Sex as a defense against transmissible tumors: a new hypothesis
Sexual reproduction imposes long and annoying formalities, such as searching and courting a partner. But it is definitely worth it: the genetic mixing prevents the appearance of dangerous mutations and increases the chances of fighting parasites and pathogens. Once again, someone has spoken in favor of sex: a group of researchers from the University of Montpellier has claimed that its evolution could have been driven by the need to combat transmissible tumors.
Why do we reproduce ourselves through sex? Many single-cell organisms and even some multicellular organisms show us that also another type of reproduction exists, the asexual one. From a single organism, others are formed through various modes of cell division, but all genetically identical to each other and to the parent.
Genetic variability is the best
However, evolution has established the superiority of sexual reproduction, which is prevalent among higher organisms. It begins with the formation of male and female gametes, each carrying 50% of an individual's genetic heritage. Half of the progeny genes are then inherited from the mother and the other half from the father. But it is not as simple as that: the mechanisms of genetic recombination create sequences of genes that are always different from parental ones, so that not even two brothers are completely identical to each other, except for the homozygotic twins.
By mixing our genes with those of other individuals (non-relatives) we reduce the likelihood of accumulating deleterious mutations. Mutations appear spontaneously in all organisms, but an individual that reproduces itself only by asexual division will transmit every mutation to one hundred percent of its progeny. All future generations will inherit that mutation, and then perhaps another and yet another: an increasingly bulky load, which over time could lead to the population becoming extinct. Furthermore, the reassortment of genes increases genetic variability: natural selection will determine who will adapt best, for example by rewarding genotypes that are more resistant to infections or parasites.
A new hypothesis
And yet, asexual reproduction also has its advantages: you can do everything by yourself and your energy investment is much less. There are individuals using both, passing from one to the other modality according to the circumstances, even if the so called “occasional” sexual reproduction is surprisingly rare. The vast majority of organisms adopt "forced" sexual reproduction: it is necessary to be in two and of opposite sex.
Researchers from the University of Montpellier believe that in addition to the reasons already mentioned, the overwhelming evolutionary victory of sex would hide something else. It could be a defense mechanism against transmissible tumors, able to spread from one individual to another by contact as a virus or a parasite would do.
Against transmissible tumors
Cases like these, fortunately, are rare in nature, limited to some animals, such as the Tasmanian devil or clams. But they could have been much more common without sex. The life of these abnormal and contagious tumor cells would be much easier if we were all genetically identical: after having formed into an individual, they could infect all the others in a flash, because they would already be completely adapted to that type of organism. For the unfortunate guest, however, it would be much harder to recognize the invaders: cells almost completely identical to healthy ones.
Currently, transmissible tumors are a rarity in the animal world. Cancer is not normally a contagious disease, precisely because we are all different and our immune system recognizes and destroys any foreign entity. There are exceptions, but they only confirm the rule. But now there is this new suggestive hypothesis on the reasons of sexual reproduction, which seems to agree with a series of observations: will they be able to demonstrate it experimentally?
Thomas, F., et al. (2019). Transmissible cancer and the evolution of sex. PLOS https://doi.org/10.1371/journal.pbio.3000275