Benefit Of Detrimental Mutations

The overwhelming majority of genes are represented in two copies in the cells of animals: one is received from the mother, the other – from the father. In many cases, the organism endures with difficulty the damage to one of the copies: a single dose of a gene may be insufficient for normal evolution. However, for some genes a single dose is not simply endurable but also welcome. Such cases deal primarily with the genes that increase life span and stress resistance. Individuals with a single normal copy of such gene (the second copy is desperately spoiled as a result of mutation) live longer and better endure unfavorable conditions. But, the owners of a double dose of the mutant gene are either nonviable or more frequently than others die of some disease such as malignant growth.

Investigation of mutations in stress resistance genes is only possible on models, and the researchers worked with drosophilas. They studied the lgl tumor suppressor gene. Human beings have the gene with the same functions, it is called Hugl-1. If normal copies of the gene are absent in the cells, frequency of cancer occurrence is high. It may seem that such detrimental mutation should be abolished by selection, however, the lgl gene mutations are rather frequently found in the population. Why?

It has turned out that the drosophilas containing one mutant copy and one normal copy of the gene possess increased embryonic survival rate if they develop at stressful temperatures (16 and 29 degrees centigrade). At the temperature of 25 degrees (which is optimal for flies), viability of mutation carriers is slightly lower that the one of owners of the double gene dose. Besides, mutation carries have a longer life span. These useful properties become normally apparent in case if mutation was received from the mother. Apparently, increased viability of flies is being formed in the early embryonic evolution stage and depends on the maternal genotype. Mutation carriers are more stable to viral infections.

Thus, if the gene does not work at all in case of two mutant copies, then probability of cancer development in the organism is high, and a single copy of the gene (in case of one mutant copy) in some way ensures optimal state of the organism in stress conditions.

Cancer development with human beings and animals is a complicated, multi-stag process connected with the loss or damage of certain genes. The drosophila investigations carried out by the Russian researchers help to understand why the mutations that can cause cancer do not disappear from the population but are preserved in it.