The Origin of Gender: Adam & Eve – themedideas

  • Sunday, June 16th, 2013

Updated on 31.10.2016.

So God created mankind in his own image, in the image of God he created them; male and female he created them … the sixth day.

Genesis 1, New International Version. http://biblehub.com/niv/genesis/1.htm

 

How and why did the gender start? What is the theory behind the origin of sexual reproduction? It is quite fascinating!

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The theory explaining the origin of Eukaryotic cells suggests that Eukaryotic cells were formed by merger of two bacteria (Prokaryotic cells). Following merger some of the DNA materials of the engulfed bacteria were incorporated into the nuclear DNA materials of the engulfing bacteria and the rest of the DNA materials of the engulfed bacteria formed the mitochondria. There are two theories for the origin of sex. One is that sex helps us in our evolutionary battle against parasites. The other theory is that sex helps to remove bad (mutant) genes (Kondrashov, 1988; Ridley, 2000).

The Theory (Leda Cosmides and John Tooby)

Origin of Eukaryotic cells

* Eukaryotic cells 2000 mya

* Multicellular eukaryotes, including animals before 580 mya

* The history of life contains two difficult steps – from prokaryotic cell to eukaryotic cell, and from single-celled to multi-celled eukaryotic life.

* Merger of two bacteria –> Incorporation of some DNA materials of the engulfed bacteria into the nuclear DNA materials of the engulfing bacteria, the rest of the DNA materials of the engulfed bacteria formed the mitochondria.

Origin of complex life

* Formation of gender to avoid the conflict between different organelles.

Origin of Gender

* Formation of mutant gene in the gamete nucleus e.g. “organelle-excluder” (developed into the egg) and “organelle-ejector” (developed into the sperm). The “organelle-ejector” mutant gene ejects its own organelles before fertilising another cell. The “organelle-excluder” mutant gene excludes organelles of other cell.

Validity of the theory

Bacteria + Bacteria

Eukaryotic cell

Incorporation of some of the DNA of the engulfed bacteria into the nuclear DNA materials of the engulfing bacteria, the rest of the DNA materials of the engulfed bacteria formed the mitochondria

Presumably no conflict existed between the DNA of the engulfing bacteria and the DNA of the engulfed bacteria as probably they complemented each other (synergistic merger)

Replication

More eukaryotic cells (with nuclear and mitochondrial DNA without any conflict)

Queries

How and why did the gamete cells develop. It has not been explained. It is important because the theory is based on appearance of nuclear mutant genes in the gamete cells. The gamete cells in mammals contain half the number of chromosomes that is present in a somatic cell (e.g. 23 in humans). In life forms using sex (merger of two gametes) the number of chromosomes would have been half that of the somatic cells. No explanation has been given about their origin.

According to the theory, a gamete containing a nuclear mutant gene “organelle-ejector” (sperm) ejects its organelle and fertilises a gamete containing a nuclear mutant gene “organelle-excluder” (egg) that retains its own organelle and excludes organelles of other gametes. If the organelles in the gamete containing the “organelle-ejector” gene (sperm) are ejected before the gamete fertilises another gamete containing the “organelle-excluder” gene (egg), the “organelle-excluder” gene would be superfluous as there would not be any organelles left in the gamete containing the “organelle-ejector” gene (sperm). In fact, it has been shown that in humans, hundreds of sperm mitochondria may enter the egg at fertilisation, and destroyed afterwards. It may be explained by the presence of “organelle-excluder” gene in the egg nucleus. Probably, there is “organelle-excluder” gene in the nucleus of all cells that excludes organelles of any other cells. From experiments in nuclear transfer (cloning), where the nucleus from one somatic cell is transferred to an enucleated cell, it is apparent that the mitochondria in the recipient cell are not excluded. Therefore, the theory of the presence of an “organelle-excluder” gene in the nucleus of all cells that excludes organelles of any other cells, cannot be supported. Similar findings have also been observed in experiments on artificial oocytes developed by nuclear transfer (transfer of the nucleus of an oocyte from an older woman to an enucleated oocyte from an younger woman). So, it is apparent that the egg nucleus does not exclude mitochondria of another egg. The available evidence does not preclude the presence of “organelle-excluder” gene in the egg nucleus that excludes sperm mitochondria only.

If the “origin of gender theory” is accepted as it is, following fertilisation, the sperm mitochondria will be destroyed, and the fertilised ovum (subsequently the embryo and fetus) will contain maternal mitochondria only. The sperm nucleus fuses with the ovum nucleus along with the fusion of the chromosomes restoring the diploid chromosomal complement (46, in the humans). The “organelle-ejector” gene in the sperm nucleus will have been incorporated in the embryo genome along with the “organelle-excluder” gene in the ovum nucleus. Therefore, when the baby is born and becomes an adult it will contain both the “organelle-ejector” and the “organelle-excluder” genes. If it were a girl, both the “organelle-ejector” and the “organelle-excluder” genes will be present in the ovum, which is self-contradictory to the “origin of gender theory” that states presence of “organelle-excluder” gene in the ovum. If “organelle-ejector” gene were present in the ovum it will eject its own mitochondria, which does not happen in reality. On the other hand, if the baby were a boy, both the “organelle-ejector” and the “organelle-excluder” genes will be present in the sperm, which is again self-contradictory to the “origin of gender theory” that states presence of “organelle-ejector” gene in the sperm. If “organelle-excluder” genes were present in the sperm it will exclude the ovum mitochondria.

If we were to accept the presence of “organelle-ejector” gene in the sperm, the plausible explanations would be the following:

* Suppression of the “organelle-ejector” gene action in the fertilised ovum by the ovum nucleus

* Suppression of the “organelle-ejector” gene action in the fertilised ovum during the fusion of the male and female pronuclei

Discussion

According to the organelle theory of gender, following fusion of two gametes conflict appeared between the organelle (mitochondrial) genes of the gametes. If the conflict was not resolved, their organelles fought it out and the cell was damaged. Now two nuclear mutant genes arise. A gamete containing a nuclear mutant gene “organelle-ejector” ejects its organelle (male) and fertilises a gamete containing a nuclear mutant gene “organelle-excluder” that retains its own organelle and excludes organelles of other gametes (female). Natural selection will favour the fusion of an “organelle-excluder” gamete (female) with an “organelle-ejector” gamete (male) as they are complementary to each other without any conflict between the organelles (Ridley, 2000).

If the organelles in the gamete containing the “organelle-ejector” gene (male) are ejected before the gamete fertilises another gamete containing the “organelle-excluder” gene (female), the “organelle-excluder” gene would be superfluous as there would not be any organelles left in the gamete containing the “organelle-ejector” gene (male). In fact, it has been shown that in humans, hundreds of sperm mitochondria may enter the egg at fertilisation, and eliminated afterwards by a ubiquitin-dependent mechanism (Sutovsky and Schatten, 2000). This phenomenon of elimination of the paternal mitochondria has been shown by experiments in mice. Rejection of paternal mitochondria by the embryo normally occurs at the 4- to 8-cell stage in mice and is apparently dependent on mutual recognition between the mitochondria and the nuclear genome (Cummins et al, 1999). This raises question about the existence of “organelle-ejector” gene in the male gamete that is supposed to eject its organelles before fertilising a female gamete, not after fertilisation. It may, however, be explained by the presence of “organelle-excluder” gene in the egg nucleus, which following fertilisation, recognises the organelles originating from the sperm and destroys them.

Following fertilisation in humans, the ovum would produce either a female or a male zygote depending on whether it is fertilised by a sperm containing an X or Y chromosome respectively. The chromosomes of the sperm nucleus containing ‘organelle-ejector’ mutant gene fuse with the nuclear chromosomes of the ovum containing ‘organelle-excluder’ gene to form the diploid genome of the zygote. If the ‘origin of gender’ theory were correct, the nuclei of the gametes of the resulting offspring would have the ‘organelle-excluder’ gene if it is a female or the ‘organelle-ejector’ gene if it is a male. It is not clear how this could happen when the sperm-nucleus containing ‘organelle-ejector’ gene (paternal) fuses with the ovum-nucleus containing ‘organelle-excluder’ gene (maternal) during fertilisation. The plausible explanation would be that, following fertilisation, in a female zygote ‘organelle-excluder’ gene in the ovum-nucleus suppresses the ‘organelle-ejector’ gene in the sperm-nucleus (containing X chromosome) and in a male zygote the ‘organelle-ejector’ gene in the sperm-nucleus (containing Y chromosome) suppresses the ‘organelle-excluder’ gene in the ovum-nucleus (hypothesis).

Now the other question is which chromosome(s) in the nucleus carries the mutant genes ‘organelle-excluder’ and ‘organelle-ejector’? As these are supposed to be characteristics of the gametes, autosomes are unlikely to contain them. Even if we presume that the autosomes contain them, then the ‘organelle-ejector’ genes present in the sperm could be passed to the daughters unless there is a mechanism by which these are selectively absent in the sperm containing the X- chromosome. This would be highly unlikely.  Even if we presume that the mitochondria contain them, then the ‘organelle-ejector’ genes present in the sperm would not be passed to the sons as the paternal mitochondrial DNA would be destroyed following fertilization.

Let us presume that the X-chromosome in the ovum-nucleus contains the ‘organelle-excluder’ gene, the Y- chromosome in the sperm-nucleus contains the ‘organelle-ejector’ gene and the X- chromosome in the sperm-nucleus does not contain any of the mutant genes:

If the ovum is fertilised by a sperm containing Y-chromosome and forms a male offspring, the ‘organelle-excluder’ gene in the ovum-nucleus and the ‘organelle-ejector’ gene in the sperm-nucleus will act synergistically to eliminate the sperm-organelles. But what is not clear is what happens to the ‘organelle-excluder’ gene in the ovum-nucleus after fusion with the sperm-nucleus. According to the organelle theory of gender, the ‘organelle-ejector’ gene should remain active as it is a male offspring and the ‘organelle-excluder’ gene should be suppressed. The mechanism of suppression of the ‘organelle-excluder’ gene is unknown. If the ovum is fertilised by a sperm containing X-chromosome and forms a female offspring, the ‘organelle-excluder’ gene in the ovum-nucleus will eliminate the sperm-organelles. The concept of the presence of the ‘organelle-excluder’ gene in the X-chromosome of the ovum-nucleus and the absence of the ‘organelle-excluder’ gene in the X-chromosome of the sperm-nucleus is contradictory in itself as the X-chromosome of the sperm-nucleus was inherited by the male from his mother and thereby should contain the ‘organelle-excluder’ gene.

Let us presume that the X-chromosome in the ovum-nucleus contains the ‘organelle-excluder’ gene, the X- chromosome in the sperm-nucleus contains the ‘organelle-ejector’ gene and the Y- chromosome in the sperm-nucleus does not contain any of the mutant genes:

The X-chromosome in the sperm was actually inherited by the male from his mother, and cannot contain the ‘organelle-ejector’ gene (a characteristic of male gametes). In contrast it may contain the ‘organelle-excluder’ gene originating from his mother’s ovum-nucleus. Therefore, the X-chromosome in the sperm-nucleus cannot contain the ‘organelle-ejector’ gene.

Let us presume that there is no ‘organelle-ejector’ gene and the X-chromosome contains the ‘organelle-excluder’ gene:

If the ovum is fertilised by a sperm containing Y-chromosome and forms a male offspring, the ‘organelle-excluder’ gene in the ovum-nucleus will eliminate the sperm-organelles. The difficulty now is that the ‘organelle-excluder’ gene will be present in the X-chromosome containing sperm of this offspring and will create problems when the sperm fertilises ovum (with the ‘organelle-excluder’ gene) in future. This problem could be avoided if, in the male offspring, the Y- chromosome has a suppressing effect on the ‘organelle-excluder’ gene in the ovum-nucleus after fertilisation. If the ovum is fertilised by a sperm containing X-chromosome and forms a female offspring. The ‘organelle-excluder’ gene in the X-chromosome of the sperm must be inactive to avoid the problem of destroying the organelles of the ovum. This raises another question. If in the resultant female offspring the ‘organelle-excluder’ gene in the X-chromosome contributed by the sperm remains inactive, there will be problems in future when her ovum containing the inactive ‘organelle-excluder’ gene in the X-chromosome gets fertilised by sperm as the sperm-organelles will not be eliminated after fertilisation. There are two possibilities by which this problem could be avoided. Firstly, the inactive ‘organelle-excluder’ gene in the X-chromosome gets activated in presence of an active ‘organelle-excluder’ gene in the other X-chromosome contributed by the ovum. Secondly, the X-chromosome containing the inactive ‘organelle-excluder’ gene gets eliminated in the polar body during meiotic division of the oocyte and does not get involved in fertilisation. The first one is more probable as accepting the second explanation means that the paternal X-chromosome reaches a dead end in the female offspring, which is not true.

The hypothesis that there is no ‘organelle-ejector’ gene and the X-chromosome contains the ‘organelle-excluder’ gene would be valid if the ‘organelle-excluder’ gene becomes inactivated in the male.

Let us presume that there is no ‘organelle-excluder’ gene  and the Y-chromosome contains the ‘organelle-ejector’ gene:

If the ovum is fertilised by a sperm containing Y-chromosome and forms a male offspring, the ‘organelle-ejector’ gene in the sperm-nucleus will eliminate the sperm-organelles. If the ovum is fertilised by a sperm containing X-chromosome and forms a female offspring, the sperm-organelles would be eliminated as there is no ‘organelle-ejector’ gene in the sperm containing X-chromosome.

There are other explanations why the sperm-mitochondrial-DNA is eliminated from the fertilised ovum. Allen hypothesises that the sperm sacrifices the mitochondrial-DNA to maximise energy production for motility (Allen, 1996). This is not supported by the fact that paternal mitochondrial-DNA is excluded from the embryo in cases of nuclear transfer and Intra Cytoplasmic Sperm Injection where motility of the donor cell/sperm is not required (Evans et al, 1999; Houshmand et al, 1997). The possibility of selective elimination of the paternal mitochondrial-DNA by nuclear DNA of the ovum is not supported by the fact that it happens in nuclear transfer where the nucleus of the recipient cell is absent (Evans et al, 1999). This indicates that the oocyte mitochondria or the mitochondrial-DNA may have a role in eliminating the paternal mitochondrial-DNA. In an experiment to determine the fate of mitochondria and mitochondrial-DNA after entry of human sperm into transformed cells containing mitochondria devoid of endogenous mitochondrial-DNA ( rho0 cells ), very few somatic cells harbouring functional mitochondrial-DNA survived and proliferated (Manfredi et al, 1997). It may indicate an inherent weakness of sperm mitochondrial-DNA in terms of survival. The hypothesis that a deficiency of RNA in the spermatozoal mitochondria may contribute to the loss of paternal mitochondria has been refuted by Alcivar et al (1989). Another posibility would be that the mitochondria and/or mitochondrial-DNA, irrespective of whether it is in the sperm, ovum or somatic cell, may not survive in a foreign environment when it enters a different cell (hypothesis). In a study in nuclear transfer cloned sheep, it was observed that the mtDNA was derived exclusively from the recipient enucleated oocytes, with no contribution from the respective donor cells. The mtDNA of each of the ten nuclear-transfer sheep was derived exclusively from recipient enucleated oocytes, with no detectable contribution from the respective somatic donor cells (Evans et al, 1999). This study indicates the vulnerability of the mitochondria in somatic cells when these enter a foreign cell. Another study, however, showed the transmission of donor mtDNA into somatic sheep clones. MtDNA heteroplasmy was detected in seven of 12 clones tested, whereby all but one case revealed less than 1% mtDNA contribution from the nuclear donor cell (Burgstaller et al, 2007).

In conclusion, there are several complex issues regarding the organelle theory of gender. The presence of the ‘organelle-ejector’ gene in the male gamete is highly questionable. The presence of the ‘organelle-excluder’ gene in the female gamete, although probable, requires a lot of explanations regarding its location and mechanism of action. A clearer explanation from the evolutionary Biologists would be useful.

References

Alcivar, A.A., Hake, L.E., Millette, C.F., Trasier, J.M., Hecht, N.B., 1989. Mitochondrial gene expression in male germ cells of the mouse. Dev. Biol. 135, 263-271.

Allen, J.F., 1996. Separate Sexes and the mitochondrial theory of ageing. J. Theor. Biol. 180, 135-140.

Burgstaller JP , Schinogl P, Dinnyes A, Müller M and Steinborn R. Mitochondrial DNA heteroplasmy in ovine fetuses and sheep cloned by somatic cell nuclear transfer. BMC Developmental Biology 2007, 7:141.

Cosmides, L.M., Tooby, J., 1981. Cytogenetic inheritance and intrgenomic conflict. J. Theor. Biol. 89,  83-129. doi:10.1016/0022-5193(81)90181-8

Cummins, J.M., Kishikawa, H., Mehmet, D., Yanagimachi, R., 1999. Fate of genetically marked mitochondrial DNA from spermatocytes microinjected into mouse zygotes. Zygote 7, 151-156.

Evans, M.J., Gurer, C., Loike, J.D., Wilmut, I., Schnieke, A.E., Schon, E.A., 1999. Mitochondrial DNA genotypes in nuclear transfer-derived cloned sheep. Nat. Genet. 23, 90-93.

Houshmand, M., Holme, E., Hanson, C., Wennerholm, U.B., Hamberger, L., 1997. Is paternal mitochondrial DNA transferred to the offspring following intracytoplasmic sperm injection? J. Assist. Reprod. Genet. 14, 223-227.

Kondrashov, A., 1988. Deleterious mutations and the evolution of sexual reproduction. Nature 336, 435-440.

Manfredi, G., Thyagrajan, D., Papadopoulou, L.C., Pallotti, F., Schon, E.A., 1997. The fate of human sperm-derived mtDNA in somatic cells. Am. J. Hum. Genet. 61, 953-960.

Ridley, M. ed., 2000. Mendel’s Demon [Gene Justice and the Complexity of Life], Phoenix, London.

Sutovsky, P., Schatten, G., 2000. Paternal contributions to the mammalian zygote: fertilization after sperm-egg fusion. Int. Rev. Cytol. 195, 1-65.

 

Dr Sudipta Paul

Originally written in January 2005

 © Dr Sudipta Paul, themedideas.com, 2013

The Origin of Gender: Adam & Eve – themedideas
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