Writing for Quanta, Christie Wilcox delves into the introduction of DNA from one living thing to another by ‘horizontal transfer’:
It isn’t surprising, then, that herrings and smelts, two groups of fish that commonly roam the northernmost reaches of the Atlantic and Pacific Oceans, both make AFPs. But it is very surprising, even weird, that both fish do so with the same AFP gene — particularly since their ancestors diverged more than 250 million years ago and the gene is absent from all the other fish species related to them.
A March paper in Trends in Genetics holds the unorthodox explanation: The gene became part of the smelt genome through a direct horizontal transfer from a herring. It wasn’t through hybridization, because herring and smelt can’t crossbreed, as many failed attempts have shown. The herring gene made its way into the smelt genome outside the normal sexual channels…
The genes’ introns — stretches of noncoding DNA, which generally mutate faster than coding regions — are more than 95% identical. “The only conclusion we could come up with was that the gene was transferred horizontally,” she said.
But when the team tried to publish their findings, they faced significant pushback. “We sent it to journal after journal, and we really had to fight to get it in,” she recalled…
The disbelief in their findings was understandable because the barriers to horizontal transfer in eukaryotes looked insurmountable. Horizontal transfers are common and easy in bacteria, whose DNA is just within their cytoplasm. If a DNA fragment can make its way through a bacterium’s cell wall and membrane, there’s not much to bar its integration into the genome. But eukaryotes keep their genome cloistered inside a second barrier, the nucleus, and most of the time, their DNA is tightly condensed into chromosomes that limit the opportunities for splicing into the genome. Moreover, for a horizontal transfer to establish itself in a eukaryotic species, it can’t integrate into just any cell’s DNA; it needs to end up in a germ cell, be passed on to offspring and persist in the general population. That chain of events seemed wildly unlikely to many scientists.
Thing is, we don’t just have the potential to absorb genes from other mammals— apparently the DNA from any other organism that has DNA (which is most living things, possibly all) is up for grabs, as long as enough factors line up just right:
Successful conjugation or transduction requires vector compatibility between donor and recipient, which often depends on recognition of and interaction with recipient surface proteins (Thomas & Nielsen 2005). Nonetheless, conjugation between distantly related organisms such as bacteria and eukaryotes (plants and fungi) has been demonstrated experimentally (Heinemann & Sprague 1989); Agrobacterium tumefaciens, for example, can be induced to transfer its Ti plasmid to non-plant hosts including fungi and human cells in culture (Lacroix et al. 2006). Plasmids of the self-transmissible IncP-1 group have been isolated from both clinical and environmental (particularly wastewater) settings and harbour a wide range of resistance genes (Schlüter et al. 2007). These plasmids utilize mechanisms that allow their transfer, replication and maintenance in diverse Gram-negative hosts and can mobilize the transfer of other plasmids into an even wider range of target organisms. Under certain selective conditions, plasmids can expand their host range, often via a relatively small number of genetic changes (De Gelder et al. 2008). Broad-host-range phages are known as well: promiscuous immunoglobulin-like domains are found in three families of phages and can attach to a wide range of bacteria including Bacillus, Escherichia, Klebsiella, Lactobacillus and Staphylococcus (Fraser et al. 2006).
So, we didn’t just evolve by random mutations selected by environmental pressures, DNA has been smuggled in by pathogens and parasites over the past few billion years:
Although many studies have shown that HT in vertebrates is more common than previously thought, it is unclear how HT of homologous sequences could occur multiple times in distant species. One of the hypotheses is that pathogens might act as vectors capable of capturing host sequences and donating them to other hosts [17, 19]. This hypothesis is supported by several observations [41]; for instance, several of the transposable elements found in tetrapods have also been observed in several parasites [37, 39], such as trypanosomes. These parasitic protozoa cannot only capture but also donate sequences to their hosts: HT of trypanosome sequences has been reported in infected human beings, who vertically transmitted them to their children [42]. In the genus Drosophila, a parasitic mite appears to be responsible for the HT of P elements among different species [43]. Pathogens such as bacteria and viruses could also play a key role as HT vectors; for instance, Wolbachia (an intracellular parasitic bacteria that is horizontally transmitted) can donate genetic material to the nuclear genome of its insect and nematode hosts [44, 45]. Moreover, Wolbachia can be infected by bacteriophages, and it has been suggested that these viruses might mediate HT among intracellular bacteria [17]. Viruses might also mediate the HT of transposable elements from lepidopteran hosts to their parasitic wasps [19, 46].
In fact, viruses are potential HT vectors not only among vertebrates, but also in other eukaryotes and prokaryotes. The genomes of large DNA viruses, such as poxviruses, contain many genes derived from their bacterial and eukaryotic hosts [9, 10] (including transposable elements [47]). Poxviruses also seem to have changed hosts recurrently during their evolution [48], broadening their opportunities to transfer sequences among distantly related species. For instance, a transposable element related to snake sequences has been found in the genome of a taterapox virus isolated from a rodent [49]. Therefore, poxviruses may be good HT vectors due to their large genomes and low host and cell specificity. Other potential vectors are double-stranded RNA viruses, which are likely the donors of multiple sequences found in the nuclear genome of very diverse eukaryotes, including plants, fungi, protozoa, arthropods, and nematodes [50–52].