Human beings have assumed that sex is a purely binary condition that is determined by the sex chromosomes; XX for a female, XY for a male , the egg always carries an X chromosome, and the sperm is what determines the sex of the embryo that will be formed, depending on whether it carries an X or a Y chromosome.
However, if you delve into the subject, you discover that the alleged binary categories are not really binary , but rather two bimodal categories in a spectrum . In other words, there is actually a range where people fall, and most of the cases, but not all, are in one of these two extremes.
There are exceptions, such as people with XY chromosomes who are born with female genitalia , or even have a mosaic of cells in their body, some XX and some XY, and who develop gonads that are neither ovaries nor testes, but some kind of intermediate form. called ovotestis .
Other ways to determine sex
If in the human being things are not as simple as is claimed, they can be much more complex in other animals . There are many ways of determining sex, other than the primate XY chromosome system.
Birds and many reptiles also use chromosome determination, but in a different way. They have the W and Z chromosomes ; the chromosome pair ZZ designates the male and ZW the female . In this case, the sperm, therefore, only contributes a Z chromosome, and it is the ovule that makes the difference, depending on whether it contributes a Z or a W chromosome.
But in other animals, sex is not even determined by a pair of chromosomes. In bees, wasps, and ants , sex is determined by the complete set of chromosomes; the female is diploid – it has pairs of chromosomes – while the male is haploid – it has single chromosomes. In certain species of fish , sex is determined by age; the young are males and after a certain moment, they become females .
Even in some frogs there can be a change from female to male as a result of living in an environment in which females are much more abundant.
This background is important to understand how pollution can affect the sex of some animals. And it is that certain animals can have a sexual determination based on hormones .
The human being has long been emitting pollutants in all its possible aspects. We expel gases that accumulate in the atmosphere, solid waste that we pile up on the ground, we pollute the water —directly and indirectly, from what infiltrates through the ground from landfills and dumps with solid waste—… If we have even left garbage in space!
Among the pollutants, the so-called endocrine disruptors stand out. They are chemical products that can penetrate animals and alter their endocrine system , either by mimicking natural hormones or by inhibiting their activity. These metabolic changes can lead to multiple harmful effects in humans and other animals.
Some of these contaminants are polychlorinated biphenyls (PCBs) , used as an additive in lubricants, resins, paints or hydraulic fluids, and although they were banned in 1986, they are still present in equipment and materials built before. Another contaminant is the so-called “lindane”, or hexaclococyclohexane (HCH) , a pesticide banned in the European Union in 2008 due to its high toxicity, or the famous dichloro-diphenyl-trichloroethane (DDT) , banned since 1972.
However, despite the ban, they continue to appear as contaminants in the natural environment, due to their quality of bioaccumulation in food chains , that is, they remain in the tissues of animals and pass from one to another through the feeding relationship. This makes them very difficult to remove .
The feminization of fish
Although these contaminants appear in the water, they do not always come from discharges or leachate. They often constitute atmospheric pollution that has been dissolved in the water. Substances that travel through the air and end up deposited in lakes or rivers, far from the original source of the contaminant .
Fish , and specifically salmonids, are a good alarm system for the presence of these products, since males are exceptionally sensitive to them. In them, endocrine disruptors generate an increase in estrogen levels, which leads to a feminization of animals . They begin to produce certain proteins that are characteristic of females, and that normally have the function of forming eggs —as if male fish were taking birth control pills. As the sexual determination of fish is different from that of humans, in the long term, it can cause a change of sex in entire populations.
These effects can be easily observed from a small number of fish caught, allowing reliable relationships to be established with respect to air pollution. Endocrine disruptors have disastrous effects on ecosystems, but not only. There are also negative effects on human health.
In our case, the feminization suffered by fish will not happen —because our determination of sex is given by chromosomes and not by the environment—, but the undesirable effects of endocrine disruptors on our species continue to be serious: lack of sexual appetite, impotence, defects of development, mental deficiencies, malformations, various types of cancer or sterility .
These observable events in fish should therefore be taken as a warning system for the presence of endocrine disruptors in the atmosphere , which can have significant effects on public health.
Jarque, S. et al. 2015. Background fish feminization effects in European remote sites.Scientific Reports, 5(1), 11292. DOI: 10.1038/srep11292Villafañe, V. E. et al. 2011. Hermafroditismo verdadero. Un caso 46XY/46XX. 3.Yilmaz, B. et al. 2020. Endocrine disrupting chemicals: exposure, effects on human health, mechanism of action, models for testing and strategies for prevention. Reviews in Endocrine & Metabolic Disorders, 21(1), 127-147. DOI: 10.1007/s11154-019-09521-z