The analysis of genomes of different mammals showed that, although they all have a similar catalog of genes, they can be ordered differently and turned on and off differently. proverbs chromosomal rearrangements affect the function and gene regulationand also contribute to defining the identity of the species, although its origin was unknown until now.

A study conducted by the Autonomous University of Barcelona (UAB) and the kent university (United Kingdom) shows that the formation of male germ cells is the key to determining which regions of the genome are rearranged within and between chromosomes during evolution.

“The main objective of this study is, ultimately, to understand the origin of the biodiversity that surrounds us. All species that inhabit the planet share genes, which can be arranged differently in each species, in the form of chromosomes”, he explains to SINC. Aurora Ruiz Herreraresearcher at the Institute of Biotechnology and Biomedicine (IBB) from UAB and lead author.

The main objective of this study is to understand the origin of the biodiversity that surrounds us. All species that inhabit the planet share genes, which can be organized differently in each of them, in the form of chromosomes.

Aurora Ruiz-Herrera (UAB)

“In nature, the diversity of species that we find is also reflected in a different number of chromosomes. Humans, for example, have 46 chromosomes, but the mouse has 40 or the rhino is characterized by 86 chromosomes. So there are a wide variety of ways to package the genome inside cells,” he adds.

“Like everyone mammals we share a common ancestor, during the evolution of the species there were rearrangements of specific genes in each taxonomic group that can expose the diversity in number and structure of chromosomes that we detect today in current species”, highlights Ruiz-Herrera.

And these rearrangements take place in the germ line – during the formation of eggs and sperm – so they can be passed on to succeeding generations, to offspring. Specifically, they are associated with specific physical and biochemical processes in the final stages of food production. gametes males, after completion of meiotic cell divisions.

Understand species diversity

the entire sequence of DNA of an individual is packaged into a specifically adapted dynamic three-dimensional (3D) structure, the chromatininside the cell nucleus. This arrangement determines which genes are ‘on’ and which are ‘off’ in each cell type and also occurs in gametes.

The oocytes s sperm occur in any organism with sexual reproduction through the meiosis. This process involves a round of genome replication followed by two consecutive cell divisions to generate haploid cells (gametes), which carry a single copy of each chromosome.

These results indicate that the process of sperm formation is an important factor in the evolution of the species genome.

During meiosis, genes are “shuffled” between the chromosome copies inherited from the mother and father, a process known as meiotic recombination. These complex events occur while the genome is being packaged in a highly precise and regulated manner.

As Ruiz-Herrera explains, “The dynamics of chromatin remodeling during the formation of male gametes is essential for understanding which regions of the genome are located close together within the nucleus and therefore are more likely to be involved in chromosomal rearrangements at different times in spermatogenesis.

These results indicate that the process of sperm formation is an important factor in the evolution of the species genome. Determining which genomic regions are affected and at what point in sperm formation is important to understand species diversity,” says the researcher.

Study in detail the evolution of the genome

The team compared the genomes of 13 different rodent species and deciphered the rearrangements that distinguish them. Although the study was conducted in rodents, spermatogenesis is a highly conserved process, and so this principle likely applies to other species, the researchers note.

“This made it possible to elaborate the genome configuration of the common ancestor of these rodents and to determine the location of the genomic regions that participate in the rearrangements”, he emphasizes. Marta FarreProfessor of Genomics in the School of Biosciences at the University of Kent and co-director of the study.

On the other hand, Peter Ellisalso a researcher at Kent and co-director of the study, points out that “the genomic regions involved in rearrangements are generally activated in the later stages of spermatogenesis, when the developing male germ cells are called spermatids. We found that the evolutionary rearrangements that occur in genomic regions are physically close together in the nucleus of these cells,” she says.

The team compared the genomes of 13 different rodent species. However, as spermatogenesis is a highly conserved process, this principle is likely to apply to other species.

Furthermore, the genomic regions involved in evolutionary rearrangements are not associated with meiotic recombination hotspots, indicating that they probably do not occur during meiosis. Instead, they correlated with the location of DNA damage later in the process.

“We show that spermatids retain a ‘memory’ of previous genomic configurations. There are regions of DNA that used to be part of a single chromosome in the rodent ancestor, but are now found on different chromosomes in the mouse genome, but remain in physical contact within the nucleus of the developing sperm (the spermatids),” says Farré.

Differences between males and females

Spermatids are cells that are in the final stage of sperm development, since the cell division, and the events that occur during this process are male-specific. This implies that males and females are not equal in their impact on genome evolution.

“Of all the rearrangements that distinguish a mouse from a rat, a squirrel or a rabbit, most appear to have arisen in a sperm rather than an egg. This demonstrates that the male germ line is the general engine of the structural evolution of the genome”, points out Ellis.

The authors propose that differential events that occur during egg and sperm formation could explain the results. Spermatozoa undergo a process of DNA compaction to adapt to a very small cell volume, located in the head, something that does not occur in eggs (a larger cell in comparison).

Germline research can help us solve this mystery: determine which genomic regions are affected and at what time during sperm formation.

Aurora Ruiz-Herrera (UAB)

This compression can cause DNA breaks, which are usually repaired by a mechanism capable of generating errors. Some of these errors can result in genomic rearrangements, which would explain why the process of sperm formation is a critical factor in the evolution of the species’ genome.

A still little known aspect is why some species have very stable genomes with few rearrangements, while others have undergone multiple chromosomal alterations. “Germline research can help us solve this mystery: determine which genomic regions are affected and at what time during sperm formation,” concludes Ruiz-Herrera.

Reference:

Alvarez-González et al. 3D chromatin remodeling in the germline modulates the evolutionary plasticity of the genome. nature Communications, May 2022, https://doi.org/10.1038/s41467-022-30296-6

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