Monday, March 24, 2014

Are Males Worth It?

Selective pressures are the driving mechanisms for change in populations, and at one point in time, selective pressures chose the mechanisms of sexual reproduction over those of asexual reproduction. To help understand what types of things can decide whether the selective pressures sexual to asexual reproduction, let's take a look at the advantages and disadvantages of sexual reproduction.

Disadvantages

There are significant disadvantages to sexual reproduction, and while these are not all of them, here are some important ones. Most of these disadvantages are lessened, if not mitigated, by reproducing asexually.

Finding a Mate:

Superb Bird of Paradise
http://www.enlargeyourplaylist.com/wp-content/uploads/2011/12/superb-bird-of-paradise.jpg
Everything has an energy budget that it must balance by the end of the its life. Even on a smaller scale, if more energy than usual is consumed in one aspect of activity, then another such aspect suffers from a deficiency. Finding a mate is a very strenuous activity. For males, as can be seen in Superb Bird-of-Paradise (birds of paradise), elaborate mating rituals are very costly to the organism [4].This cost may be superficial as first, such as spending energy showing off to potential mates takes away energy from gathering food, but can also be less ostensible, such the price payed for such extravagant male characteristics - testosterone [1] - has its own set of adverse effects. Asexual mating removes the need to impress, and therefore the energy expenditure required to show off to the opposite sex. That very energy could be diverted towards reproducing more offspring, increasing the reproductive success of that organism.

The Video from class showing the intense mating rituals of birds of paradise:


Ain't Nobody Got Time for That:

Potential Octopus Threats
http://fc05.deviantart.net/fs4/i/2004/223/7/b/Gangster_Octopus.jpg
Sex itself consumes time. As can be seen from these dolphins, mating can happen very quickly. And as can be seen in these octopi, staying in the mating encounters could potentially become dangerous (fellow on the right). The primary reason why this is true, is because both partners are vulnerable to attack during intercourse. Besides copulation taking up time, sexual reproduction is usually coupled with time spent investing in the next generation. This may be anything from sitting on eggs as they incubate, to gathering food, to having to work two jobs to make sure tuition is paid. This idea returns to the one mention above: energy budget. Time is as important a resource as energy when it comes to budgeting and managing it. Organisms that have to allocate time to make sure their future generation is successful, are taking that time away from other activities. Say in some species, such as the bonobo, males are extremely polygamous [2]. If a male in that species had to expend extra energy and time to ensure that its progeny would survive, it would be directly impacting its ability to have access to other females and the potential increase in the total number of offspring it could have had. In fact, it is very difficult to find examples of creatures that have polygamous mating rituals and strong male parental care. This plays into the uncertainty males have about the father of the child.

http://2.bp.blogspot.com/-jU1X6anOvpE/TcpXXCyWLfI/AAAAAAAAACc/YGVP1gRiANE/s1600/19beautiful-bonobo-baby.gif

Not 100% Thrust into the Next Generation:

An innate problem with sexual reproduction is that not all of an organism's genes are present in the next generation. Naturally, about a half of the genes in an individual comes from one parent, the other 50% coming from the other parent. The goal of reproduction is to make sure that the organism's own genes are successfully transmitted, yet sexual reproduction places a theoretical limit of only half said organisms' genes continuing. As successive generations reproduce, one's progeny becomes less and less representative of those original genes. Asexual reproduction guarantees 100% transmittance.

Whenever I hear the concept of only 50% of genes being transferred to the next generation, I remember a movie I watched when I was still in grade school. The movie is called Rabbit Proof Fence, and it told the story of three Aborigine girls who escaped from the "Moore River Native Settlement" to return to their families. The movie details the events of the Stolen Generation, a plan run by many agencies in Australia to remove Aborigine children and raise them in a western fashion, eventually having them bear children that were half Aborigine and half European in an attempt to breed out the Aborigine. Here is short clip film someone adapted and annotated for what seems to be a school project. I warn that some of the scenes are very intense.


A Higher Chance for Infection:

Purple Martins
http://www.surfbirds.com/media/gallery_photos/20050710021606.jpg
Parasites play an interesting role in the ultimate selective pressure, opting for either sexual or asexual reproduction. Sexually Transmitted Diseases are commonly cited as disadvantages to sexual reproduction, but I hesitate to include them as something that was determinant of sexual or asexual reproductive strategies. STD's are opportunists that have evolved to become successful in their own right. By being passed from host to host through the exchange of fluids, STD's are just taking advantage of a easy bridge. Other forms of infection, such as maggots in Purple Martins, are similar in this regard (opportunists) to other notable STD's such as HIV [3]. What can be considered influential in the determination of which strategy, sexual or asexual, is the extended concept of STD's. The passing of something deleterious. Harmful things can be anything from viruses and bacteria to simply destructive DNA. Say, in a hypothetical sense, a mutation occurs in a generally asexually reproducing bacteria that leads to a gene that would cause the cell to die. Suppose this gene is usually deactivated, but say in the presence of lactose,  the mechanisms that would allow for the generation of lactase were defective and resulted in a protein that either did not digest the lactase, or otherwise led to harmful chemicals to persist in the cell. The cell, before the external pressure of lactose, would reproduce normally, and have many progeny. All of a sudden, there is a significant percentage of the population that would die because the harmful element was passed generation to generation. Asexual reproduction would not contact above the baseline, average intraspecies interactions (potentially even more reduced as contact and proximity required in mate selection would also be gone).

I wild Operon Appears

Boys:

There wouldn't be boys in asexual reproduction. While some would call this an advantage to asexual reproduction, I chose to ambivalent. Regardless, there is an impact of boys being in a population. Say the distribution between males and females was equal, 50-50. Compared to population of a 100 asexually reproducing "females," there are half the amount of reproductive females. The reproductive success of females is restricted by the number of eggs they hold, and the reproductive success of males is, similarly, dependent on how many mates they can encounter [10]. The theoretical maximum is therefore the total number of eggs females can consecutively get fertilized in their life times. The asexually reproducing population, would by default have twice as many females, and therefore a greater theoretical maximum.


There is snail native to New Zealand called the Mud Snail, Potamopyrgus antipodarum. The snail has the ability to reproduce both sexually and asexually [5]. The asexually reproducing snail creates clones, while the sexually reproducing produces genetically varying offspring. Parasitic influences from shallow water trematodes select for sexually reproducing snails that would have a varying genetic resistance to the parasite. In deeper water, the asexually breeding snail dominates due to the lack of parasitic pressures [9]. 


Advantages

Despite these substantial negatives against sexual reproduction, it was selected for, and still is. As I am sure you are aware, the purpose of all adaptations is to help a population survive. It is interesting that regardless of all the downsides, the positives make up for it.

Cleaning up DNA Mutations:

Crossing over during Meiosis
http://www.phschool.com/science/biology_place/labbench/lab3/images/crossovr.gif
"Cleaning up" is a loosely defined term in regards to DNA. The replication of genetic material has many steps and can be condensed into two main types of DNA replication: mitosis and meiosis [6]. The focus of this post is not the differences between the two, but if you want to learn more, click here. The main relevant difference is the result of the two mechanisms. Mitosis results in two daughter cells, each genetic copies of their mother, while meiosis results in four haploid daughter cells, each a different genetic combination of the mother, and only half the genetic material. It is not to say that genetic information is lost in the process, but simply redistributed. Sexual reproduction requires the conjugation of a haploid egg and a haploid sperm, coming from the female and male respectively. What does this have to do with "cleaning up?" It all has to do with recombination [7]. In sexual reproduction, genes have the chance to switch it up. In a mother cell of a sexually reproducing organism, half of the DNA comes from its mother and the other half from its father. So, when it divides into the four daughter cells, it could be assumed that it splits into two cells with the mother's genetic information, and two with the father's. This is not the case. During meiosis, pairs of chromosomes rearrange themselves, resulting in new combinations of maternal and paternal genetic material. Moreover, DNA can also partake in crossing over, which allows for individual chromosomes exchanging genetic material, increasing the genetic diversity of the resulting four daughter cells. These processes allow for new combinations of traits in progeny, causing the focus of selective pressures to fall upon individual traits and not just the successfulness of the entire genome. Overtime, mutations that could result in less favorable traits would be selected against, essentially cleaning the genetic material of a population. Evidence of this can be found in a study conducted by R. Stephen Howard & Curtis M. Lively, here.

Genetic Variability:

http://mentalfloss.com/sites/default/files/styles/article_640x430/public/105235698-565x399_6.jpg
One undeniable benefit to sexual reproduction is genetic variability. In a population that doesn't suffer from selective pressures doesn't necessarily feel the need for genetic variability. Certain selective pressures are time dependent. These could be environmental pressures, which are comparatively short-lived pressures. An example of a constant selective pressure is good old parasitism. As I was mentioning earlier, parasitism has a kind of two way street deal with sexual reproduction. Because of sexual reproduction, organisms, and males specifically, are more exposed to parasites. Increased energy expenditure on masculine displays of virility draw energy from other processes, such as immune response. This male immunity, which is already relatively weaker than a female's due to testosterone (necessary for secondary male characteristics), is further weakened by increased energy consumption elsewhere, increasing the compatibility filter for assailing parasites [10]. But, because of genetic diversity, the likelihood that all individuals in the species are susceptible to a parasite decreases, thus selecting for a future generation to have a greater percentage of resistant individuals. This benefit has enormous benefits. An example where the lack of sexual diversity affected humans was the Irish Potato Famine. Almost all of the potatoes in Ireland were of the same lumper variety. These plants had be planted vegetatively, meaning that they were all clones [8]. When the potato blight hit Ireland, nearly eradicated the entire potato population. The lack of genetic diversity led to the parasite infecting the entire population unrestricted. This is a major concern today, as farmers continue to plant the same variety of genetically modified grain in an attempt to increase their yield [9]. There is a very real threat to these genetically modified crops, as they are already resistant to many generic parasites, and the hypothetical infection which could attack, would already be resistant plenty of preventative measures.
http://familygonehealthy.com/wp-content/uploads/2013/10/Genetically_Modified_Corn.jpg

Conclusion:

Are males worth it? To many species, the answer is yes. The benefits of sexual reproduction outweigh the costs for many species. And while there are still many species of organisms that continue to reproduce asexually, it is as nature intended. Parasitic pressures drove organisms to reproduce in such a way that resulted in increased genetic variability in the population as well as the steady removal of unsuccessful traits. Sexual reproduction doesn't speed up evolution, and doesn't benefit an individual, but the entire population. An interesting thing to consider as well, some of the very downsides of sexual reproduction have become positives in that they give a certain degree of satisfaction. Finding a mate, for example, isn't time consuming, as it is usually the goal.

References:

[1] http://www.ncbi.nlm.nih.gov/pubmed/16671011

[2] http://pin.primate.wisc.edu/factsheets/entry/bonobo/behav


[3] http://www.purplemartin.org/update/Parasites.html


[4] http://www.australiangeographic.com.au/topics/wildlife/2011/04/birds-of-paradise-ritual-of-seduction/


[5] Fox J., Dybdahl M., Jokela J., Lively C. (1996). Genetic structure of coexisting sexual and clonal subpopulations in a freshwater snail (Potamopyrgus antipodarum). Evolution. 50 (4): 1541-1548


[5'] http://nas.er.usgs.gov/queries/factsheet.aspx?SpeciesID=1008


[6] http://www.diffen.com/difference/Meiosis_vs_Mitosis


[7] http://www.evolutionary-philosophy.net/sex.html


[7'] http://www.indiana.edu/~curtweb/Pubs/Howard%26Lively%201994%20Nature.pdf


[8] http://evolution.berkeley.edu/evolibrary/article/agriculture_02


[9] Zuk, M. Riddled with Life: Friendly Worms, Ladybug Sex, and the Parasites That Make Us Who We Are. Orlando: Harvest, 2008. Print.
[10] Combes, Claude. The Art of Being a Parasite. Chicago: University of Chicago, 2005. Print.

8 comments:

  1. When speaking of the disadvantages, you bring up a good point by talking about how mating is very energy costly. By going into detail describing how the energy cost is devastating to the population in regards to gathering food and health, you make a strong argument as to why sexual reproduction could possibly be bad. The biggest argument towards sexual reproduction that you have made, I believe, is the one that explains how genetic mutation helps to clean up and get rid of less favorable traits. A great article that may be good to explore further would be: http://www.diffen.com/difference/Asexual_Reproduction_vs_Sexual_Reproduction

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  2. At the end of your paper, you gave the example of how farmers are worrying about a genetic parasite killing all of the corn since the corn is genetically identicle. There are many organisms besides plants which produce asexually and have not extreme circumstances like the Irish Potato Famine. What would be some of the defenses organisms other than plants that produce asexually use to avoid having an entire population eradicated?

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  3. In your post you explain that sexual and asexual reproduction occurs in nature where it beneficial to a species of organisms. Your example of the Mud Snail is interesting because this species switches between sexual and asexual based on environmental pressures. When parasites increase in the snails, sexual reproduction is better because the population can evolve resistance. If the snails reproduced asexually, they would be continuously susceptible to the parasites. Therefore, as humans, sex is beneficial in a similar manner. Everyone would be suffering from the same diseases and parasites if we were asexual. There would be no evolution of genes to outwit the parasites and diseases...for a little time at least.

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  4. Great blog on this specific topic! You were very detailed and your videos were amazing to watch also. I really like how you described the genetic variability and gave the example of the Irish potato to demonstrate how the lack of genetic variability is not always a good thing; asexual reproduction has disadvantages also. When it comes to parasites and not being able to close the compatibility filter because the organism doesn't have the selection for survival genes, it is not beneficial to be asexual. One topic that I did think you should have addressed in your “Boys” section was the second of the three hypotheses about the relationship between parasitism, sexual dimorphism and longevity: “the stronger the bias toward males in rates of parasitism, the more pronounced is sexual dimorphism” - Combes. I think it would have flowed smoothly because you did discuss the connection with parasites and sexual dimorphism and I just feel that the hypothesis was a strong point that also connects with your topic of “Are Males Worth It?”

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  5. I really enjoyed discovering the different upsides and downsides to sexual reproduction. I noted that in the end you explained that sexual reproduction is the best option for most species as it eliminates parasites due to genetic variability, and that this wider array of genes allows for the exploitation of multiple niches and habitats You also mentioned that sexual reproduction eliminates mutations. However, this made me think; if sexual reproduction is so advantageous (despite having many disadvantages) why would organisms actively become asexual beings? Strikingly, some parthenogenic species are actually able to produce a limited amount of genetic variation by the very process of genetic mutations. I found it interesting that an advantage of sexual reproduction could be to get rid of mutations and increase variability, however these very mutations are the source of genetic variability in asexual reproducers.

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  6. I found in interesting in your post how you bought up points about how in an asexually reproducing spruces mutations can be detrimental to an entire population of bacterial cells. I had never thought of the possibility of a mutation that causes cells death. For me, this lends more evidence to why sexual reproduction evolved as a means of "cleaning up" mutations in genes. Not to mention, in sexual reproduction just because a parent has a mutation or a gene that is less favorable does not mean that it will be passed down to the progeny. Which is the opposite of the outcome of what happens in mutations that arise in asexual reproduction.

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  7. To help answer Austin's question, there are other methods of gene transfer other than traditional reproduction. Horizontal gene transfer, which includes conjugation (transfer of genetic material via a plasmid), transformation (uptake of genetic material from a cell's environment) and transduction (genetic transfer of material to another cell via a viral vector) can occur in viruses, prokaryotes, and eukaryotes. Through these mechanisms, bacteria can develop antibiotic resistance, proving horizontal gene transfer to help lessen the vulnerability of asexual reproduction.

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  8. I agree with how you have mentioned that sexual reproduction was selected for for multiple reasons including diversity which allows the ability to be more resistant to parasites. I also think that it is interesting to look at organisms that prefer asexual reproduction in order to maintain the relationship with certain parasites. An example of this can be seen between the nematode and bacteria Xenorhabdus discussed in "The Art of Being a Parasite." The nematode injects itself into an insect and then releases the bacteria that is stored in its gut. The bacteria will then digest the inside of the insect. Both the nematode and the bacteria get to eat and reproduce inside of the insect. When the nematode is ready to leave, it packs some of the bacteria away and leaves to find another insect. It seems that in this mutualistic relationship, being rid of the bacterial parasite would be detrimental to the nematode. The nematode cannot survive without the bacteria that lives inside of it and vice versa. So it seems that organisms can select for sexual reproduction to rid themselves of parasites like the New Zealand snail that you mentioned, but organisms can also select for asexual reproduction to keep their relationship with their parasites.

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