On the trail of an astonishing exchange of genetic information: Researchers have demonstrated horizontal chromosome transfer in insect-infecting fungi. An entire genetic material is exchanged between individuals in an asexual manner. The system is apparently similar to horizontal gene transfer in bacteria: By transferring the chromosome, the fungi can apparently acquire genetic programs that make it easier for them to infect their victims. This concept may play an important role in the adaptability of some fungi, the scientists say.
For us and other higher living beings, vertical transfer is the rule: genetic material is passed on from parents to the next generation. In addition to this form of transmission, there is also another form of transmission in nature: With horizontal gene transfer, genetic material is passed on between different, even non-specific, individuals of the same generation. This system is particularly common – and notorious – among bacteria. In their case, horizontal gene transfer occurs through the transfer of a ring of genetic material – a so-called plasmid. In this way, the single-celled organisms can quickly acquire useful gene programs from other microbes. For example, bacterial pathogens use this concept to spread genes among themselves that give them resistance to antibiotics.
Horizontal gene transfer in eukaryotes
However, horizontal gene transfer is rare in eukaryotic organisms, which include animals, plants and fungi. This is due, among other things, to the fact that in these organisms, in contrast to bacterial prokaryotes, the genetic material is present in a cell nucleus. It is also packed there in genetic carriers – the chromosomes. Nevertheless, some cases of horizontal gene transfer in eukaryotes are already known. In most cases, certain genetic sequences from other organisms were “collected” and firmly integrated into the genome. In fungi, however, there have already been indications that horizontal transfer of an entire chromosome can occur under certain circumstances. However, so far this has only been shown under special laboratory conditions. The researchers led by Michael Habig from the University of Kiel have now demonstrated the phenomenon under natural conditions and have gained initial insights into its function.
They discovered horizontal chromosome transfer by chance through genetic studies of fungi that are used in biological crop protection: various members of the genus Metarhizium attack insects and are therefore specifically released to kill certain pests.
It is noteworthy that the victims apparently have difficulty developing lasting resistance to the fungal pathogens. The opponents are probably engaged in an evolutionary arms competition based on genetic changes. To shed light on the mechanisms, Habig and his colleagues conducted studies on different strains of the fungus Metarhizium robertsii, which is used to combat an invasive ant species in South America.
An entire chromosome is transferred
As part of the study, the team examined the genomes of various strains of the fungus to determine their infection potential against ants. When the genetic data was analyzed, the exciting discovery came about: It turned out that a single chromosome was very frequently exchanged horizontally between two different strains. Since the Metarhizium fungi only reproduce asexually, this cannot have been a vertical transmission. “The analysis of the genetic information of the fungal strains shows that a single chromosome was independently transferred a total of five times in the course of the co-infection experiments, but no further transfer of genetic information from one strain to another via horizontal transfer took place,” says Habig. Further investigations also provided evidence that the same chromosome can also be found in the distantly related fungal species Metarhizium guizhouense, which also attacks insects. “The chromosome was passed on naturally between different fungal species,” says Habig.
Insect killers shift weapons to each other
As the researchers explain, this special genetic carrier could have a function that is similar to that of plasmids in bacteria. “The experiments showed that under certain conditions the fungi that had received the chromosome had competitive advantages over fungi of the same strain that had not received the chromosome and were able to prevail against them,” says Habig. Specifically, the scientists found evidence that the chromosome carries genetic programs that are important for successful insect infestation. In addition, there could also be genes on it that are linked to the horizontal transferability of the chromosome, say the researchers.
The team is now on the trail of an exciting aspect in the genome evolution of fungi: It is possible that these eukaryotes can use bacteria-like mechanisms of rapid evolutionary adaptation, for example to increase their virulence or harmfulness for their host organism and to share genetic information across species boundaries transmitted. That’s why the researchers are now planning to explore the significance of this system in more detail through further investigations.
Source: Christian Albrechts University of Kiel, specialist article: PNAS, doi: 10.1073/pnas.2316284121