Incest prolongs life
To test this, Swedish scientists tracked 10 generations of siblings who mated exclusively with each other. Females began to live less in full accordance with the hypothesis. But males also began to live longer, and this hypothesis of asymmetric inheritance is no longer explained.
Evolution and rigid natural selection manifest themselves not only in a variety of shapes and sizes, but also in the smallest details worked out over millions of years. Sometimes even at first glance inexplicable – for example, in very stable sexual differences in characteristics that seem to have nothing to do with sex and reproduction.
An example of the external manifestations of such differences, visible to the naked eye, is a lion's mane, peacock tail or formidable walrus tusks, as well as the advantage of men in height, weight and strength. Another difference that can be noticed only when studying demographic data is the difference in the average life expectancy between men and women that cannot be logically explained.
Swedish Trina Bilde from Uppsala University and her colleagues from Denmark, Australia and the USA have found an unexpected way to compensate for this "sexual injustice" – through incest.
In fact, the authors of the experimental work published in BMC Evolutionary Biology did not set goals to equalize the duration of sex in the length of life. They were only trying to understand the intracellular reasons that women in developed countries live on average 3-8 years longer than men, even after adjusting for higher mortality in infant boys.
One possible explanation is the effect of the female hormone estrogen on the content of low–density lipids associated with atherosclerosis and subsequent cardiovascular diseases; they affect men to a much greater extent. The same group includes risk factors such as stress, smoking and alcohol, which, according to an unkind tradition, are more typical of the stronger sex.
But then why is the same difference in life expectancy characteristic not only for humans, but also for other mammals, and even for some insects?
After all, from the point of view of natural selection, it would seem that everything should be exactly the opposite – it is females who should live less, since they exit the reproductive age earlier than males who are able to fertilize and with gray hair on their heads. "Evolutionarily" this can be explained by the fact that it is enough for the male to "invest" in the spread of his genes for a short period of time, while the female has to nurture her children for a long time – for her this is the only chance to consolidate her genes in generations.
One of the hypotheses that resolves this paradox is the so–called protected X-chromosome theory. According to this hypothesis, males with a single copy of the X chromosome are much more likely to suffer from mutations in the genes located on it to one degree or another. But the females have a kind of duplication: even if some mutation disrupts the synthesis of the desired protein on one of the chromosomes, the second, full-fledged backup copy "protects" the body from an unfavorable effect.
An extreme case of such an effect is gender–related diseases: hemophilia, Duchenne myodystrophy, which are transmitted through the female line, but boys suffer from them.
Bilde decided to test this hypothesis in a more general way, not only for the mentioned illustrative examples, but also for "hidden" mutations, the effect of which has not yet been studied in detail.
One of the ways to achieve the manifestation of these mutations is inbreeding, closely related crosses. In this case, the "concentration" of genes with unfavorable mutations increases (and not only on the sex chromosomes). After all, with closely related relationships, one of the main mechanisms, negative natural selection, is essentially turned off from the evolutionary process: individuals with harmful mutations that reduce survivability are not eliminated.
In order to test the hypothesis of a protected X chromosome, biologists have bred 20 inbred lines of grain beetles Callosobruchus maculatus by long-term closely related crosses. In wild populations, they have a fairly stable difference in life expectancy between the sexes: males live on average 7 days, females - 10 days. After 10 generations of crossbreeding between siblings (in each generation), males equaled females.
Both lived on average for 8 days. To achieve this, scientists had to record "acts of civil status" – birth, all crosses and death – for more than 6 thousand individuals. According to scientists, by the 10th generation, beetles in each of the 20 lines should be homozygous for an average of 89% of the genes.
However, despite the fact that the shortening of the life of females is quite consistent with the hypothesis of the "protected X chromosome", it does not explain the increase in the life of males. After all, they receive the X chromosome from their mother, and closely related crossing should not have had any effect on them.
In any case, inbred males could not escape punishment for violating the "family code": like females, they had much lower fertility. If a typical grain in the control non–inbred group left behind 90 offspring, then in inbred lines these indicators decreased - three times in females and twice in males. So the chances of the latter leaving behind the same "long-livers" are very small.
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06.02.2009