Epigenetics Comes of Age

Dangers of Preamture Extrapolation in Biology (Part 2)

The Dogma of Genetic Sex Determination   For more than a century conventional textbook knowledge has been that sex is genetically determined by sex chromosomes. It could be determined by male heterogamety (this is the case with humans and other mammals where combination of chromosomes XY produces males and XX- females) or by female heterogamety (ZZ combinaiton  males and WZ- females).  Another form of sex determination is the system of haplodiploidy in many insects, where males develop from haploid eggs and females from diploid zygotes. However, recent evidence shows that in most cases studied sex in animals is not genetically determined. Environemental factors such as the temperature in the sea turtle Lepidochelys olivacea determine whether an embryo develops into a male or a female individual. In this species it is the embryo’s brain, the diencephalons that senses the temperature during the sex determination. It is concluded that“The spinal cord and the innervation derivating from it could play a role in driving or modulating the process of temperature-dependent gonadal sex determination and/or differentiation” (Gutierrez-Ospina et al. 1999).Transplantation of male brains alone into abdomens of some insect females induces presumptive ovaries to develop into into apical tissue and display male secondary characters (Gorbman and Davey, 1991) (see for further details in chapter 6 of the Epigenetic Principles of Evolution in this website).

In contrast to mammals and birds, teleost fish display an amazing diversity of sex-determination systems. Male heterogamety (males are XY and females XX, as is generally the rule in mammals) and female heterogamety (females are WZ and males ZZ, the system at work in birds) are sometimes observed within the same fish genus and even the same fish species. Moreover these fishes reversibly change their phenotypic sex that is the individuals of the same female genotype during their lifetime develop functionally normal male phenotype and revert again to their female phenotype. Similar reversions of sex occur in genotypically male fishes. Such facts clearly exclude the genotype as a determinant of sex in these species

But if genotype is responsible for the sexuality, what is that determines sex in these fishes? Adequate evidence shows that in these fishes, as well as in other reptiles and birds, and even in mammals, sex is determined by the brain and sexual differentiation of the brain during the individual development takes place before differentiation of gonads, the “sexual organs” (until recently biologists believed that the sexual differentiation of the brain was result of the action of hormones produced by these organs). It is very important to remember that to emphasize the fact that the most common factors of sex reversion in fishes are social and behavioral stimuli.

Investigators of the sex conversion in fishes have concluded that

“The initiation of the sex reversal is often controlled by social, behavioral factors, and since the only way behavior can affect the gonads is through the brain, there must be central neuronal mechanisms underlying the gonadal change” (Elofsson et al., 1997)

Which is the sexual organ then, the brain or the gonads? Epigenetics, not genetics or genes, seems to be in control of sex reversion of fishes.

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