I never had problem to accept evolution, but in the same time I never was comfortable the way it is often explained. So I may understand some concern, had expressed not by creationists only, about limited explanatory ability of natural selection mechanism. Many years ago, my interests in theoretical biology led me to one controversial hypothesis about DNA molecule genetic activity, which if correct, among other things, should explain the sex emergence phenomenon. Below is an essay devoted to this investigation.
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WHY DO TWO SEXES EXIST?
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All Children are believers in world harmony. To them, everything is beautifully created and for a reason -the sun is to catch sunbeams in a mirror, the wind is to blow the sails of toy-ships, and night provides an opportunity for dreams. Perhaps, a cub once shared a similar belief. Winnie the Pooh, the popular animated cartoon series critter, did. One day, while sitting under a tree, he thought about the bees, which lived in the tree and their destination to collect honey. Because he also believed in a supreme purpose, he quickly found his answer: "The bees are destined to collect honey for me, Winnie the Pooh." This realization encouraged him into action, but there were consequences that Winnie the Pooh, stung all over, will remember for a long time.
In spite of very impressive achievements in evolution Biology, many problems still lack the answers and some of them do not have even approach for resolution. However if we assume that a supreme purpose is the driving force for evolution, the question "What is it for?" will help to find an answer. This is why the Philosophy of Supreme Purpose prompts some biologists to look for a predestination of all natural events. They tried to find purposes even in case of some animals eat their own posterity, or when certain worm embryo destroys its mother's body. Even death, due to aging, was found by these scientists to be a greatly advantageous adaptation, which emerged during evolution. However, evolution is a development that does not happen without mistakes or defects and may not be a show case of a total harmony and so, we should not always ask the question "What is it for?" when studying Nature.
One of the main phenomenons of life is propagation. Regardless endless consideration biologists have given to this phenomenon; one aspect of it does not give them any rest - what are two sexes for? Many scientists suggest that this question hasn't had an answer, until now.
In the 1930’s, it seems that the answer about "unknown goals" was found. Ronald Fisher, owner of a London insurance agency, the father of mathematical statistics and population genetics, demonstrated that sexual propagation combines advantageous inheritances of both parents, therefore accelerating evolution. The diversity of inheriting combinations gives to the species evolutionary flexibility, which is the capability to change quickly. "However the interbreeding of hermaphrodite organisms, as in some worms, yields double the combination that heterosexual organisms do" - the scientist of Armenia Institute of Genetics, Vachtang Geodakjan objected. He questioned: "Why didn't Nature follow that way." He also believed in a supreme purpose of two-sex existence and proposed a hypothesis about distribution of evolutionary roles: for the male "design and test" of new varieties and for female, to save the best ones. This was one of the most elegant and convincing scientific theory I ever read.
However, the history of the sexes starts with accounts of the earliest life on Earth. It means that the roles between "him" and "her" were designated already, when only microbes totally dominated the planetary surface. What for did nature decide to distribute evolutionary roles to microbes? I asked, as I used to . yet the all-over-sting physiognomy of Winnie the Pooh reappears. His methodology and unfortunate experience prompt me to look into different direction by asking another question: Why in the process of evolution two sexes may emerge and where is the source for sexual differences? For this reason, we arrive at the cellular level of organism, where the mystery of all living things is hidden.
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HYPOTHESYS
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Many years ago, my interests in the theoretical biology led me to one controversial hypothesis about DNA molecule genetic activity. As reader may know, the double strand DNA molecule contains one DNA strand inherited from the parental DNA and one newly synthesized DNA strand. Both strands carry genetic information, however according to my hypothesis only one, newly synthesized strand are genetically active in the double stranded DNA molecule. As a result, two sister DNA molecules (emerged as result of single DNA autoreplication), having identical genetic code, should have a very dramatic difference in genetic expression because they have different newly synthesized DNA strands.
Unfortunately, this hypothesis contradicts one of the essential paradigms of molecular biology that identical DNA molecules must have identical genetic activity. We should respect the paradigm, but I could not understand how this paradigm was verified, i.e. how the genetic activity pattern of single DNA molecules was determined. Even the modern methods, because of low resolution, are unable to determine a single DNA molecule genetic activity and therefore the cumulative genetic activity pattern of "DNA soup", consisting at least of thousands or even millions DNA molecules, is tested instead. Regardless my reservation, I still expected to find plenty evidences to support this paradigm, yet despite a thorough literary search and numerous discussions with experts in the field, I came upon an unexpected result: nobody could uncover any proof, regardless of a general confidence in their existence. This irresistible simple paradigm seems to be self-evident.
I have continued developing this controversial idea that seems to contradict not only Paradigm, but common sense also. According to this hypothesis, only the newly synthesized DNA strand is genetically active, but genes on the "old" DNA strand are blocked from genetic expression. As a result, our hypothetical cell has limited access to its own genetic code. Because to survive, a cell needs all the genetic information it contains, the fate of our hypothetical cell and this idea accordingly, seems to be doomed.
However, I argue that cell may not worry about genetic information if it has the products of this information available. For example, sister cells may obtain all necessary genetic products by inheriting them from a parental cell. These products for a while may provide them not only with vital capacity, but also make them practically indistinguishable. Nonetheless, parental ingredients, like any other ingredient, become consumed and "our" cell must start another cycle of auto-replication to refill the missing genetic products. Looks like "our" cells propagate, not because they "worry" about future generations, but because they worry about they own vital capacity.
I have been allays puzzled by the supreme purpose of propagation. Cell division is a dramatic event because during that time, the cell is most prone to injury. However, it is widely accepted that the cell must sacrifice its own interests and propagate in order to "pay its debt" to species welfare. It is why I was glad to find out that cell division might serve not the species interests only, but the cell’s interests also, providing cells with a vital capacity. Dependence of "our" cell on propagation must have forced their breeding and eventually, this might have caused the predominant spreading of those cells, compared to the cells, which are organized in a more "advisable" manner: i.e. whose survival doesn't depend on autoreplication. Thus, "our" cells may have been the main materials for evolutionary steps that followed and they must have been the brick, which had been laid in basis of the living Nature of our planet.
Under conditions of low temperature or starvation, the cells cannot grow and auto-replicate themselves. So, under this condition, our hypothetical cell must die after parental product’s depletion. By the way, there are numerous observations (but no explanation) of massive cell deaths occurring during a long artificial block of cell auto-reproduction. What should “our”cells do to survive in this circumstance? Fortunately, there is an escape. If under such conditions, the cells of the opposite type (having complementary active DNA) merge with each other, the resulting cell can generate a complete set of necessary genetic products and is capable of survival. There is an interesting similarity between the cells of our model and well-studied single-cell eukaryotic organisms. In their populations, each cell produces two sister cells of opposite type and as a result, the population consists of two type cells. Under conditions preventing cell growth and auto-replication, the cells of opposite types merge to start the most ancient rituals, referred as the sexual process.
Sexual reproduction (when the new generation carries the mixed genetic material from both parents) was a significant evolutionary step because it has provided the genetic variety of species. However, I think that, in the beginning, cell merging was divorced from reproduction and had served only for cell survival during unfavorable environmental conditions. Sex emerged, because cell-merging process led to recombination of genetic material, which probably, had been occurred primarily randomly. During evolutionary stages, recombination proved its advantage and has become more regular in character. However, even now we can often observe cell couples, who neglect their evolution roles. These couples go through conjugation, but like a reminder of past times, they do not mix their genetic material and don't form a family (zygote).
According to our model, the DNA molecule can be regarded as the simplest family. In this family, "male" and "female" complement strands co-exist, but only one of them works while the other strand is kept dormant. DNA molecules in which the "male" strand works we’ll call the "patriarchal" DNA and the opposite type of DNA molecule in which the "female" strand works - "matriarchal" one. According to our model, DNA replication results in both, "patriarchal" and "matriarchal" DNA. During cell division they move to different cells and determine "patriarchal" and "matriarchal" types of the sister (better to say, sister and brother) cells and thus provide the numerical equality of both sexes.
It is well known that two complementary DNA strands carry unequal amount of genetic information. Therefore, the "patriarchal" and "matriarchal" cells are unequal in respect to amount genetic information they can access. Because the shortage of "active" genes make cell more depended on auto replication the cells, which engage fewer genes, have to divide more frequently and in such a way provide mostly the reproduction of the population. As expected, this role better fits "matriarchal" cells. In turn the cells of opposite "patriarchal" type that engage majority of genes should have slower division rate and therefore represents the majority of the cell population. Therefore these cells must have become the main material for future evolution experiments and natural selection. In such a way, nature might have distributed the evolution roles between "Her" and "Him."
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IMPLICATION FOR MULTI-CELLULAR ORGANISM
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Nature has made an enormous step from a single cell to the highest level of organization and finally, Adam and Eve enter this wonder world. This famous couple first faced the challenge of sex, but no deep understanding of the problem was achieved at that time. The first success was obtained much later, thanks to the chromosome theory. According to this theory, an infant’s sex is definitely determined by a distinct sex-chromosome pair when the women’s X-chromosome joins with the man’s X- or Y-chromosomes. Depending on the combination XX or XY, the parents delight in a girl or boy. This theory has been proved by a good deal of facts, but there are number of examples, which are exceptions. For instance, the genetic factors of sex in plants often depend upon surrounding conditions and in some animals (for example media Crenomytilus grayanus dunker) depends on age. Experiments on fruit flies revealed that not only sex chromosomes, but also other chromosomes could determine sex, independently from the zygote’s sexual constitution. Another very important factor for sexual differentiation is the sexual hormone. By injecting female hormone into a chicken embryo, it is forced to become a female, even by its chromosomal constitution, it was destined to become a male. When it hatches, this artificial female cannot be distinguished from an ordinary, "normal" female. In the beginning of this century, the idea of organism bio-potentiality was formulated. According to this idea, an organism has inclinations for both, male and female sexes, with determination occurring during development. For example, in the beginning stages of embryo development, human gonads of both sexes are non-distinguishable. Such gonads contain two main structures - the cortex and medulla. During development of male embryos, the cortex degenerates, and the medulla forms a testicle. In female embryos, a medulla degenerates, and the cortex forms an ovary. Based on the idea of bio-potentiality and our model, we can build a speculate scenario of an organism sex development.
The plan that determines our organic development is ready when the sex cells of our parents merge and the zygote is formed. The zygote starts dividing and soon millions of cells, genetically identical "patriarchal" and "matriarchal" cells evolve that are determine male and female properties of an embryo. On early stages of organism development, the number of "patriarchal" and "matriarchal" cells should be near to equal, that is actually a root of hermaphroditism. The sexual development must be caused by numerical prevalence of one of two cell types that achieved by discontinue division and proceed to specialization by one of the cell type while the cells of opposite type continue to divide. In the male organism the "patriarchal" cells prevail and perform most of the duty, but the "matriarchal" ones are responsible for organism growth. In the female organism, the "matriarchal" cells prevail and perform most duties, but the "patriarchal" are responsible for organism growth. The magnitude of numerical prevalence of one cell type determines the degree of sex expression that, as we know, has a wide spectrum.
The numerical ratio of two cell types can be changed only through cell division and specialization. These processes have a maximum rate on the early stages of development and therefore, this is the best time for sex to be changed. The older an organism, the slower the division rate and the smaller the chance to succeed in sexual changes.
Cell division and specialization can be affected by various factors: temperature, radiation, organic and inorganic compounds, etc. It is why these factors can change the numerical representation of both cell types and influence the process of sex determination. Plants or cold-blooded organisms are sensitive to surrounding factors that may play notable role influencing sex determinations. In turn, warm-blooded organisms are shielded from these factors by regulatory mechanisms that maintain internal homeostasis. Therefore the influence of the environment on the sex determinations in warm-blooded organisms is practically eliminated. In this case, the sex constitution of the zygote can be the only remaining factor, which influences the numerical ratio of both cell types and determine the sex of an organism.
In the process of organism development, the growth rate is the highest on the early stages and gradually decelerated with age. That reduces a need in dividing cells (mostly represented by cell type that cannot proceed to specialization) and therefore their surplus must have occurred. The highest surplus level of unemployment dividing cells must have been reached up to the moment of sexual maturity because at that time the decelerating of the growth rate is the greatest. At that time, the critical level of unemployed dividing cells may threaten the integrity of the organism and to prevent this .
. What could have happened in an organism during that dramatic period is another, not written yet, story that shed light on a mystery of Sex.
Edited by miosim, : No reason given.
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