A Theory of the Evolution of Eukaryotes, Crossing Over and Sex
Copyright ã 1997 by James Michael Howard.
(New Support from 2007: Some new support for my explanation of eukaryotic evolution arrived in 2007. Here is how I wrote about it for here and some other places on the internet:
“It has been suggested that "Staphylococcus aureus appears adapted to intracellular survival in non phagocytic cells." (BMC Genomics 2007; 8: 171) These findings of Garzoni, et al., may be explained by DHEA use of both the host cells and S. aureus. It has been determined that DHEA "can induce an increase in resistance to vancomycin in methicillin-sensitive and methicillin-resistant clinical isolate of Staphylococcus aureus" (Chemotherapy 2007; 53: 181-184). The effects of internalization of S. aureus reported by Garzoni, et al., may directly relate to DHEA levels within the host cells.
There is, moreover, a very important issue regarding a significant event in evoution, that is, evolution of eukaryotic organisms, which this report and that of the one cited above concerning DHEA and S. aureus, Chemotherapy, mentioned above, may support. I suggest these two reports may support my explanation of the evolution of eukaryotes dependent upon DHEA. (This is derived from my hypothesis that DHEA was selected by evolution because it optimizes replication and transcription of DNA.) In 1997, I first suggested that the evolution of eukaryotes resulted from a sharing of DHEA between the nucleus and the organisms which eventually became mitochondria. When the host organisms evolved the ability to provide DHEA sufficient to optimize duplication and transcription of DNA in both the nucleus and mitochondria, a symbiosis could occur that resulted in a viable cell in which nuclear and mitochondrial DNA are reproduced in subsequent, daughter cells.
Garzoni, et al., reported that "Following internalization, extensive alterations of bacterial gene expression were observed. Whereas major metabolic pathways including cell division, nutrient transport and regulatory processes were drastically down regulated, numerous genes involved in iron scavenging and virulence were up regulated. This initial adaptation was followed by a transcriptional increase in several metabolic functions. However, expression of several toxin genes known to affect host cell integrity appeared strictly limited." I suggest DNA and genes compete for DHEA which results in differential replication and transcription. The findings of Garzoni, et al., support this hypothesis. Since DHEA is used by both the host cells and internalized S. aureus, cell division is reduced in the host cell while some genes in S. aureus increase in transcription. The use of DHEA by S. aureus genes reduces the available DHEA for the host cell.
During evolution, I suggest an event occurred which was similar to the internalization of S. aureus as reported by Garzoni, et al. Subsequently, host cells evolved the ability to produce sufficient DHEA for themselves and the internalized organisms. Once this occurred, I suggest the first eukaryotes existed.”
Evolution of Eukaryotes
Steroids, histones, and mitochondria appear in relatively close proximity in the evolution of eukaryotes. It is my theory that the hormone, dehydroepiandrosterone (DHEA), may be the reason that mitochondrial-like organisms (MLOs) successfully invaded "host cells," the combination, of which, eventually produced eukaryotic organisms. My principal hypothesis of my theory is that DHEA is directly involved in the opening of the DNA helix, during transcription and replication. Based on this hypothesis, the host cell produced DHEA for transcription and replication of its own DNA. When MLOs were included into the milieu of the host cells, the DNA of the MLOs benefited from the DHEA. In the following quotation, note that not only is mitochondrial respiration stimulated by DHEA, but protein synthesis is also stimulated. This is an indication that DHEA positively affects transcription of the mitochondrial DNA.
"These findings indicate that mitochondrial respiration is the earliest factor affected by DHEA and may be associated with protein synthesis." (Journal of Nutrition 1991; 121: 240)
The entry of MLO DNA into the host cell would create a competition for DHEA between the two genomes. This would produce a selection pressure that would affect DNA and DHEA. (Over time, I suggest, this would stimulate an increase in endothermic organisms, as it would also increase the numbers of mitochondria in the organisms.) My theory suggests that DHEA exerts its effect on DNA by hydrogen bonding between thymine and adenine bases in DNA. Therefore, the effect of competition for DHEA in the host cell genome would be an increase in thymine-adenine bases. An increase in T-A bases would increase the effects of DHEA in the host genome. I suggest this is the reason that E. coli has approximately 25% of each of the nitrogenous bases, while humans and other mammals have about 21% cytosine and quanine and 29% thymine and adenine. It may be that this increase in thymine and adenine was accomplished by development of multiple sites for DNA replication in the nuclei of eukaryotes. Replication sites should exhibit increased areas of T-A bases. Increased replication sites should, therefore, increase competition for DHEA by nuclear DNA.
It is known that certain levels of nuclear DNA and mitochondria must exist before eukaryotic cells can divide. It is my theory this occurs, because of the competition between the two DNAs for DHEA, i.e., not enough DHEA is produced for nuclear DNA replication during the time of mitochondrial growth, and vice versa. That is, nuclear and mitochondrial DNA compete for a limited supply of DHEA. One waits on the other, i.e., a time lag in mitochondrial growth and nuclear DNA replication would develop. Eukaryotes would accommodate this lag in DHEA availability by developing a mechanism which would inhibit nuclear DNA replication until mitochondrial replication has finished. Following mitochondrial use of DHEA, DHEA and mitochondrial energy could then be used for mitosis.
I suggest the mechanisms that evolved for separating the use of DHEA, by the two genomes, are H1 histones and the nuclear envelope, hallmarks of the eukaryotes. H1 histones, according to my theory, developed to repress nuclear DNA replication. That is, these histones are a mechanism to shut down DNA opening by DHEA, therefore, freeing up DHEA for mitochondrial use. Mitochondria do not produce histones. Since a nuclear envelope is also characteristic of eukaryotes, the nuclear envelope may have evolved to sequester H1 histone gene translation in the nuclear envelope, so H1s could not turn off mitochondrial DNA, and, therefore, mitochondrial energy. H1s are, indeed, synthesized directly in the nuclear milieu. (In 1979, work on H1s had established that H1s could repress transcription. Also, in 1979, I developed a detailed explanation of how the H1s can be affected by phosphorylation and nonhistone chromosomal proteins to open DNA for transcription. For sake of brevity, at this time, I will post it at another time.)
In 1992, I had the occasion to respond to an author of the Journal of Reproduction and Fertility 1991; 93: 467, about the "fluctuation of histone H1 kinase activity during meiotic maturation in porcine oocyes." I wrote to explain how my model of transcription control by H1s may fit their findings in meiosis. (That letter included my mechanism of H1 control of transcription, in detail) One of my important hypotheses, generated by my letter to them, was a new explanation of crossing over. For this part of my theory, you need to know that phosphorylation of H1s removes H1s from chromatin. This phosphorylation of H1s, according to my theory, is necessary before DNA synthesis in mitosis and meiosis. In the same manner as my transcription theory suggests, H1s must be removed before replication may occur. (H1 kinase phosphorlyates H1s.) Naito and Toyoda contains a diagram of H1 kinase activity, Fig 3, page 471, that shows that H1 kinase activity is low in the germinal vesicle, rises in prometaphase I, and reaches a maximum in metaphase I. The activity then declines to just above that of the germinal vesicle at anaphase and telophase I. It then increases, again, at metaphase II, essentially as high as that of metaphase I.
As I reread some textbooks about meiosis in preparation for writing Naito, it occurred to me that crossing over of pachytene during meiotic prophase may result from relatively incomplete phosphorylation of H1 histones at this time. That is, as H1 histones are removed by phosphorylation, detached unphosphorylated H1 histones may actually recombine with core histones of other stands of DNA and cause crossing over. (It is established that H1s are attached at one end to the core histones and at the other end in the "linker region of DNA" between core histones.) During pachytene the levels of H1 kinase, according to my theory of crossing over, is not so high; this would explain why crossing over occurs at pachytene and not later in meiosis, when H1 kinase activity is so high. If my idea is correct, not only will this explain crossing over, but will provide a powerful evolutionary selection pressure for development of H1 histone formation in evolution of eukaryotes. That is, the core histones probably evolved first to act as support structures for DNA replication and condensation in eukaryotes, somewhat advanced beyond yeasts. (Yeasts do not produce H1 histones.) This extremely useful function of crossing over would evolve along with sex and the true eukaryotes.
The idea is that if only a small fraction of H1 histones are not phosphorylated on time, crossing over occurs. If the vast majority are not phosphorylated, then nondisjunction occurs; the chromosomes are literally glued together by the histones. This is a general mechanism, supported by the following quotation.
"(1) Nondisjunction of the sex chromosomes is largely confined to the first meiotic division; (2) nondisjunction involves chromosomes 2 and 3 as well as the sex chromosomes; this argues against a specific geometrical problem with sex chromosomes pairing and segregation and in favor of a more general physiological effect on the control of meiosis; ..." (Genetics 1984; 107: 609)
If I am correct, this effect should be relatively global, i.e., it should not be isolated to one particular chromosome. Also, it should occur during the first meiotic division.
The Evolution of Sex
I suggest that protein kinase evolved to remove H1 histones totally from DNA for replication. Since cell division would benefit from this, it seems appropriate to assume that the activity had selection value. That is, cells that were better at it, divided more. My theory of the interaction of H1s and DHEA also explains multicelluarity. That is, specialization in the manipulation of H1 removal probably occurred over time in different cells. Differential removal of H1s could produce differences in transcription in different cells. Eventually, the non-histone chromosomal proteins, that I utilize to explain differential transcription, would arise.
Differential transcription, at some point, must have produced problems for cell division, within multicellular clumps of cells. In cells in which the chromatin exhibited very little transcription, cell division would be a problem. I suggest the protein kinase evolved to over come this division problem. That is, some of these cells over-expressed, or perhaps produced a more powerful protein kinase for allowing cell division. Since these cells would be characterized by highly repressed DNA, the DHEA would be more readily available for the mitochondria. This would increase the energy and cytoplasm available for cytoplasmic division, while the nuclear DNA remained repressed.
A single event, or a single cascade of events, involving protein kinase could have evolved that resulted in a rapid DNA replication in cells that have increased tremendously in size, due to increased mitochondrial function and reproduction. This scenario could eventually produce a large cell with a haploid genome, due to a rapid division. Since these cells would be nutrient enriched, fusion of two of them could rapidly produce a diploid cell. This fused cell would exhibit rapid cell divisions, because of extra mitochondrial energy and mass. The daughter cells could produce "normal" multicelluarity as the mitochondrial and nuclear genome reestablish a normal ratio of nuclear and mitochondrial DNA to DHEA. Eventually, the formation of the severely repressed cells would occur, and the mechanism would repeat itself.
All that would be necessary for "sex" to evolve are mechanisms that would enhance the process. Such things as chromosomal part deletions, crossing over, etc. may enhance the repression in these cells, the size of the cytoplasm, etc. These could produce the large egg and small sperm that we see today. Of course nature is full of the variations leading from simple cell division, to fusion of two similar, haploid cells, to the formation of the egg and sperm that would increase the favorable aspect of fusion of two large haploid cells from two different organisms.