It has long been assumed that an infant receives half her genetic material (DNA) from her mother and half from her father, but new research suggests that bacteria makes three. While studying intestinal disorders such as Crohn’s disease, researchers at Washington University discovered that a form of resistance developed in the offspring of subject mice, and the only possible source of the genes conferring resistance was bacteria in the mother’s gut.
How is this possible? The answer is an odd phenomenon called Lateral Gene Transfer (LGT), where genes are passed not just from a parent to an offspring but from one mature, independent organism to another.
Writing in American Scientist, Carlos Amábile-Cuevas and Marina E. Chicurel describe this unusual process. The secret to LGT lies in the unique nature of the bacteria genome. Bacteria have almost two genomes, the “regular” genome encoded on chromosomes and a separate, circular piece of DNA called a plasmid. Since bacteria have no nucleus, both the plasmid and the chromosomal DNA float about in the cytoplasm.
Under certain circumstances, these free-floating plasmids can move from one bacteria into another, carrying their genes with them. Those plasmid genes may then be inherited by offspring. If the plasmid carries beneficial genes, the recipient bacteria and all its descendants are permanently changed. Lateral Gene Transfer was believed to be a phenomenon only between bacteria, but Amábile-Cuevas and Chicurel found examples of LGT in fruit flies and believed that it might be more widespread.
In the case of a developing embryo and maternal bacteria, the plasmid will not be replicated whole, but sections of the plasmid DNA will become part of the embryo’s genetic code. An embryo which receives bacteria DNA will replicate that DNA as its own, and when eventually that embryo grows up and has its own offspring, they, too, will have sections of bacterial DNA.
The implications for evolution are huge. Evolution by natural selection has been assumed to proceed when beneficial mutations get fixed in a population, conferring a competitive advantage. However, if LGT is widespread, mutations may not be responsible for evolution at all; instead beneficial traits may actually be donated through bacteria.
Heaven forbid bacteria start offering parenting advice, we’d never hear the end of it.
Correction: The mechanism being discussed in the original St. Louis research is not Lateral Gene Transfer between the bacteria and the mammal embryo, but the heritability of maternal bacteria. In other words, you don’t just inherit your mother’s DNA, you get her bacteria also. The genome of the inherited bacteria influences the physical characteristics of the offspring to a degree comparable to that of mutations in the offspring’s genome. Lateral Gene Transfer has not yet been observed between a bacterium and a mammal, but it’s still cool! My apologies for the error, and thank you to an astute reader for pointing it out! (James McDonald)