sex in Microorganisms When Anthony van Leeuenhoek observed the animalcules, as he called them, from different dips of water or from his own body, he did not discuss how they formed. Most of his contemporaries would have said thattheyformedfromspontaneousgeneration.Theideaisasoldaswritten thought. Aristotle believed in spontaneous generation, and so did anyone watching rotting food or meat swarming with maggots. Before Rudolph Virchow and Robert Remak’s cell doctrine, biologists did not think of life coming from preexisting life. At least they conceived the process as far back as life goes: Genesis for the pious; after Charles Darwin , some sort of event that led to the formation of the first living cell; or afterH.J.Muller,theformationofthegene,thefirstreplicatingmolecule that could copy its errors. Microscopy flourished in the last half of the nineteenth century. It spunoffthefieldofhistologyinmedicalschoolsandthefieldofcytology that led to inquiries about heredity. It was a necessary tool for the field of microbiology that flowed from germ theory. Louis Pasteur (1822–1895) and Robert Koch (1843–1910) introduced the germ theory of infectious diseases in the 1870s and 1880s. It revealed even smaller organisms than those seen by Robert Hooke and Leeuenhoek. Pasteur and Koch’s theory brought microscopy back to the medical school to study infectious diseases caused by bacteria and other microorganisms. By the end of the nineteenth century, scientists inferred the existence of even smaller organisms, which slipped through filters that barred passage of bacteria. In 1892, the first virus, tobacco mosaic virus, was identified.1 The Discovery of Sex in Microorganisms 77 Bacteria were clearly not typical cells. They were much smaller. Hundreds of them could fit into a typical cell of the histologist’s tissues. It took a while before microbiologists agreed that bacteria arose from preexisting bacteria, but they did not show signs of mitosis. They were instead described as dividing by simple fission: some sort of amorphous material grew, split into two, and that was it. But not all microorganisms were amorphous in substance like bacteria .Fungi,algae,andprotozoaallhadcells,andineachcasethosecells hadclearnucleiandchromosomes.Mostbiologistsinthelatenineteenth century believed that these microorganisms multiplied by mitosis; the idea of microbial sexuality did not enter their thinking. That changed in 1904, when Albert Francis Blakeslee (1874–1954) obtained his PhD at Harvard working on the bread mold, Mucor mucedo.2 This mold forms a white fluffy coating on stale bread and produces zygospores that look like sprinkled pepper. When Blakeslee worked out the life cycle in M. mucedo and other varieties of Mucor that he obtained, what he found surprised him. If he took two different samples of Mucor and let them formafluffymatofhyphae,thetwomatswouldusuallyjoinandproduce no zygospores, but on occasion one strain of hyphal colony encountering a different hyphal colony would form a row of zygospores along their margin of encounter. Blakeslee called the two types (+) and (-) strains. In his mind, this was sex without masculinity and femininity. The work earned him a fellowship to travel to Germany, where he spent two years at Halle studying fungi. He much enjoyed those years except for one incident when he was arrested for sweeping horse manure into a paper bag.ThepolicemansawmischiefafootandBlakesleecouldnotconvince him that he was sweeping up manure to look for new varieties of fungi.3 Blakeslee’s work created a controversy because he had chosen the term “sexual strains” to describe his (+) and (-) strains. Many of his fellow mycologists felt that the term sexual should apply only to those organisms that have distinct male and female partners. Neither (+) nor (-) strains differed in appearance. For this reason, most microbiologists of his generation rejected Blakeslee’s use of the term “sexual strains”, instead referring to his findings as the discovery of “mating types” in microorganisms. 78 The 7 Sexes Blakeslee returned to the US to teach at the Connecticut Agricultural Station (later the University of Connecticut) and had neither time nor opportunity to do research on Mucor. He switched to trees and then to flowering plants, spending most of his life studying the Jimson weed (also known as the thorn apple), Datura stramonium. He isolated aneuploid forms for all twelve of its haploid chromosomes. He returned to his Mucor work in the 1920s, but it was secondary to his interest in cytogenetics in Datura and Oenothera. It would be another 30 years after Blakeslee’s first papers on Mucor that microbiologists would be discussing sex in microorganisms or even mating types in them. Why was there so little interest in studying genetics...