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New research shows that the immune system can stop hybridization within the same species, thus leading to the creation of new species

Apparently there are reproductive barriers that prevent the transfer of genes not only between different species, but also between couples of the same species. If we want to understand the source of nature's diversity, then it is important to understand how these barriers were created. A research group led by Detlef Weigel from the Max Planck Institute for Developmental Biology in Germany and Jeff Dangel from the University of North Carolina showed that an immune system that has 'lost the north' can establish reproductive barriers and in fact constitute the first step towards separating different species. The joint international research examined genetic incompatibility known as 'hybrid necrosis', using the model plant - Arabidopsis thaliana [1].

The new study, reported in PloS Biology, is based on the observation that when plants of different species are mated together, their offspring look very similar. Their growth is stunted, the leaves turn yellow and die, the tissue collapses and they usually do not survive long enough to produce flowers. This syndrome is called 'hybrid necrosis' [2]. "We suspected that hybrid necrosis is always caused by the same biochemical mechanism," explains Weigel, director of the Max Planck Institute.

To test the hypothesis, the scientists took 280 genetically different strains of Arabidopsis from around the world, and paired them with each other in 861 different combinations. Most of the hybrid plants were vigorous and grew normally. Twenty of the matings - two percent - resulted in the creation of small, sickly and necrotic offspring. Experiments that examined the genome of those plants showed that all hybrids have a similar profile of gene activity.

A common set of 1000 genes were more or less active in the hybrid offspring than they were in their healthy parents. Furthermore, a similar pattern of gene action exists in strong immune responses that are activated against pathogens [3] during normal infection. The plant's immune response usually involves the sacrifice of several cells in and around the infected area. In this way, the plant isolates the pathogen in dead tissue. In the weak hybrids, on the other hand, the healthy tissues also suffered - but without any sign of pathogenic infection. Apparently the hybrid plants decided that their cells were actually dangerous bacteria.

Although the genes that caused the abnormal autoimmune activity [4] were different in most pairings, the researchers found that often only two genes were needed to cause the hybrid necrotic response. One of the lethal genes came from the father, and the other came from the mother. In one case that the researchers looked into more deeply, they found that the function of the gene that causes necrosis in the hybrid offspring, but not in the parents, is to sense the presence of pathogens.

The scientists emphasize, however, that the hybrids are not victims of defective genes. Unlike many inherited diseases, the necrosis is not the result of each parent carrying a defective copy of the same gene. It is simply a case of a destructive interaction between two different genes, each of which has evolved differently in both parents. The genes, in themselves, are not harmful and even beneficial to the plant - and proof of this is the fact that both parents are healthy. Only the combination of the different genes creates problems. These cases of genetic incompatibility are known as 'Dobzhanshy-Muller incompatibilities' (Dobzhanshy-Muller incompatibilities) after the two giants of the young modern genetics science, who first studied the hybrid necroses in fruit flies.

The results of the German-American group challenge the classical definition of a species, according to which individuals of the same species can mate with any other individual of the same species and produce fertile offspring. Apparently there are barriers to free gene exchange even within the same species - after all, one out of every 50 matings in this study was unsuccessful. "The formation of new species must be understood as a gradual process, in which barriers within the same species keep increasing, until two groups can no longer produce fertile offspring," says Weigel.

This worldview is widely accepted today, but it is still not clear why such genetic barriers are created in the first place. What advantage does the plant have, when sometimes all the seeds from a certain hybrid die? The current study provides a possible explanation, according to which the genome of the plant changes due to stress from pathogens.

"The plant and the pathogen are locked in a race," says Dangel, a professor and expert in plant pathology genetics at the University of North Carolina. The pathogen constantly develops new strategies to attack the plants and evade their immune system. The plant in turn tries to be ready against all the possible 'weapons' of the various microbes. The well-armed plant may decide that a different and innocent version of a protein, originating from a distant relative, is actually a source of danger and attack the protein.

The scientists believe that the implications of their research are also valid for other species. Similar characteristics indicate that hybrid necrosis in crops such as wheat is caused by the same mechanism as in Arabidopsis. Dangel believes that Arabidopsis can be used to create a model that will help understand the phenomenon of hybrid necrosis in a more comprehensive way. "Such a model would be very useful for breeding, because genetic incompatibility prevents some of the hybrids that breeders would like to do," according to Dangel.

The results - according to which only a small number of genes are responsible for each case of hybrid necrosis - are particularly encouraging. It appears that only a small number of genetic changes are necessary to bypass the existing barriers within the species and achieve a new and sought-after combination of genetic traits. On the other hand, it turns out that all that is necessary to create a new species is a minimal change in the genome, which is enough to stop the free exchange of genes between relatives.

Links:
The press release of the Max Planck Institute

[1] Arabidopsis thaliana is a model plant that is used for many biological studies, similar to mice or fruit flies.
[2] Necrosis from the Latin word 'necro' which means death. Necrosis means death of a tissue or cell.
[3] Pathogen - creates a disease - bacterium, virus, fungus, etc.
[4] Autoimmune activity - an abnormal condition, in which the body's immune system attacks the body itself.