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7 The Discovery of Sperm in Higher Eukaryotes

Elof Axel Carlson Indiana University Press ePub

Semen has long been recognized as necessary for producing offspring. It is liquid, somewhat viscous, and usually clear or slightly cloudy in appearance; certainly the unaided eye can see no visible body within it. The Greeks, especially through Hippocrates and later Galen, embraced a theory of vital fluids, which they called humors. Blood was considered the major constituent of life, at least among vertebrates. It was considered the progenitor of semen in the male body, and believed to be the hereditary material that allowed a species to generate offspring in its likeness.

Semen was endowed with a capacity to impose form on the pliable material supplied by females. That material was also thought to be blood: sometimes it was associated with menstrual blood, and sometimes it was thought to be another type of semen. Female semen was not clarified, like male semen, but still bloodlike and clotted—a type of miniscule clay ready to be molded into shape by the empowering effect of male semen. For more than two thousand years, arguments were made about the relative roles that males and females play in forming a new individual through their fluids, which were commingled after copulation. There were inside–outside theories in which the male supplied the outer components of the new baby. There were theories in which the female role was passive, being shaped exclusively by the male, forcing some observable phenomena, such as the equal contributions made to the skin color of the offspring of a black person and a white person, to be swept under a mental rug.

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4 Monotheistic Religious Interpretations

Elof Axel Carlson Indiana University Press ePub

Countries with substantial populations of Christians and Muslims follow a monotheistic tradition derived from the Jews and their scriptures, the Old Testament or Hebrew Bible. Sex determination of humans is introduced in the book of Genesis as two separate events: Adam is first created as a male and later given a female companion, Eve, derived from his rib. The first strange feature of this separate creation of the two human sexes is that it occurs after the creation of sea, air, and land animals. These creatures are created in an unspecified number and commanded to multiply their kind. By implication, male and female representatives of these other forms of life were created without comment about their having two sexes, as were species that produced their progeny by parthenogenesis, budding (or other cloning mechanism), multiple mating types (e.g., paramecia), or hermaphroditic mutual gametic exchange (e.g., earthworms and snails).

The second unusual feature of the Genesis account is the creation of a female, Eve, presumably having a 46,XX chromosomal composition from the rib of Adam, presumably having a 46,XY chromosomal composition. Barring some miraculous act and assuming the Creator was using an XY mechanism for sex determination, the rib would have had to contain this 46,XX tissue. This would mean that Adam was some sort of chromosomal mosaic (or chimera, if the XX tissue was some type of embedded twin), with a region of his body (the rib area used for Eve) containing a karyotype that required two separate, nondisjunctional events to bring about. If one attributes miraculous acts for this formation of Eve, why was the rib necessary? If the human female has her origin as a second sex from Adam, the chromosome difference has to be reconciled (at least for those calling themselves “Creationist scientists”). One such possibility would be the lagging of the Y shifting a cell to 45,X, followed by a delayed separation of chromatids of the X producing the 46,XX cell from which the rib area was derived. The two X chromosomes would necessarily be identical in nucleotide sequence (except for a few new spontaneous mutations).1 Note the coincidence (or possible association) between the hermaphroditic nature of Adam before Eve is extracted from his rib, and the Greek myth cited by Plato in The Symposium, where heterosexual couples were produced from an initially hermaphroditic state.

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22 The Quest for a Unified Theory of Sex, Gender, and Sexuality

Elof Axel Carlson Indiana University Press ePub

I have attempted to explore the history of sex determination. As a biologist, my outlook is comparative, because the human story was largely built from findings about other mammals, insects, plants, and even viruses. Although there are a scattered few species that have resisted the most common mode of exchanging genetic information, the term “sexuality” applies across all of life (Table 22.1). We think of that common mode when we use the term sex determination. It implies a two-sex system, although, as we saw in paramecia, there can be several more than two mating types. There are also non-sexual (or more accurately, female-only) species of rotifers, such as Philodina roseola, that use horizontal transfer of DNA to supply an influx of new genes, either by ingesting other rotifers or from other things that they eat.1 When we apply sexuality to humans, the nuances increase because we invoke cognate terms like “gender” which is not applied to bacteria, or even fruit flies. We can speak of “feminism” as a human academic study, but the term has no meaning when applied to most of the animal and plant kingdoms. Among many animals, there are atypical hermaphrodites or intersexes, some arising as accidents of cell division, like “gynandromorphic” fruit flies. We do not use that biological term for chromosomal chimeras that are XX/XY, or for mosaics, like XY/X, in humans. The older literature calls them hermaphrodites or “true hermaphrodites,” defining such individuals as having both testicular and ovarian tissue. We do not apply the term “freemartin” to our offspring. That is an intersex associated with twinning in cattle. Instead we use the term “female pseudohermaphrodites” (XX or ovarian DSD) to describe the androgen-stressed embryo in its first and second trimester of development.

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17 The Identification and Role of Sex-Determining Genes

Elof Axel Carlson Indiana University Press ePub

Life can be resilient and vulnerable at the same time. We rejoice at stories of Olympic medalists who overcome severe injury or a childhood marked with tragedy. At the same time, nature can dish out genetic disorders resulting in births incompatible with life—severely limiting in organ function or leaving an adult with chronic illness. What makes life so vulnerable is the nature of genetic material. Something as simple as altering or removing one nucleotide pair out of some three billion present in a sperm or an egg can result in one of those debilitating or lethal genetic conditions. That doesn’t happen if one pulls a single brick out of a multistoried building. It won’t collapse no matter where that brick is removed. If the gene happens to involve the sex-determining programs in the embryo, the results can be quite dramatic.

The sex-determining genes can be found on the sex chromosomes and the autosomes. This is no surprise, because many organ systems are involved in sex pathologies. They can result from abnormalities of the pituitary gland, the hypothalamus, the adrenal cortex, the gonads, or those embryonic structures that will form the internal and external genitalia. A major gene involved in male sex determination is the SRY gene. It is on the Y chromosome and located at Yp11.3, i.e., on the short or p arm of the Y chromosome at band region 11.3 (Figure 17.1). Andrew H. Sinclair found the SRY gene while he was working with Peter Goodfellow’s laboratory at Cambridge University in 1990.1 Prior to that, in 1987, David C. Page believed that what was called the “testes determining factor” (TDF) was a zinc finger gene in a different region.2 The TDF was known to be Y-associated from cytological studies of certain partial or complete sex reversals. Thus, a baby with X isoYp sex chromosomes has a duplication of the p arm. Such an individual is a male and sterile, because the q arm of the Y provides the genes associated with spermatogenesis, but otherwise functional. But a baby with X isoYq has a duplication of the q of the Y chromosome and an intact X chromosome and is missing the short arm of the Y chromosome. Because such babies are born as females with no testes, that is where the testes determining factor has to be located. The gonads of such babies are streaks, like those of babies with Turner syndrome. They also show other symptoms similar to those associated with Turner syndrome, suggesting that some growth factor is associated with the Yp region.

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12 The Discovery of Sex in Microorganisms

Elof Axel Carlson Indiana University Press ePub

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 that they formed from spontaneous generation. The idea is as old as written 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 after H. J. Muller, the formation of the gene, the first replicating molecule that could copy its errors.

Microscopy flourished in the last half of the nineteenth century. It spun off the field of histology in medical schools and the field of cytology 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

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