Sex Determination

Reproduction: Molecular and Cellular Biology

In recent decades, science fiction has grappled with startling discoveries about the molecular and cellular basis of sex determination and fertilization. How do some vertebrate females reproduce without fertilization? Why does evolution link fertilization with aging? Why is the molecular determination of sex linked with the molecular development of other structures throughout the body?

Jurassic Park, by Michael Crichton (1990)

In Crichton's classic novel, dinosaurs are reconstructed by cloning their DNA sequences from dinosaur blood cells obtained from ancient blood-sucking insects trapped in amber. The scientists attempt to control their dinosaur population by rearing only females, thus preventing uncontrolled breeding and reproduction. The plan goes awry when some of the cloned females reproduce by parthenogenesis, the development of an offspring from an unfertilized egg.

Discussion Questions:

1. Parthenogenesis in fact occurs in a number of reptile and amphibian species, as well as in fish. The environmental signals inducing parthenogenesis in nature are not well understood. What factors might have induced parthenogenesis in Crichton's story?

2. Like all biological capabilities, the capacity for parthenogenesis must be determined by genes in the genome. How is it possible that the dinosaurs possess the ability for parthenogenesis, yet the scientists who cloned the animals do not know it?

3. Vertebrates show a surprising range of sex-determination mechanisms. In crocodiles, sex is determined not by different genes but by the temperature of development of the egg. In certain fish species, all individuals are born female but later metamorphose into males, or vice versa. How might you rewrite Crichton's story assuming one of these other mechanisms of sex determination?

A Door into Ocean, by Joan Slonczewski (1986)

An all-female race of genetic engineers inhabits a planet called Shora, covered entirely by ocean. The ocean-dwelling women, called Sharers, maintain an uneasy relationship with more traditional humans from the nearby planet Valedon. When the Valans attempt to establish military control over Shora, they face unexpected cultural and biological obstacles. The Sharers use nonviolent techniques of resistance based on their unisexual society and their advanced genetic engineering, called "lifeshaping."

Discussion Questions:

1. The female Sharers manage their reproduction by "fusion of ova." In actual human reproduction, what molecular problems would occur when two haploid ova are combined to produce a diploid zygote? Why does this process have to be performed in vitro by the Sharer lifeshapers?

2. What techniques of nonviolent resistance do the Sharers employ to defend themselves and their planet? To what extent are these techniques based on biology, psychology, or cultural assumptions? Can males as well as females employ these techniques? How might this book have been written with Sharers who were all male, or of both sexes?

Daughter of Elysium, by Joan Slonczewski (1993).

An advanced race of humans, the Elysians, has evolved to live without aging. The price of eternal youth, however, is the total loss of fertility. Elysians can generate children only by genetic engineering in artificial wombs. Sexuality, therefore, is reduced to a social function, with no reproductive significance. But medical scientists seek to "cure" the Elysian defect by restoring their natural fertility through genetic engineering.

For a free copy of this book, contact the author at slonczewski@kenyon.edu (limited quantity available).

Discussion Questions:

1. The Elysian scientists cite the example of the first anti-aging mutants of C. elegans, which showed impaired fertility. Why does evolution link fertility with aging?

2. An obstacle the Elysian scientists face is that genes linked to fertility inevitably are linked to functions of other parts of the body. For example, a gene enabling the proto-germ cells in the embryo to migrate to the site of gonad formation (male or female) may also be involved in development of the heart and brain. Why does this linkage evolve? Explain in terms of the function of molecular transcription factors.

3. The daily work of the Elysian Fertility Project is tinged with irony because many of the lab workers come from "developing planets," where overpopulation of "normal" humans remains a serious problem. How do the political dynamics and the research atmosphere mirror those of actual laboratories of molecular biology today? Do you think the ageless Elysians should regain their fertility, or should they limit their reproduction to artificial wombs?

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