The effect on fertility of mutations in genes employed in meiotic recombinational repair and the increased expression of these genes in testes
The origin of meiosis and its adaptive function in eukaryotes, and the related problem of the origin and adaptive function of sex in eukaryotes, are fundamental issues in biology. Among eukaryotes, meiosis and sexual reproduction are widespread, occurring in single-celled eukaryotes (including protozoans such as paramecium), and fungi (e.g. yeast) and in most multicellular organisms including animals and most plants. Accumulating evidence indicates that meiosis arose very early in the evolution of eukaryotes (reviewed in Bernstein & Bernstein, 2010). Thus, basic features of meiosis were likely already present in the prokaryotic ancestors of eukaryotes.
Central features of meiosis are the pairing of homologous chromosomes of different parental origin, recombination (information exchange) between these chromosomes, and the passage of the recombined chromosomes to progeny. In bacteria, the sexual process of transformation has these same essential features (Michod et al., 2008). In a further parallel, key enzymes that catalyze meiotic recombination are homologous to enzymes that serve similar functions in the recombinational steps of transformation. The earliest eukaryotic organisms were single-celled protists, similar to bacteria. In this chapter we review the evidence that meiosis in the earliest single-celled eukaryotes evolved from the sexual process of transformation in their bacterial ancestors.
Among extant organisms, both single-celled bacteria and single-celled eukaryotes tend to enter the sexual cycle under conditions of environmental stress (Bernstein & Bernstein, 2010). These are conditions that can cause DNA damage. DNA damage appears to be an important fundamental problem for all organisms. We review evidence here that DNA damaging agents induce sex in prokaryotes and microbial eukaryotes. We summarize evidence in bacterial transformation and in eukaryotic meiosis that recombination serves the adaptive function of removing DNA damages that are potentially lethal to progeny (see also Bernstein & Bernstein, 2004; Bernstein et al., 2011).
Thus in this chapter we explore the reasoned likelihood that meiotic recombination arose from bacterial transformation and that both transformation and meiosis are adaptations for repairing damage in the DNA to be passed on to progeny.
2. The common ancestor of all eukaryotes was likely capable of meiosis
Eukaryotes emerged from prokaryotic ancestors more than 1.5 billion years ago (Javaux et al., 2001). The oldest taxonomically resolved eukaryote in the fossil record,
Thus, the common intestinal parasite
Another group of eukaryotes, commonly referred to as a member of the “ancient asexuals,” the arbuscular mycorrhizal fungi, were thought to have propagated colonally for over 500 million years. However, recently, several members of this group (
Thus, the common ancestor of current day eukaryotes was likely capable of meiosis.
3. Natural bacterial transformation as a form of sex
Natural bacterial transformation involves the transfer of DNA from one bacterium to another through the surrounding medium. Transformation depends on the expression of numerous bacterial genes whose products appear to be designed to carry out this process (Chen and Dubnau, 2004; Johnsborg et al., 2007). Transformation is ordinarily a complex, energy requiring developmental process. In order for a bacterium to bind, take up and recombine exogenous DNA into its chromosome it must enter a special physiological state, referred to as competence. Development of competence in
Transformation is similar to eukaryotic sex involving hydrophilus pollination in plants, in which water is a vector in the transportation of pollen (Cox, 1988). In both bacterial transformation and sex in such plants, DNA is passed from one individual to another through the intervening liquid medium, rather than by direct contact. On the other hand, an intermediate stage in the evolution of prokaryotic to eukaryotic sex may have been similar to sexual interaction in the extant archaebacterium
The transformation systems and archaebacterial systems appear to differ fundamentally from the more well-studied
4. Transformation and meiosis are similar at a molecular level
Bacterial transformation and eukaryotic meiosis are similar in their central molecular processes, and these processes are catalyzed by homologous gene products. There are three major steps in bacterial transformation: (1) DNA derived from a donor cell enters into a recipient cell; (2) the two homologous chromosomes (or homologous portions of the two chromosomes) derived from the two bacterial cells align and undergo genetic recombination (exchange of genetic information); (3) the new recombined chromosome is passed on to progeny bacteria. Meiosis in diploid eukaryotic cells can similarly be viewed as occurring by three steps. These steps are: (1) gametes undergo syngamy/fertilization so that chromosomes of different cellular origin share the same nucleus; (2) homologous chromosomes from different cells (i.e. non-sister chromosomes) align in pairs and undergo recombination; (3) two successive cell divisions (without chromosome duplication) lead to haploid gametes, which can repeat the cycle in subsequent generations. In meiosis, as in transformation, the central step (step 2) is the intimate alignment of non-sister homologous chromosomes followed by genetic recombination. In bacteria, recombination between non-sister homologous chromosomes is catalyzed by the RecA protein, and in eukaryotes this same reaction is catalyzed by orthologs of RecA, such as Rad51 and Dmc1 (see section 16, below).
5. The prokaryotic ancestor of eukaryotes was likely capable of transformation
The hypothesis that meiosis evolved from transformation depends on the assumption that the prokaryotic ancestor of the eukaryotic cell lineage was able to undergo transformation. A crucial event in the emergence of the eukaryotic cell was likely the establishment of a stable association of an anaerobic host bacterium and a smaller, internalized aerobic bacterium. We next consider the likelihood that this progenitor of the eukaryotic cell lineage was capable of transformation. The internalized aerobic bacterium is assumed to have provided the capacity for respiration, and to have eventually evolved into the mitochondrion. We will first focus on the nature of the internalized aerobe, and then on its anaerobic host.
On the basis of genome sequence analysis, extant mitochondria are most closely related to α-proteobacteria, suggesting that mitochondria are descended from an α-proteobacterium (Gray et al., 1999; Muller and Martin, 1999). Based on a computational analysis, Boussau et al. (2004) concluded that the common ancestor of α-proteobacteria likely had a genome consisting of between 3000 and 5000 genes, and was an aerobic, motile bacterium with pili and surface proteins for interacting with host cells. A gene sequence data analysis by Gray et al. (1999) strongly indicated a monophyletic origin of mitochondria from an α-proteobacterial ancestor, and also implied that mitochondria evolved only once. We can assess the plausibility of the idea that such an ancestor was capable of transformation by knowing whether present day α-proteobacteria are capable of transformation. In fact, several present day α-proteobacteria are capable of natural transformation, including
The ancient anaerobic bacterial host of the internalized proto-mitochondrion was determined on the basis of a phylogenetic analysis to likely be an archaebacterium (Cox et al., 2008). Transformation has been reported among currently living archaebacteria including
Evidence indicates that, during the evolution of mitochondria from an ancestral α-proteobacterium, much of the genetic information of the α-proteobacterium was transferred to, and became integrated into, the host nuclear genome. Phylogenetic studies by Gabaldon and Huynen (2003) suggest that at least 630 genes were transferred to the nuclear genome from the α-proteobacterial genome. If the ancestral α-proteobacterium was capable of transformation, as are some of its current day relatives, its genes necessary for transformation may have been integrated into the early eukaryotic nuclear genome. In current-day organisms, the gene family
6. DNA damage is a basic problem for life
A DNA damage is an alteration in the molecular structure of DNA, such as a break in one or both DNA strands, a missing base, or an oxidized base (e.g. 8-OHdG). Damage to DNA often results from natural processes. As noted by Haynes (1988), DNA is comprised of rather ordinary molecular subunits that are not endowed with any unusual quantum mechanical stability, and this “chemical vulgarity” makes DNA vulnerable to all the “chemical horrors” that might befall any such molecule in a warm aqueous environment. The particular types of DNA damage occurring when organisms were undergoing the prokaryotic to eucaryotic transition cannot be determined directly, but can be indirectly surmised from the types of DNA damage occurring in present day organisms. In extant cellular organisms, metabolism releases numerous compounds that damage DNA including reactive oxygen species, reactive nitrogen species, reactive carbonyl species, lipid peroxidation products and alkylating compounds, among others, while hydrolysis cleaves chemical bonds in DNA (De Bont and van Larebeke, 2004). In eukaryotes such as mammals, tens to hundreds of thousands of naturally caused DNA damages occur per cell per day (see next section). While most of these DNA damages can be repaired, such repair is not 100% efficient. Unrepaired DNA damages accumulate, especially in non-replicating or slowly replicating cells.
One indication that DNA damages are a major problem for life is that DNA repair processes, to cope with ubiquitously occurring DNA damages, have been found in all cellular organisms in which DNA repair has been investigated. For example, in bacteria, a regulatory network aimed at repairing DNA damages (called the SOS response in
Another indication that DNA damages are a major problem for life is that cells make large investments in DNA repair processes. As pointed out by Hoeijmakers (2009), repairing just one double-strand break may require more than 10,000 ATP molecules, since ATP is used in signaling the presence of the damage, the generation of repair foci, and the formation (in humans) of nucleofilament intermediates in homologous recombinational repair by RAD51, a homologue of bacterial RecA.
In plants, dormant seeds accumulate DNA damages which can be largely repaired during germination (Cheah and Osborne, 1978; Koppen and Verschaeve, 2001). Multiple effective pathways for DNA damage signalling and repair have evolved in plants for dealing with endogenous and exogenous sources of DNA damage (reviewed by Bray and West, 2005).
Sagan (1973) examined the flux of solar UV irradiation penetrating the primitive reducing atmosphere of earth prior to the formation of a shielding ozone layer, and concluded that a mean lethal dose would be delivered to unprotected microorganisms of the type existing today in 0.3 seconds or less. Since DNA damage is the main cause of UV-induced lethality, it appears that DNA damage was likely a problem for even the earliest microorganisms.
7. Frequency of occurrence and consequences of DNA damage
An idea of the magnitude of the biologic problem posed by naturally occuring DNA damage can be obtained by considering the frequency of occurrence and consequences of DNA damages in present day organisms. The estimated frequency of occurrence of oxidative DNA damages per cell per day is about 10,000 in humans (Ames et al., 1993; Helbock et al., 1998) and about 74,000 to 100,000 in rats (Fraga et al., 1990; Ames et al., 1993; Helbock et al., 1998). For depurinations, the estimated frequency is 9,000 to 13,920, per mammalian cell per day (Nakamura et al., 1998; Tice and Setlow, 1985); for single-strand breaks, the frequency is about 55,200 per cell per day (Tice and Setlow, 1985). Double-strand breaks, which are difficult to repair accurately, occur in human cells at a frequency estimated to be 10 (Haber, 1999) to 50 (Vilenchik & Knudson, 2003) per cell cycle. Other types of DNA damage, such as formation of O6-methylguanine and cytosine deamination are also frequent.
An unrepaired DNA damage may block replication of the DNA, and when such a damage occurs in the transcribed strand, it may also block RNA polymerase catalysed transcription (Kathe et al., 2004). Blockage of DNA replication can be lethal to a cell, and blockage of transcription is deleterious because it can interfere with the synthesis of a protein coded for by the gene in which a blockage occurs.
Also, during DNA replication, as the DNA polymerase copies a DNA strand containing a damaged site, it may inaccurately bypass the damage and in so doing generate a mutation. Although damages and mutations are both errors in DNA, DNA damages are distinct from mutations. DNA damages are structural and chemical alterations in the DNA, whereas mutations ordinarily involve the normal four bases in new arrangements. Furthemore, whereas DNA damages are altered structures that cannot be replicated, mutations can be replicated when the DNA replicates. In aerobically growing bacteria, reactive oxygen species (ROS) seem to be an important source of DNA damage, as indicated by the observation that 89% of spontaneously occurring base substitution mutations are caused by inaccurate replication past bases damaged by ROS (Sakai et al., 2006). Thus another harmful consequence of DNA damages is that they likely generate a substantial portion of spontaneous mutations.
Further consequences of DNA damages for eukaryotes and prokaryotes are the expenditures of energy, time and material resoures (e.g. nucleotides) required by the multiple processes that repair DNA damages. Five major pathways are employed in repairing various kinds of DNA damages. These processes are nucleotide excision repair, base excision repair, mismatch repair, non-homologous end joining and homologous recombinational repair (HRR) [reviewed in Bernstein et al. (2002)]. Only one of the five pathways, HRR, is able to accurately repair double-strand damages, such as double-strand breaks (DSBs). The HRR pathway depends on the availability of second homologous chromosome for restoring the information lost by the first chromosome due to the DSB. As detailed below, in both prokaryotes and eukaryotes, sex promotes the conditions needed for especially effective HRR of double-strand damages.
Overall, it is clear that, in eukaryotes and prokaryotes, DNA damages are ubiquitous and thus are a major problem for cellular and organismal survival. Furthermore, over time, DNA damages have selected for the evolution of numerous complex, specialized DNA repair pathways.
8. Competence for transformation is induced by stress in prokaryotes
Both transformation and meiosis (in microbial eukaryotes) are induced by stressful conditions. Thus transformation may have evolved by natural selection as an adaptive response to stress in prokaryotes and been maintained for this purpose subsequent to the transition to meiotic sex. In this section we describe stressful conditions that induce competence for transformation and in section 10 we describe similar stressful conditions that induce meiotic sex in eukaryotic microorganisms.
Among present day bacteria, competence for transformation is induced when bacteria are grown to high cell density and/or under nutritional limitation, conditions characteristic of the stationary phase of bacterial growth. As an example, transformation in
As discussed above in section 3, certain archaebacteria, namely
9. The adaptive function of transformation is likely repair of stress-induced DNA damage
Competence for transformation is induced specifically by DNA damaging conditions. For instance, the DNA damaging agents mitomycin C (a DNA interstrand cross-linking agent) and the fluoroquinolones norfloxacin, levofloxacin and moxifloxacin (topoisomerase inhibitors that causes double-strand breaks) induce transformation in
DNA damaging UV irradiation increases transformation in
The number of genome copies in logarithmically growing bacteria typically differ from the number of genomes in stationary phase bacteria, and this has implications for the capability of bacteria to carry out homologous recombinational repair (HRR). During logarithmic growth, two or more copies of any particular region of the chromosome are ordinarily present in a bacterial cell, as cell division is not precisely matched with chromosome replication. HRR is effective at repairing double-strand damages, such as double-strand breaks. This repair process depends on interaction of the damaged chromosome with a second homologous chromosome. During logarithmic growth, a DNA damage in one chromosome may be repaired by HRR using sequence information from the other homologous daughter chromosome. As bacterial cells approach stationary phase they typically have just one copy of the chromosome, and HRR requires input of an homologous template from another cell by transformation (Bernstein et al., 2012).
A series of experiments were carried out using
10. The sexual cycle is induced by stress in single-celled and simple multicellular eukaryotes
In eukaryotic microorganisms, sex occurs under stressful conditions as it does in bacteria. Among currently existing unicellular and simple multicellular eukaryotes, sexual reproduction is ordinarily facultative. These organisms usually reproduce asexually in a favorable environment, but reproduce sexually when under stress. Sex is induced in these organisms by starvation, mechanical damage, desiccation and heat shock. One example is the paramecium tetrahymena that can be induced to undergo conjugation (sexual mating) by starvation (Elliott and Hayes, 1953). Another example is the unicellular green alga
11. Meiosis is induced by DNA damaging conditions in microbial eukaryotes
In sections 2 and 5, above, we reviewed evidence that sex was present early in the evolution of eukaryotes, raising the likelihood that eukaryotic sex arose from ancestral prokaryotic sex. We also reviewed evidence in section 9 that bacterial sex (transformation) is an adaptation for repair of DNA damages. In this section we present evidence that sex in eukaryotes, particularly the meiotic stage of the sexual cycle, is also an adaptation for response to DNA damage.
Hydrogen peroxide produces oxidative stress that causes a variety of DNA damages including modified bases and single- and double-strand breaks (Slupphang et al., 2003). When the yeast
As mentioned in the preceding section, sex in the green algae
12. Recombinational repair of DNA damages in the germ line
Studies from a wide range of eukaryotes indicate that meiotic recombinational repair is an adaptation for repairing germ-line DNA damages. For example, exposure of eukaryotes to a DNA damaging agent causes increased meiotic recombination (expected as a result of HRR) as measured by exchange of allelic markers. Thus X-irradiation increases meiotic allelic recombination in
In another type of experiment, Preston et al. (2006) studied the repair of double-strand breaks in the male germ line of
In some protozoans, vitality declines over the course of successive asexual cell divisions by binary fission. However, if sexual interaction (conjugation) occurs, vitality is restored. Evidence indicates that meiosis leads to rejuvination, and that this rejuvenation is associated with removal of DNA damages. The ciliate protozoan
Auferheide (1987) clarified the cause of clonal aging in
A similar phenomenon to clonal aging in paramecium is also found in yeast. In the budding yeast
In the nematode
Coogan and Rosenblum (1988) measured the repair of double-strand damages after γ-irradiation of rat spermatogenic cells in sequential stages of germ cell formation (i.e. spermatagonia, preleptotene spermatocytes, pachytene spermatocytes and spermatid spermatocytes). The greatest repair capability occurred in pachytene spermatocytes, the meiotic stage in which recombinational repair occurs. These findings suggest that a function of meiosis, expressed during the pachytene stage, is the repair of double-strand damages. The most likely natural sources of the double-strand damages normally repaired at pachytene are reactive oxygen species generated by active metabolism.
Recombinational repair of DNA damages during meiosis also likely occurs in plants and depends on
The observations summarized in this section imply that meiosis is an adaptation for recombinational repair of DNA damages in the germ line.
13. Unrepaired DNA damage during mammalian gametogenesis causes infertility
In mammals, germ cells are exposed to natural causes of DNA damage. For instance, several germ cell stages, including pachytene spermatocytes, have the potential to produce levels of reactive oxygen species sufficient to cause oxidative stress (Fisher and Atkin, 1997). Reactive oxygen species can produce double-strand damages in DNA. Also, heat can cause DNA damage. Paul et al. (2008) showed that mild heat stress (40 or 42ºC for 30 minutes) applied to the mouse scrotum causes DNA strand breaks in germ cells leading to infertility and abnormalities in embryonic development of progeny.
In the US, about 15% of all couples are infertile. A major cause of male infertility is oxidative stress during gametogenesis (Makker et al., 2009). Oxidative stress causes a variety of DNA damages including oxidized and ring fragmented bases and single- and double-strand breaks (Slupphang et al., 2003). Nili et al. (2011) showed that subfertile men have an increased frequency of sperm chromosomal aneuploidy as well as increased DNA damage (i.e. DNA strand breaks and alkali labile sites). Lewis and Aitken (2005) reviewed evidence that an increased level of DNA damage in the germ line of men is associated with poor semen quality, low rate of fertilization, impaired pre-implantation development, increased abortion, and a higher incidence of disease in progeny including childhood cancer. These authors also noted that the natural causes of this elevated DNA damage are unclear, but that the principal candidate is oxidative stress.
Genes whose products are employed in recombinational repair (e.g.
|Females and males in both mice and humans are infertile||4-fold increased mRNA expression in human testes vs. somatic cells||Barlow et al., 1998; Galetzka et al., 2007|
|The few surviving male mice are infertile||3-fold increased mRNA expression in human testes vs. somatic cells||Cressman et al., 1999; Galetzka et al., 2007|
|Female and male mice are infertile; ||Expression specific for|
|Pittman et al., 1998; Mandon-Pepin et al., 2008|
|Female and male mice are infertile||1.7-fold increased mRNA expression in human testes vs. somatic cells||Wei et al., 2002;|
Galetzka et al., 2007
|Male infertility and sperm DNA damage||2 to 4-fold increased mRNA expression in human testes vs. somatic cells||Galetzka et al., 2007;|
Ji et al., 2012
|Female and male ||Expression specific for|
|Edelmann et al., 1999; Mandon-Pepin et al., 2008|
|Female and male mice are infertile||Expressed at a high level in mouse testes||Hsia et al. (2003)|
In humans, the mismatch repair genes
In women, about half of fertilized eggs fail to produce embryos that survive, as is typical of mammals generally (Austin, 1972). Roberts and Lowe (1975) estimated that about half of all post-implantation embryos are lost, often before the first missed period. Also, Wilcox et al. (1988) noted that, after implantation, about 31% of embryos miscarry, often before the woman is aware that she is pregnant. Although the basis for these malfunctions is not yet established, it is likely that unrepaired DNA damage is an important contributing factor. Adriaens et al. (2009) reviewed several studies showing that the mammalian oocyte can repair various kinds of DNA damage occurring either spontaneously or as a consequence of exposure to external agents. However these repair processes are not 100% efficient.
Chemotherapy used in cancer treatment can result in infertility in young female cancer survivors. Soleimani et al. (2011) showed that exposure to the chemotherapeutic agent doxorubicin causes massive double-strand breaks in human and rodent oocytes leading to apoptotic cell death. Activation of the recombinational repair pathway appeared to allow only a minority of oocytes to survive.
The inheritance of a mutant
Reproductive capacity of women begins to diminish after young adulthood (i.e. about age 37 years). Recently, Titus et al. (2013) showed that DNA double-strand breaks – a measure of DNA damage – accumulate with age in primordial follicles (immature primary oocytes) of mice and humans. Paralleling this increase in breaks, expression of key recombinational repair genes
The findings described in this section indicate that DNA damage during spermatogenesis and oogenesis can lead to infertility, a significant clinical problem for humans. Furthermore, meiotic recombinational repair likely plays an important role in repairing such DNA damages and avoiding infertility.
14. Homologous recombinational repair functions similarly during meiosis and mitosis, but has greater scope for repairing double-strand damages during meiosis
Homologous recombinational repair occurs during mitosis, but is largely limited to interaction between nearby sister-chromosomes subsequent to replication (but prior to cell division). During mitosis, the frequency of recombination between non-sister homologous chromosomes is only about 1% of that between sister-chromosomes (Moynahan and Jasin, 2010). In contrast, during meiosis, recombination between non-sister homologous chromosomes is frequent and is indeed a key characteristic of meiosis. Despite these differences concerning which homologous chromosomes are involved, the machinery of recombinational repair appears to be closely similar during mitosis and meiosis. Numerous common gene products are essential to both processes. The parallels in the machinery of recombinational repair across meiosis and mitosis in eukaryotes, and even extending to the mechanism of recombination during transformation in bacteria, suggests similarity of function.
In the budding yeast
Mutants of the fruit fly
Homologous recombinational repair during meiosis provides a unique advantage compared to mitosis. During meiosis, systematic pairing and recombination between non-sister homologous chromosomes is promoted compared to mitosis, where recombination between non-sisters chromosomes is infrequent (Moynahan and Jasin, 2010). Consequently, during mitosis, homologous recombinational repair is largely restricted to the portion of the cell cycle in which DNA replication is occurring (S phase) and after DNA replication is complete (G2 phase) so that a closely adjacent homologous chromosome is available. During this restricted period of the mitotic cell cycle, double-strand damages may be accurately removed by homologous recombinational repair between sister homologs (Tichy et al., 2010). However, during the portion of the mitotic cell cycle after cell division but prior to DNA replication (G1 phase), double-strand damages, such as double-strand breaks and interstrand crosslinks, are not ordinarily repaired by accurate homologous recombinational repair. Rather, they are either repaired by the inaccurate process of non-homologous end-joining that generates mutation, or else the double-strand damages cause cell death.
In contrast to the constraint on DNA repair during mitosis, during meiosis, homologous recombinational repair can accurately remove double-strand damages that arise at any stage of the cell cycle because of systematic pairing of non-sister homologous chromosomes. Cells in the G1 phase of meiosis are more resistant to the lethal effects of X-rays than cells in the G1 phase of mitosis (Kelley et al., 1983). This finding suggests that types of damages caused by X-rays, which include double-strand damages, are more readily repaired during meiotic G1 than mitotic G1. Repair capability during meiosis is likely more effective than during mitosis, because there is greater access to a homolog for the source of needed redundant information and also because the proteins that catalyze homologous recombinational repair are present at an increased level (Table 1).
In this section we reviewed evidence that DNA damages caused by a variety of exogenous agents are repaired by homologous recombinational repair during mitosis. Since this repair process is closely similar to the analogous process during meiosis, we infer that homologous recombination during meiosis also functions to repair a variety of DNA damages.
During each cell cycle in humans, 30,000 to 50,000 DNA replication origins are activated (Mechali, 2010). Thus chromosomes are replicated in segments. During pre-meiotic replication, a double-strand damage in any segment may block completion of replication of the segment until the damage is repaired. During the subsequent prophase I stage of meiosis when the blocked segment becomes paired with a non-sister homolog, the damage may be accurately repaired by recombinational repair using the intact information from the non-sister homolog. After the damage is removed, replication of the segment can be completed. Hence meiosis can provide a means for accurately repairing double-strand damages present in all stages of the preceding cell cycle.
15. The key role of RecA repair protein in bacterial transformation
In this section we focus on the role of the RecA protein and its orthologs in catalyzing key steps of recombination during transformation. First, RecA interacts with ATP and single-stranded DNA to form a helical filament. This filament then binds to double-stranded DNA, searches for homology and next catalyses exchange with the complementary strand of the duplex DNA producing a new heteroduplex (Chen et al., 2008). Pairing of homologous regions of the two parental genomes leads to exchange of information between the genomes (recombination).
The RecA protein is essential for transformation in the bacteria
The molecular interactions of bacterial RecA protein with DNA were analyzed by Cox (1991; 1993) who concluded the RecA protein evolved as the central component of an homologous recombinational repair system for dealing with DNA damage. Cox concluded that DNA repair is the most important function of homologous genetic recombination. This conclusion is in accord with the idea that, in bacterial transformation, RecA protein functions to remove DNA damages in the resident chromosome by homologous recombinational repair using intact (undamaged) information from the donor chromosome. As noted above, bacteria typically become competent for transformation in late log phase, when most cells are haploid and the only available source of intact homologous DNA is another bacterium.
16. The key role of RecA orthologs in eukaryotic meiosis
In this section we present evidence that orthologs of the RecA protein have a similar role in eukaryotic meiosis to that in bacterial transformation, the facilitation of recombinational repair between two homologous DNA molecules of different parental origin.
Protein components of the homologous recombination machinery appear to be highly conserved from bacteria to eukaryotes. We first consider unicellular, and then multicellular eukaryotes. Genes
We next review evidence that orthologs of bacterial RecA play a key role in meiotic recombination in multicellular animals and plants, suggesting evolutionary continuity of the central role of recA orthologs in sexual processes from microorganisms to multicellular life forms.
Orthologs of the RecA protein play a key role in meiosis of animals (e.g. nematodes, mice and humans) and of plants (e.g.
In the testis and ovary of the mouse, a homolog of the
Rad51 and Dmc1, RecA-like recombinases, are required for meiosis in the plant
17. Another key repair protein employed in both transformation and meiosis
Prior to the assembly of the RecA recombinase or an equivalent ortholog recombinase on single-stranded DNA to form a presynaptic filament, a single-strand-binding protein [termed SSB in bacteria and RPA (replication protein A) in eukaryotes], processes the single-stranded DNA.
18. Meiosis, sex and outcrossing
The essential feature of meiosis is information exchange between two genomic DNA molecules derived from different individuals (parents). This feature is shared with bacterial transformation, which we have proposed here to be the ancestral precursor to meiosis. The two different individuals that participate in mating events leading to meiosis may be closely or distantly related to each other. When the two individuals are distantly related to each other they are likely to differ more genetically than if closely related. Thus in matings of distantly related individuals there is a greater potential for generating genetically varied progeny than in matings of closely related individuals.
Evolutionary biologists have often assumed that the potential benefit of producing genetically varied progeny is of substantially greater consequence than the advantage of DNA repair and is, by itself, sufficient to explain the adaptive advantage of meiosis specifically, and the adaptive benefit of sex generally. Critical evaluations of this view have been presented elsewhere (e.g. Bernstein et al., 1987; Birdsell & Wills, 2003; Michod et al., 2008; Gorelick & Heng, 2011; Horandl, 2009; Horandl, this volume). However, our view, in brief, is that genetic variation is a byproduct of homologous recombinational repair during meiosis, and that any benefit of producing varied progeny is a long-term population level effect that would supplement the advantage of DNA repair. DNA repair is an immediate benefit that occurs at each sexual generation. In the short term, the benefit of variation, we think, is unlikely to be adequate, by itself, to maintain sex, especially in those organisms where the costs of sex are high (see Michod et al., 2008; Horandl, this volume).
In many microbial eukaryotes, it is likely that, in nature, mating occurs most often between members of the same clonal population and that outcrossing is uncommon. For instance, Ruderfer et al. (2006) analyzed the ancestry of natural yeast
Among early facultatively sexual eukaryotes, the requirement of sex that two different parental genomes come together in a common cytoplasm would have led to a brief diploid interval. However, these early facultatively sexual organisms were likely primarily haploid, reproducing asexually as haploid organisms and only experiencing diploidy transiently during sex. As eukaryotes evolved further, the diploid stage of the life cycle became more extended, and eventually became predominant as in present day mammals and higher plants. As the diploid stage increased in importance, outcrossing also became more important because genomes of distantly related individuals are less likely than genomes of related individuals to contain common mutations. Thus outcrossing allows the masking of expression of deleterious recessive mutations in the diploid stage (Bernstein et al., 1985; Bernstein et al., 1987; Birdsell and Wills, 2003). The mutual masking of deleterious recessive mutations is referred to as complementation, a phenomenon underlying such concepts as hybrid vigor, heterosis, avoidance of inbreeding depression, and the incest taboo. We consider that these advantages of outcrossing are secondary benefits of sexual reproduction that arose with the development of a significant diploid stage, and that the primary advantage has remained homologous recombinational repair of damages in the DNA to be passed on to progeny.
19. Summary and conclusions
In bacteria, transformation involves transfer of DNA from a donor bacterial cell to a recipient cell, and can be regarded as a primitive form of sex. Competence for transformation arises by a complex developmental process that requires expression of numerous bacterial genes. Therefore competence appears to be an evolved adaptation that is of substantial benefit to the bacterium. Competence ordinarily develops during exposure to stressful environmental conditions such as growth to high cell density and nutritional limitation. Such stress conditions tend to cause increased DNA damage. Studies of transformation in several different bacterial species indicate that transformation is an adaptive response to DNA damage, and that it functions to repair DNA through the process of homologous recombinational repair.
Eukaryotes emerged in evolution from prokaryotic ancestors over 1.5 billion years ago. Based on recent evidence, meiosis, and thus sexual reproduction, appears to have arisen very early in the evolution of eukaryotes. Transformation and meiotic sex both involve the coming together of DNA molecules from separate individuals, recombination between these molecules, and passage of the recombined DNA to progeny. Thus transformation and meiosis are similar at a fundamental level. This similarity suggests that meiotic sex evolved from ancestral prokaryotic sex, that is, from bacterial transformation. In extant eukaryotic microorganisms sex is generally facultative and tends to occur under stressful conditions that are similar to the conditions that induce competence for transformation in bacteria. Also in extant eukaryotic microorganisms, sex is induced by agents that cause DNA damage. Damage to the genome (DNA in most species) appears to be a fundamental problem for life. In multicellular eukaryotes, defective recombinational repair during meiosis causes infertility that is likely due to accumulation of excessive DNA damages.
The RecA protein catalyzes key steps in recombination during bacterial transformation. Orthologs of RecA (e.g. Rad51 and Dmc1 proteins) in eukaryotes catalyze similar steps in recombination during meiosis. The RecA protein and its orthologs interact with single-stranded DNA to form a presynaptic nucleofilament that initiates recombination. This nucleofilament is thought to scan the partner chromosome for regions of homology in preparation for the informational exchange reactions of homologous recombinational repair. Other proteins that interact with the RecA protein or its orthologs also seem to carry out similar functions in transformation and meiosis. These similarities in the machinery of recombinational repair during transformation and meiosis suggest continuity in the evolution of sexual processes through the prokaryote to eukaryote boundry.
As the early eukaryotes evolved from their prokaryote ancestors, the diploid phase of the sexual cycle became increasingly prominent compared to the haploid phase. With the increasing prominence of diploidy, avoidance of expression of deleterious recessive mutations in diploid cells became advantageous. The two individuals that participate in a mating may be either distantly or closely related. A consequence of outcrossing, the mating of distantly related individuals, is that deleterious recessive mutations occurring in one parent are not likely to occur in the other, and so the expression of deleterious mutations tend to be masked in the progeny’s diploid cells. This mutual masking of deleterious recessive mutations is referred to as complementation, a phenomenon that underlies hybrid vigor. In contrast, closely related individual are likely to share the same deleterious recessive mutations, and matings between them are more likely to lead to expression of deleterious recessive mutations in progeny. Thus, adaptations have evolved to promote outcrossing. Outcrossing organisms are also more likely to produce genetically varied progeny than inbreeding individuals and this also may provide advantages over time at the population level.