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Microscopic Heat-Resistant Fungus Proved To Be The Champion Of Mutations

Microscopic Heat-Resistant Fungus Proved To Be The Champion Of Mutations

The rate of accumulation of mutations in the fungus Neurospora thick (Neurospora crassa) was two orders of magnitude higher than in any cellular organism, according to BMC Genome Biology. 96-98 percent of mutations are associated with mobile genetic elements. However, the mechanism of their appearance was initially associated with repetitions of DNA sections that can occur during sexual reproduction, so the neuro-spore's genes also often mutate-and this does not always go to the body's benefit.

Neuro-spore thick (Neurospora crassa; often referred to simply as neuro-spore) is a widespread microscopic fungus from the group of Ascomycetes. The fungus survives on nutrient-poor environments, and its ascospores (spores of sexual reproduction) germinate if at least an hour is spent at a temperature of 65 degrees Celsius — the cells of other organisms die at this temperature. For this reason, neuro-spores often populate cultures of various bacteria and fungi, and microbiologists can't get them out of there for a long time.

On the other hand, the peculiarities of reproduction of N. crassa make it a convenient object for geneticists: it is a haploid organism (that is, each gene is normally represented by a single copy), it willingly reproduces asexually (in this case, the descendants, in theory, do not differ from their parents), but with a lack of some nutrients, it goes to sexual reproduction. In this case, a part of the cells divides, the DNA from the same chromosomes, and the neuro-spores have 7 of them, exchange similar sites, as a result, usually, eight ascospores are formed. They lie in the ascospore (bag) one after the other, and since the neuro-spore is a single-cell-thick thread, it is easy to track where any ascospore moves during formation.

If recombination results in a pair of identical DNA fragments on the same chromosome, the neuro-spore initiates mutations in both of these fragments and thus neutralizes them. This is called repeat-induced point (RIP) mutation — point mutations induced by repeats. In addition to the genes of the neuro-spore itself, such mutations could be used to deal with mobile genetic elements (MGE) — sections of DNA that can move through the genome (and even pass from one organism to another) and thus sometimes disable normal genes. But until now, it was not known whether the neuro-spore uses RIP mutations to get rid of mobile genetic elements.

Geneticists from Nanjing University, led By Sihai Yang, in collaboration with Laurence Hurst from the University of Bath, decided to find out by crossing five neuro-spore lines and sequencing the genomes of parent organisms and their descendants. A total of 273 ascospore genomes were read. The average coverage (the number of times that a single genome nucleotide got into a fragment of the last one) was 37, and 96-97 percent of the DNA sequences of each individual ascospore were recovered. The researchers looked at which sections of DNA mutated from generation to generation: their own neuro-spore genes or mobile genetic elements. They also measured the mutation load — the number of DNA changes that potentially reduce the body's fitness.

During sexual reproduction, N. crassa had an average of 136.6 mutations per genome per generation. If we take into account the length of the neuro-spore genome, this is 3.38 × 10-6 mutations per pair of nucleotides per generation. More often, mutations occur only in viruses, and in cellular organisms, the same value is two orders of magnitude lower. Most of the changes in DNA are related to mobile genetic elements and are similar to RIP mutations-it finds repeats in such elements, changes one nucleotide to another, and as a result puts the MGE out of action. However, there are many mutations (5.34 per genome per generation) that reduce the fitness of neuro-spores-serve as a mutational load.

Thus, neuro-spore uses the mechanism of point mutations induced by repeats to combat mobile genetic elements. However, this affects not only repeats in the MGE, but also in the genes of the Neurospora crassa itself, so their sequences also change, and this is not always beneficial for the body. It turns out that the neuro-spore is an exception to the general rule that the rate of accumulation of mutations in the genome should be minimal (as long as it does not harm adaptation to existing conditions).

Probably, mobile genetic elements are former stationary sections of DNA that at some point "escaped" from their cells. The same thing can happen to whole cells, for example, a transmissible venereal tumor of dogs has become, in fact, an independent single-celled organism; although it appeared 6.2 thousand years ago as any other tumor in the body of some canine. It is interesting that now it is almost not evolving.