Everyone reading this (except you robots out there) inherited genes from their mother and father. This is the predominant way, in multicellular animals and plants, for genes to be transferred from one organism to another, from parent to child, and is called vertical gene transfer. But less commonly, genes can be transferred from an individual unrelated to you, possibly from a different species, and is called horizontal gene transfer. Viruses accomplish horizontal gene transfer naturally, while in the lab, genetic engineers use viruses to transfer genes horizontally to create genetically modified organisms.
The gene syncytin-2, which produces an essential membrane between the mammalian placenta and the developing fetus, appears to have come from retroviruses, who use the gene to produce a membrane around their virus capsule. If our ancestors had not acquired this retrovirus gene, you and I would not be here today. We have to be grateful for horizontal gene transfer.
Now for the speculative part of this article. Velvet worms (Onychophora) are a whole phylum (major group) of animals most people have never encountered. They look kind of like a cross between an earthworm and a millipede.
Nowadays, they are tropical and terrestrial, but their marine relatives once occurred 500 million years ago. Unusual for their bizarre habit of shooting strings of glue at their prey, some (not all) velvet worms have placentas. That leads me to two questions, the answers to which I do not know. (1) Did retroviruses transfer this essential membrane-producing gene to the velvet worms, as they did for mammals? (2) Do the velvet worms that have a placenta also have a belly button?
To address the question whether retroviruses transferred the gene, researchers could examine whether the syncytin-2 gene occurs in velvet worms, and if so, determine whether the gene’s DNA in velvet worms matches that of the retroviruses? Finding a close DNA match for the syncytin-2 gene in both groups of organisms would be a strong case that the retroviruses are responsible. To determine whether they have a belly button, let’s get some velvet worms and scrutinize their bellies with a microscope.
Timothy A. Pearce, PhD, is the head of the mollusks section at Carnegie Museum of Natural History. Museum employees are encouraged to blog about their unique experiences and knowledge gained from working at the museum.