t-gene epidemics drive mouse populations extinct

The t-gene epidemic is a case of genetic conflict in which an inheritance-biasing gene gains ground at the expense of the mice that carry it. A male mouse with one t gene can produce sperm in which up to 95 percent contain that gene rather than the normal allele. This advantage can make the t gene spread rapidly, even as the resulting increase in mice with two copies threatens the population with death, sterility, and extinction.

The inheritance distortion

In ordinary meiosis, the process that produces sperm or eggs, either allele in a pair has an equal chance of entering a reproductive cell. The expected outcome is a fair 50–50 division across many sperm or eggs. But meiosis is a physical process that genes can influence. A mutant gene that biases the process in its own favor is called a segregation distorter; the resulting unfair transmission is known as meiotic drive.

The t gene in mice is one of the best-known examples. A male carrying one copy can be healthy and otherwise normal, yet examination of his sperm may show that up to 95 percent carry the t gene and only 5 percent carry its normal allele. The distortion is not a conventional advantage such as improved survival or bodily health: it gives the gene an edge in the lottery of inheritance.

Spread and population cost

The t gene’s transmission advantage can drive its spread through a wild population. As more mice inherit it, more individuals receive a copy from both parents. In this double-dose, or homozygous, state, the t gene is lethal or causes sterility: affected mice either die young or cannot reproduce. The gene’s success in transmission thus creates the conditions for its own population-level catastrophe.

The account describes the spread as rapid, like a brushfire. Once double-dose mice become numerous, the local population is likely to be driven extinct. There is some evidence that wild mouse populations have in the past become extinct through t-gene epidemics. This is presented as a possible historical consequence, not as a claim that every population carrying the gene inevitably disappears.

Genetic success versus organismal welfare

The episode separates a gene’s success from the welfare of the animal in which it resides. The t gene can be favored by natural selection because it enters more reproductive cells than its rival, even though its effects on bodily welfare and on the other genes sharing the body may be disastrous. Selection operating at the level of genes can therefore favor an outcome that is harmful at the level of individual organisms and destructive to a local population.

The case also makes concrete the idea of genetic cheating within a body. Instead of an organism exploiting its social companions, one gene gains an unfair advantage over the other genes with which it shares an organism. The t-gene epidemic is consequently a stark example of how transmission bias can defeat the interests of the larger genetic and bodily system: the gene spreads, while the mice—and potentially the population—pay the cost.