6.3.1. Recombinational Hotspots as a Factor for Genome Stability and Evolution
DNA recombination begins with programmed DSBs mediated by an endonuclease called SPO11. This protein is highly conserved across eukaryotic evolution [
85,
107]. Along with SPO11, other proteins are involved in the control and formation of DSBs, such as PRDM9, MEI4, REC114, or HORMAD1 in mice [
106,
107,
112]. Due to its importance, this process is highly regulated throughout the meiotic cell cycle, particularly at leptotene and prophase I. Breaks are concentrated at the called recombination hotspots, where SPO11 and other factors are involved in the formation and repair of DNA breaks in the early stages of meiosis, and it’s highly regulated including total number and genome distribution of DNA breaks [
6]. Recombination hotspots have been found in localized short regions of hundreds to thousands of base pairs in many species including yeast, plants, and all vertebrates studied to date [
113]. However, in some species such as nematodes, the recombination landscape appears more uniform and lacks these hotspots [
114]. It has been described between 150 meiotic DSBs per cell in humans and around 200-300 in mice [
115,
116]. It is known that human hotspots are normally 1–2 kb in size, spaced 50–100 kb apart, accounting for no more than 20% of the genome, and only ~5% to 30% (depending on the organism) go on to produce COs [
117]. Centromeres are normally recombination deserts, whereas in (sub)telomeres recombination rates increase. Mirroring what has been described in mice [
118], hotspots are localized in genic and intergenic regions likewise. This suppression of recombination around centromeres has been described as not related with the presence of the heterochromatic domains, which are typically associated with satellite DNA in this area, at least in horse spermatocytes [
119]. Some studies also unveil the importance of specific DNA motifs and epigenetic modifications in determining recombination hotspots [
120,
121].
6.3.2. Factors and Complexes Defines Recombinational Hotspots
As previously mentioned, the way genomes are packaged is likely another factor influencing species-specific differences in recombination rates. The mean number of DSBs per cell is influenced by DNA compaction through SC length and DNA loop size (
Figure 4E). There is a close interplay between SC assembly, the organization of DNA loops, and the formation of DSBs [
86,
122]. It has been described that chromosomes with longer DNA loops and shorter SC axes showed a reduced number of DSBs in early prophase I. An explanation could be that shorter SC axes could anchor less DNA loops, but longer, providing less scaffold for the formation of DSBs and recombination rates. Likewise, loop sizes were inversely correlated with CO density [
6,
86,
123] (
Figure 4E).
Recent studies have highlighted the importance of certain factors in shaping genomic architecture and subsequently, evolution. Cohesin-mediated DNA loops are organized along the SC axial scaffold, anchoring as long DNA loops and establishing a physical first level that defines hotspots [
93,
124] (
Figure 4D). Cohesin complexes, HORMADs, and SC proteins correlate with these recombination hotspots, as reflected by the interactions observed at shorter distances (2.5–4.5 Mbp). In this sense, a recent paper found that a majority of the cohesin complex during spermatogenesis is localized within promoter regions of genes located in DNA loops away from the axes [
104] (
Figure 4C). In this regard, meiotic-specific cohesin provides an integrated structural and functional framework for the 3D organization in germ cells and spermatocytes in mice, manifesting a fine-tuned balance among recombination, chromatin remodeling, architectural proteins, and gene expression during spermatogenesis [
104].
Among species with recombination hotspots, locations are specified by PRDM9 (PR domain 9) binding. PRDM9 is a specific early meiosis protein expressed only in testes and ovaries [
83]. This protein presents a zinc finger (ZnF) motif which recognizes specific DNA sequences in the genome and adds H3K4me3 and H3K36me3 marks at nearby nucleosomes close to DSBs in early meiosis, recruiting the DSB repair machinery and SPO-11 in early stages of meiosis and determining recombination hotspots [
125,
126]. In terms of evolution, PRDM9-ZnF motif recognizes a high variability of DNA motifs, which modification can be translated into a redistribution of recombination hotspots [
125]. In this sense, different distributions of DSBs have been described in humans with different PRDM9 alleles [
127] and in mice [
128], providing new clues on a factor that generates new DSB sites, which could be a source of genetic instability and indirectly have important consequences in evolution. PRDM9 is not found in all vertebrates, it is absent in certain birds, canids, and fishes, however, orthologs of PRDM9 have been identified in some of these groups [
8]. Interestingly, mammals with PRDM9 have the fastest evolving zinc fingers (ZF) array [
117,
129]. Then, these mammals generate novel sets of hotspots leading to rapid turnover in the recombination landscape between populations and species, and thus having a role in the evolution rate. The selection pressure to explain the fast evolution of the PRDM9 ZF remains unclear. It could be explained as an assurance of recombination hotspots and to avoid inefficient DSB repair leading to reduced fertility or sterility [
128,
130].
On the other hand, in species without PRDM9, the rapid evolution of recombination hotspots is not seen. For example, species of birds lacking an ortholog of PRDM9, the locations of recombination hotspots are conserved over long evolutionary time scales, reducing evolution over time [
8]. Species that do not use PRDM9 and have more stable recombination locations tend to direct recombination in promoter-like features, as opposed to species that use PRDM9 and experience rapid turnover of hotspot landscapes, which tend to recombine away from promoters and coding-protein sequences, even in some subfamilies of transposable elements in mammals by ZF domain recognition [
131]. These different dynamics have important consequences for genome structure and, therefore, evolution [
132,
133]. As expected, recombination hotspots have higher rates of point mutations, insertions, deletions, and translocations. Therefore, directing recombination to non-coding regions might be an advantage in avoiding genome instability. In this regard, recombination at transcription-related loci could uncouple coding regions and its regulatory elements, leading to transcriptional regulation of genes or negative epistasis [
134].
The normal progression of the meiotic cell cycle involves reshuffling of the genome, which is mediated by programmed DSBs and provides diversity among organisms with new combinations of alleles on which natural selection can act. However, sometimes large-scale genomic changes, such as inversions, translocations, fusions, and fissions, occur, leading to infertility or providing new sources of variation on which natural selection can act and pushing speciation. Several genetic and mechanistic factors, such as PRDM9 in mammals [
126], affect the recombination landscape and the combinatorial effect of chromosomal rearrangements, leading to speciation.
On the other hand, chromosomal rearrangements can play an important role in further modifying both the structure and regulation of genes located near the affected regions, as proposed by the
“suppressed recombination
” model [
135]. This is because recombination is suppressed within rearranged segments, inversions, or chromosomal fusions, resulting in a reduction of COs within these regions [
136]. As a result, there is a reduction in gene flow around reorganized genomic regions, and chromosomal reorganization could lead to a combination of alleles, polymorphisms, or haplotypes that provide a certain degree of selective advantage or accumulate genetic incompatibilities, ultimately contributing to evolution and species divergence in the long term. As a result of chromosome reorganizations, there is an alteration of the recombination maps, leading to genetic differentiation across genomes, referred to as “genomic islands of divergence” [
137,
138]. Evidence for this has been described in mammals, such as between human and macaque lineages, where recombination patterns have undergone rearrangements that could be favouring adaptive alleles in the immune response [
139]. Other studies have described similar reductions in recombination landscapes in mice due to Robertsonian translocations, also related to alterations in epigenetic signatures for heterochromatinization [
135,
140].
Future comparative research on recombination hotspot landscapes, speciation genes, and different chromosomal rearrangements, and how these changes in chromosome structures affect recombination, should be conducted to enhance our understanding of the main pathways for genome reshuffling and evolution.
6.3.4. Defects in Meiotic Recombination Leads to Chromosome Rearrangements
While it has been reported that disruptions of chromosome architecture such as inversions, fusions, or translocations are associated with genetic instability and cancer due to oncogene activation and novel gene functions, there is a parallelism in organism evolution due to intra or interchromosomal alterations in the germ line or during the meiosis program, which can alter normal segregation patterns and contribute to evolution [
8,
107]. Otherwise, failures in the meiotic program can also lead to aneuploidy, infertility, genetic abnormalities, and congenital disease [
143,
144].
One chromosomal rearrangement that affects the recombination landscape and is linked to miscarriages, infertility, or aneuploid descendance in humans and pushes evolution is the Robertsonian (Rb) fusions or translocations, the most common balanced chromosomal rearrangement in nature. Rb translocations typically involve two acrocentric chromosomes, homologous or non-homologous, and result in the fusion of both long arms to form a single metacentric chromosome, plus the loss of both short arms. Aneuploidy and loss of genetic information are responsible for miscarriages and aneuploid offspring in humans. However, recent findings indicate that the situation is not as straightforward, and Rb fusions may also have an impact on the structure of the genome in germ cells, which could lead to functional and evolutionary consequences [
145]. Indeed, Rb fusions can significantly change the 3D arrangement of chromatin in spermatocytes, potentially leading to new interactions between domains and exposing them to different regulatory environments that may impact gene expression and regulation, having implications for both fertility and evolution [
109]. Additionally, the redistribution of COs across chromosomal arms in Rb mice was consistent with the “suppressed recombination” model, showing low recombination rates at Rb fusions, also affecting both chromosomal axis length and recombination [
145].
This results in the existence of a gametic barrier due to the fixation of chromosomal rearrangements in a population, which allows for a genetic distinction between species. Genome sequencing analysis has indicated that these evolutionary rearrangements are much more numerous than initially estimated [
146,
147]. An example can be seen in the human and chimpanzee genomes, which were initially estimated to differ only in 9 chromosomal inversions and one fusion in 1980 [
148], but later, in 2005, 93 supplementary evolutionary rearrangements were described, ranging from 12 kb to 1 Mb [
149]. In this sense, more than 245 large rearrangements including translocations, inversions, fusions, and deletions have been identified in the divergence between mice and humans [
150], and several duplications involved in human-specific adaptive qualities have been recently characterized between the human and non-human primate genome [
151].
Microchromosomes, or chromosomes that are smaller in size than a typical chromosome, refer to chromosomes that have undergone structural changes, such as deletions, duplications, or inversions, resulting in a smaller size than normal, and sometimes a proper NE will form around [
61]. These changes can lead to a loss of genetic material. Micro chromosomes are not found in humans but are highly express in normal cells from birds and reptiles.
Bird and reptile karyotypes are striking in their differences, as in addition of normal size mammalian chromosomes, they are generally characterized by the presence of well-conserved tiny microchromosomes, but little is known about the progression of meiosis in them. These are gene-rich and highly conserved between species of birds and reptiles. It seems that levels of DNA double-strand break (DSB) formation in reptiles are lower compared to mammals, suggesting that low recombination rates are a distinctive feature of reptiles [
152,
153]. Oppositely, micronuclei tend to clump into a central compartment at interphase and during mitosis and meiosis, leading to stronger inter-chromosomal interactions between microchromosomes rather than macrochromosomes, probably facilitating homology search and DSB formation that is maintained even in germ cells [
153]. This suggests functional coherence. Many translocations of micronuclei and fusions with each other or with larger chromosomes have taken place independently in different clades such as turtles, snakes, and lizards, or others have been lost, leading to diversity and speciation. In mammals, they have completely disappeared. The work by Waters and colleagues [
153] has shed light on the contribution of micronuclei in evolution by comparing the genomes of different birds and reptiles, as well as mammals and an amphioxus, providing evidence that microchromosomes are highly conserved ancient animal chromosomes, whereas macrochromosomes have undergone multiple lineage-specific rearrangements, especially in mammals, probably due to its higher level of DSB formation.
Beyond rearrangements of genomic segments throughout evolution, large-scale genomic duplication may occur, but more rarely, involving a macroscopic portion of the genome or even, the complete genome, also called Whole Genome Duplications (WGDs).
WGD has a significant impact on an organism’s physical characteristics, and it is highly likely that it causes infertility and reduces the organism’s fitness, which can negatively impact the organism’s short-term survive [
154]. Therefore, most WGD events do not result in viable outcomes, and they do not become fixed in the population. However, under specific conditions, WGD can provide an immediate evolutionary advantage in extreme environmental conditions [
155,
156]. It is now understood that WGD has played a significant role in evolution. WGD can reduce the risk of extinction for the affected lineage by alleviating its intrinsic disadvantages, and it can also help boost the biological fitness of the organism or create new species. This is because ancient WGDs often occurred during periods of environmental turmoil or extinction events [
156,
157]. Such advantages could explain why so many cancers present one to several rounds of WGD [
11,
154,
156].
While WGD events are relatively common in plants, such as Arabidopsis, and have also been observed in the yeast Saccharomyces [
158,
159], identifying WGD pairs or quartets in vertebrates is more difficult. It is by now widely accepted that two rounds of WGD happened at the origin of the vertebrate lineage about 500–550 millions of years ago [
160,
161] (
Figure 2). However, how these global-scale WGD events affected the gene regulatory network is not yet fully understood. It has been proposed that a main vertebrate WGD played a crucial role in the organization of multicellularity by influencing the interaction of proteins, the selection and retention of genes, and the balance of gene dosage [
162].
Many tumors present rounds of WGD, but often stabilized to a near 3N karyotype [
11]. Some genes are selected to offer advantages. Interestingly, genes that have undergone duplication are three times more likely to be involved in cancer and autosomal dominant diseases, as they are more susceptible to dominant deleterious mutations [
163]. Duplicated genes have stricter requirements for maintaining proper dosage balance [
164], are normally involved in signaling, development, and transcriptional regulation and they are represented in gene ontology categories associated with organism’s level of complexity [
163,
165,
166]. Therefore, duplicated genes resulting from stabilized WGD tend to be essential, evolutionarily conserved genes, and are related to increased redundancy and complexity, allowing for an advantage under stressful or disturbed environments compared to non-WGD organisms [
160,
165]. This parallelism between WGD in evolution and the high incidence of WGD in cancer, both strive for increasing fitness to overcome non WGD. But it is also possible that the high incidence of cancer in human population due to WGD is caused by adverse and polluted environmental conditions. However, further analysis is needed to fully understand the molecular mechanisms responsible for the advantage of WGD genomes over non-duplicated ones in cancer or evolution.
Traditionally, studies on evolution have treated genome alterations as a result of many incremental single steps. However, recent research has discovered that genomes can be altered in a single, large catastrophic event, leading to complex genomic rearrangements. This phenomenon is called chromothripsis [
14]. Chromothripsis occurs in a single cell and results in the breaking and rearranging of one or multiple segments of chromosomes in a random manner (
Figure 1). It was initially discovered in tumors, but it has also been identified in individuals with congenital malformations, developmental disorders, or seemingly balanced chromosomal rearrangements [
167,
168]. However, it has also been described in healthy individuals [
169]. Furthermore, chromothripsis may also have a role in evolution as a source of new genetic combinations. Different hypotheses have been proposed, such as the idea that derived chromosomal rearrangements can restrict gene flow through suppression of recombination, or facilitate rapid changes in patterns of genes unrelated to recombination suppression. Chromothripsis could also lead to the formation of new gene linkage blocks and new chimeric genes, as well as disruption of the cis-regulatory machinery for gene expression [
170]. One proposed explanation for the connection between the various causes of chromothripsis and the limited scope of genomic changes it causes is that the affected chromosome(s) may be placed in a micronucleus [
61,
171]. The formation of micronuclei can happen due to a failure in chromosome segregation, but also can be caused by various types of stress during any stage of the cell cycle, and it can persist for multiple cell cycles before being eliminated or incorporated back into the main nucleus [
171].
6.3.5. Repetitive DNA Elements as Contributors for Chromosome Evolution and Speciation
To fully understand how mammalian genomes are shaped and how speciation occurs, it is important to consider the structural organization of genomes. Research has shown that certain breakpoint regions tend to cluster in specific genomic features, such as duplications or repetitive elements [
172,
173], tandem repeats [
174,
175], and transposable elements [
174,
176]. Changes in the presence or length of these DNA elements can alter the chromatin state, which in turn can affect gene expression [
3,
139]. In an interesting paper published by Capilla et al. [
6] on rodents, a family characterized by high chromosomal variability between clades, they found an association between evolutionary breakpoint regions and active chromatin state landscapes.
A repetitive DNA element that plays a role in driving genome evolution is the large tandem repeat known as satellite DNA (satDNA) [
177]. satDNA has also been linked to various genetic disorders, such as cancer and developmental abnormalities [
178]. The presence of satDNA remains a prevalent concern for various species, including both vertebrates and invertebrates. To establish comparative studies of genome divergences, satDNA is still an ongoing issue in sequencing techniques due to genome assembly and annotation. Recent studies have shown that satDNA can vary in abundance, size, and organization within a single genome, and it can also differ considerably among closely related species [
177]. This dynamic behaviour has led to speculations about the role satDNA plays in genome plasticity and chromosome rearrangements. Some studies suggest that satDNA could act as a source of genetic variation and evolution, through mechanisms such as non-identical CO, replication errors, and non-allelic homologous recombination. These mechanisms could result in changes in number and reorganization of chromosomes and genetic diversity [
177,
179].
Transposable elements, also known as transposons, play a significant role in chromosome evolution and speciation. They can act as drivers of DNA breaks and changes in chromatin conformation, both of which can compromise genomic stability [
180]. However, they can also promote adaptability in a population through changes in gene expression or rapid chromosome restructuring [
181,
182]. An example of this is the gibbon genome, where the insertion of the retro-transposon LAVA in genes involved in cell cycle progression and chromosome segregation has led to a high rate of chromothripsis-related rearrangements and the emergence of different gibbon lineages with highly rearranged chromosomes [
183,
184]. Furthermore, ZF domain from PRDM9 tends to recognise transposable elements in mammals. However, further research is needed to completely understand the role of repetitive DNA elements in the evolution of genomes [
185].