Why can’t some couples have children? A question that is resolved not only the reproductive medicine of humans. Zebra finches, commonly bred singers, are known for the fact that they form a partnership for life. Scientists have discovered that some couples have great problems with reproduction. A study by an international team of scientists, published in Nature Ecology and Evolution, clarified the fundamental influence of chromosome Z on the fertility of male birds, because even exceptionally fertile males of the zebra finches will always have half of their sons fertile and half infertile. Shouldn’t sperm gradually improve with evolution? And shouldn’t infertile individuals gradually disappear from the bird populace?

“We have found that the fertility of males depends on the specific break and inversion of one of the sex chromosomes, with birds called Z. Inversion means that part of the chromosome is twisted. And chromosome Z influences sperm,” says Associate Professor Tomáš Albrecht from the Institute of Vertebrate Biology of the CAS, whose team participated in the research. “It would not have been such an interesting result, if we had not also discovered that besides the males, who as a result of the inversion have a kind of supersperm, also males emerge, who are not far from infertility,” he adds.

In the population of the males of the zebra finch, two forms of chromosome Z appear. Strong selective pressure should be in favour of the fastest sperm and the successful fertilization of the egg. However, for the ideal morphology of sperm, both forms of chromosome Z are needed, and so males with inferior sperms will still be born.

According to Associate Professor Tomáš Albrecht, this “mutation” originated 2.5 million years ago, and the variability of good and bad sperms is maintained in equilibrium in the males of the zebra finch. “It is the mechanism of overdominance, which leads to the fact that males with superspermies cannot prevail in the population and that the males with insufficient sperms will not disappear,” he specifies.

Does a similar case of the mechanism of overdominance exist in humans? Associate Professor Tomáš Albrecht provides the example of sickle cell anaemia, a hereditary disease from the tropical and subtropical regions. The disease affects the red blood cells, but it protects the carrier from malaria. Sickle cell anaemia is inherited as well as a blood group, the colour of hair or eyes, and if one parent is infected, and the other one only carries it, there is a 50 percent chance that their offspring will be infected and a 50 percent likelihood of being only a carrier.

Prepared by: Vlaďka Coufalová, Department of Media Communication of the CAS
Photo: Institute of Vertebrate Biology of the CAS