Evolutionary pressures lead to some very complex host interactions.
We are now - often horrifyingly - watching what happens to the virus and its hosts in an evolutionary arms race. Measures to reduce infection and increase immunity are chosen for viral strains that spread more easily and inhibit at least some of the immune responses. All of this can be easily explained by the theory of evolution and is modeled mathematically.
But not all evolutionary interactions are uniform and binary. Thursday's edition of Science described a three-way struggle between butterflies, the bees that infect them, and the viruses that can infect both species. Calling the resulting reactions "complex" is trivial.
Meet the Warriors
One of the participating groups is Lepidoptera, Butterflies and Butterflies. They seem to be the victims of this story because, like everyone else, they can catch viruses. Many of these viral infections can be fatal, although some kill the animal quickly and others take time. Because they often infect the larval/larval stages, viruses need other hosts to transmit the virus to other victims. Butterflies bees lay their eggs on the worms, and the larvae that hatch simply begin to eat the worms to keep them alive.
This situation creates complex matches. For example, some viruses may depend on the bees to be transferred to a new host, but once they are there, the bees compete for the unlucky worm cells. However, worms are not completely defenseless, and some are able to respond immune to the virus. It appears that some strains can resist infestations of bee larvae. However, viruses often encode proteins that reduce the immune response in their favour, which in turn benefits their competition in cells. That a type of bee can parasitize on certain lipids, but if the worms are also infected with a certain virus, this action will be blocked. The virus enters the worms when they eat the leaves and does not depend on the bees to transmit them. Inactivating the bees costs nothing to the virus and saves the most prey.Advertising
However, the virus killed the host and could not stop any parasitic bees. Researchers have found that susceptible bee larvae are actually killed. Or rather, something in the cell worms of bee larvae forced them to commit suicide on a regular basis, called apoptosis. In any case, the research team was able to establish that the killing was done by an agent dissolved in the worm's internal fluids. Parasite killing agent, or PKF. The researchers obtained some of the protein's amino acid sequences that allowed them to identify the gene encoding it in the virus' genome. Few viruses carry more than one gene, but the PKF genes were not limited to viruses. Alternatively, many Lepidopteran species also carried, with some species carrying multiple copies. The characteristics of these genes indicate that they are derived from viruses through random transfer of DNA. (Some viruses may also have selected genes from their hosts.) For some PKFs/bee species, the larvae died. In other cases, development slowed down or stopped. In the other compounds, PKF had no effect on larval survival.
In at least one case, bees carried a virus that did not infect the worms by eating them—instead, it appeared to be dependent on the bees. Its transmission is of course the bee immune to PKF. But this PKF interferes with the development of bee species that may compete for worms. At the same time, the virus competes with the bee for the worm. There is no doubt that Lepidopteran species carry PKFs that successfully prevent bees from parasitizing them.
None of this has been fixed. While the researchers describe a complex picture at some point, changes in host capacity, gene transfer, and diversity in the PKF gene family will persist into the future. If one examines things in the next half a million years, the situation may be more complex than it is today.
Science, 2021. DOI: 10.1126/science.abb6396 (About DOIs). p>
Evolutionary chaos like butterflies, bees and viruses have a triple war
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