The Molecular Basis Behind One of Nature's Most Powerful Evolutionary Tools

Introduction: Sexual reproduction is defined as the fusion of two haploid cells during fertilization to create a diploid cell (Vacquier, 1998, p.1995). Sexual reproduction and particularly fertilization are the foundation of the determination of whether certain organisms are able to mate and create viable offspring with one another. There are many mechanisms which make it impossible for some organisms to reproduce with one another. The inability of organisms to reproduce with one another is sometimes referred to as reproductive isolation. The geographical area or the season in which an organism survives may not be compatible for them to mate with certain other organisms. Even if species are found in the same location and during the same season, they may have no sexual attraction to one another. If location and sexual attraction were to make courtship possible, it may be physically impossible for mating to occur due to a dramatic difference in genitalia size. In some cases, even when mating has occurred, specific reactions in the female to these foreign bodies will cause them to become ineffective. In cases where the sperm reached the egg and fertilization occurred, the zygote may not develop or will develop into an organism with reduced viability. Lastly, if a viable offspring is created, often they will be infertile and incapable of creating their own offspring. In this paper I will focus on the last of the barriers before a zygote is formed. (Hall and Hallgrimsson, 2008, pp. 623-625).

Support: One of the last and most intriguing barriers for prezyogtic reproductive isolation includes the ability of the sperm to reach the egg, attach to the egg, and then enter into the egg's cytoplasm. Proteins are the basic structure used at this time that make isolation possible. In many biological processes (i.e. cell division), the proteins involved are greatly conserved between very distantly related species. In fertilization however, the proteins involved are highly diverse. There is also a high level of structural diversity of fertilization associated proteins between very closely related species, possibly indicating that these proteins have a high rate of evolutionary change. The variation in these fertilization proteins between species is likely the method used to cause the mating isolation. To understand how these proteins work, we must first understand the process of fertilization (Vacquier, 1998, p. 1995).

Sperm are chemotactically attracted to swim toward the egg because of molecules released by the egg. Depending on the species, the sperm release a structure known as the acrosomal process either before or after binding to the egg. After the acrosomal process has been formed, a hole in the egg envelope is formed through which the sperm passes. The sperm then enters the egg's cytoplasm and eventually fusion of the nuclei occurs (Vacquier, 1998, p. 1995).

The attractants that are used to bring the sperm in proximity to the egg are not usually well known. However, the attractants that are known are very different from one another in unrelated species. This indicates that they likely evolved independently in different phyla (Vacquier, 1998, p. 1995).

Once the sperm reaches the egg, the Acrosomal Reaction must take place. This reaction is set off by more chemical signals provided by the egg. There are many different types of chemicals that can induce the Acrosomal Reaction in sperm. These chemicals are highly diverse and have no obvious evolutionary homology across phyla (Vacquier, 1998, p. 1995).

It is also imperative that the sperm attach to the egg envelope. This attachment is achieved through the binding of proteins on the cell surface of the sperm to sugar residues on egg glycoproteins (Vacquier, 1998, p. 1995). The proteins that are found on the egg envelope are highly divergent. Aagaard et al. (2006, p. 1703) give evidence that the proteins of abalone species can differ by as much as 66% of nonsynonymous nucleotide positions. When nonsynonymous mutations significantly outnumber synonymous mutations it is a good indication that positive selection is taking place (Swanson et al., 2001, p 2510). Aagard et al (2006, p. 1705) give two possibilities for the cause of this positive selection. The first possibility is that the egg protects itself from foreign microbes attaching to its surface by constantly changing the structure of its envelope proteins. The other possibility is that by constantly changing the structure of its envelope proteins, it allows fewer sperm to be able to attach and therefore can help prevent against polyspermy.

The sperm must pass through the egg envelope in order to be able to fuse with the egg. In order to do this, it must digest a passage through the envelope. Once the sperm has reached the plasma membrane of the egg, binding must take place. Again, this binding is dependent on proteins that are species specific. A diverse assembly of proteins are used in these processes and once again, they are very species specific (Vacquier, 1998, p. 1995).

Conclusion: The definition of a species is a difficult thing to pinpoint. One of the most common definitions is that a species is an interbreeding group distinct from other such groups (Hall and Hallgrimsson, 2008, p. 623). This definition is based on the fact that interbreeding is impossible between some populations. Part of the impossibility of interbreeding is caused by the evolution of proteins which make these sperm-egg interactions impossible. We have to give much of the credit for speciation as we know it today to these tiny particles that make such a huge difference in our world.

Literature Cited

Aagaard, J. E., X. Yi, M.J. MacCross, W.J. Swanson. 2006. Rapidly evolving zona pellucida domain proteins are a major component of the vitelline envelope of Abalone eggs. Proceedings of the National Academy of Sciences of the United States of America 103 (46): 17302-17307.

Hall, B. K. and B. Hallgrimsson. 2008. Strickberger's Evolution, 4th Edition. Jones and Bartlett Publishers, MA. 760 pp.

Swanson, W. J., Z. Yang, M.F. Wolfner, C.F. Aquadro. 2001. Positive Darwinian Selection drives the evolution of several female reproductive proteins in mammals. Proceedings of the National Academy of Sciences of the United States of America 98 (5): 2509-2514.

Vacquier, V. D. 1998. Evolution of gamete recognition proteins. Science 281 (5385): 1995-1998.