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Sex and Senescence

  1. Nov 28, 2012 #1
    I've always been curios about aging and sex. These two attributes seem to make no sense from an evolutionary perspective, at least in terms of raw numbers. I recently saw a science show that tracked some fish that could reproduce asexually and sexually, and that study pretty much showed that sex is a highly valuable attribute for the red queen scenario we see between pathogens and hosts (ie it kept the hosts alive by having greater genetic diversity and varying disease resistance, whereas the asexually reproducing fish all perished to disease). This makes complete sense to me, in fact we see this very effect in the psudosexual gene swapping in bacteria, also it's noteworthy that our own species has a built in mechanism that increases attraction between individuals with different antigen/antibody detectors/producers.

    This got me thinking about senescence, and it seems to me the two may be linked. Let's assume you have an organism that doesn't age, it lives for a long time, producing many offspring; when a pathogen comes along and that attacks that organism - many of it's offspring will be affected, which could have a profound effect on an entire population and even species. Now with senescence the parent organism dies, thus forcing more gene swapping and reproduction with other individual organisms, thus increasing the genetic differentiation in the population and presumably the overall disease resistance of the population. In other words I think aging is the price we pay for sex, at least that's how evolution on earth seems to have been driven, since all sexually reproducing organisms I know of age.

    Does this hypothesis make any sense to anyone else? How would one go about testing this hypothesis? There is some supporting evidence, such as animals with rapid senescence generally have very robust immune systems, such as opossums. I'm interested in others ideas not just my own, which is why I am posting this here.
    Last edited: Nov 28, 2012
  2. jcsd
  3. Dec 9, 2012 #2
    One problem is that there are many organisms that don't reproduce sexually that age. There are species of salamanders where all the members are female. They reproduce by parthenogenesis. There are other organisms which reproduce primarily by parthenogenesis. All of them age.

    Some large plants don't age the way animals do. A redwood tree is a sexual organism. It produces males spores and female spores. Yet, it doesn't age. There is no fixed lifespan to a redwood tree. Pine trees can last until a parasite, or fire, kills them.

    Many protozoa (e.g., paramecium) conjugate as well as divide. So they have a form of sex. However, there is no time limit on the number of generations such protozoa live. Basically, a protozoa cell line can last forever unless you equate conjugation with death.

    The equivalence of senescence and sexual reproduction has too many counter examples for me to take seriously. Organisms that live in parasite laden environments can age slowly or quickly, depending on what other challenges the organisms face.

    I would generalize a different way. All animals age. By animals, I mean a multicellular organism that is mobile at some time of their lives.

    I think it has something to do with cancer. Cancer can strike at any age. Although weighted toward later ages, it can take down someone who is very young. Cells that mutate often end up crawling through the body and reproducing without limit.

    Placing a time limit on the number of times a cell can divide can help prevent cancer among the young. The reason that aging doesn't happen in very large plants is that plant cells are not mobile. A mutated cell can't do as much damage in a plant because the mutated cell can't travel. Therefore, natural selection for senescence wasn't as great for plants as animals.

    Note that there are some multicellular organisms other than animals that age. Slime molds are amoeba that periodically gather into large single organisms sometimes called "slugs". However, these multicellular constructs don't last very long. They form fruiting bodies. One would think that if mutation wasn't a problem, some slime molds would evolve to stay slugs for most of their lives.

    On the other hand, slime molds are highly sexual. Some species have as many as five genders! So maybe the short lifetime of a slug has something to do with all the sex the individual amoeboid cells have!
  4. Dec 23, 2012 #3
    Interesting. I was not aware that there were conifers that did not have senescence (though I should have realized this due to the argument about the biggest and oldest "organism" rests on definitions, partly due to a specific pine forest of clones). By the way your first example of salamanders (there is also a parthanogenic species of lizard in california) are not good examples, as these species clearly evolved from ancestors that have senesence, and the way aging works doesn't seem like something that could be easily lost. The conifer example is a good one though, and pretty much destroys the validity of this idea. Thanks for responding though.
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