Field of Science

Genetics and Poker and Sex Oh PZ!

PZs title pic I got from here
The Minnesota Atheists talk today by PZ Myers was a lot of fun. Hopefully, he is making good progress back to Morris in the snowstorm, although I expect his Monday morning students are hoping he decides to be extra cautious. PZ's talk was on genetics and evolution, basically some of the ways genetics contributes to evolutionary changes as well as how molecular biology and genetics provides profound support for the Theory of Evolution. The particular tact PZ took was to make an analogy with poker to explain genetics and evolution. He obtained audience participants and used a deck of cards to 'play out' a poker hand with said participants, using the deal and resulting hands, to demonstrate chance and randomness, the idea of winners and losers, and to discuss the power of combinations (a gene does not act in isolation, nor does the queen of spades make a flush). The best part, to me, was the idea that everyone 'understands' poker, like everyone 'understands' evolution. Everyone may think they understand poker, and the professional poker players can make a pretty decent living off that fact. There was little new for me, but I expect it was a good talk for non-biologists, which was the expected audience.

What I wanted to touch on briefly was the discussion of sex, as in sexual reproduction. Sexual reproduction is a great way to generate genotypic diversity, half your DNA comes from mom and the other half comes from dad (or the mailman). So you are truly unique, there is no one on earth with the same genomic sequence as you (unless you are an identical twin, then there is one other). So that's great right? If you have an ok genetic repertoire, by sexually reproducing with someone else, you may be able to improve the genetic repertoire of your children. If you reproduced asexually, your children are as mediocre as you are. So now that you know the advantage of sexual reproduction, what's the problem?

The problem comes from the fact that you may have a kick-ass genetic repertoire and all sexual reproduction does is mix your awesome genes with some half-breeds genes, which moves your children back to mediocrity. If you're doing well in the environment you find yourself in, chances are your children won't be doing better if they get a mixed bag of genes. If you are dealt a flush, then you are not throwing cards away to try for the royal flush (in your children). You might get shit lucky, throw away the 5 of clubs and draw the 10 of clubs completing the royal flush, but mostly you'll draw a crap card and lose out to a pair of 3s.

So why do we have sexual reproduction? The best or at least most popular idea out there is The Red Queen Hypothesis, which comes from Lewis Carrol's Through the Looking Glass.

"...the Red Queen begins to run, faster and faster. Alice runs after the Red Queen, but is further perplexed to find that neither one seems to be moving. When they stop running, they are in exactly the same place. Alice remarks on this, to which the Red Queen responds: "Now, here, you see, it takes all the running you can do to keep in the same place"."

The idea being that environments change so rapidly that we must continually generate diversity to survive. The argument is often presented, as in Matt Ridley's book, from the perspective of hosts and parasites. Parasites grow rapidly and have hundreds to millions of generations in the time it takes a human to undergo one generation. So our children need to be different from us because our parasites may have learned how to beat us, but will not be ready to kill our (genetically) distinct children. This makes a great deal of intuitive sense and there is data to support this idea.

I think the red queen hypothesis may explain why we, and many other organisms, reproduce sexually now. However, it does not answer the question, why sexual reproduction evolved in the first place nor why it occurs in many eukaryotes today.
From here
In the above picture I want to point out that sexual reproduction, as in meiosis, only occurs in the eukaryotic (red) branch. The vast majority of life on this planet is in the bacterial (green) branch. Bacteria do not have sex. Furthermore, we can add in the archaeal (blue) branch to the organisms lacking sex. So focusing on the licentious red eukaryotic branch, we find that many organisms still do not have sex. Yes, the vast vast vast majority of creatures found in the zoo reproduce sexually, as do most plants, and many fungi (the crown groups of eukaryotes). However, the popular eukaryotes you know and love do not represent the majority of eukaryotes, nor even do all members of these groups we know and love reproduce sexually (see whiptail lizards).

From Scientific American
From the red queen hypothesis, we have to assume that whiptail lizards lack parasites since they don't have sex. In fact, numerous animals primarily reproduce asexually and others can go either way depending on environmental conditions. The point is sexual reproduction is not essential, even in multicellular animals. (Although I know of no mammals that reproduce asexually.)

What I find fascinating is that many fungi can take it or leave it. By that I mean some species that reproduce sexually have closely related sister species that do not. Take Saccharomyces cerevisiae (baker's yeast), it's can reproduce sexually. However, a related fungus Candida glabrata (a human commensal/opportunistic pathogen) does not appear to have sex.

Another fungal commensal/opportunistic pathogen of people, Candida albicans, also seems to lack sex. However, C. albicans retains many of the genes necessary for sex in fungi like S. cerevisiae and you can get C. albicans to do sex-like things in the laboratory. Regardless, if C. albicans has sex in nature, it's not often, not for several 10s of thousands of years anyway.

If we look across the eukarya and focus on those eukaryotic organisms that cause disease in people (which most eukaryotic microbiology research has focused on anyway), then we find many, dare I say most, are asexual. If sex is awesome for us to deal with parasites, wouldn't it be awesome for the parasites to deal with us? Regardless, I do find the idea of parasite warfare a great paradigm to explain sex today, but that really explains why sex is maintained in organisms like us, not why it evolved or is maintained in other eukaryotes.

When sex evolved, multi-cellular highly complex decade-long generational organisms didn't exist, so what was the selective advantage for sex? or was sex simply something that happened in an early eukaryotic lineage and was maintained fortuitously? Were environments more dynamic when sex evolved? If so why didn't sex evolve in the world-wide powerhouses, the bacteria?

We reproduce sexually as do most animals, a couple of lizard species excluded, therefore the biology of sex and sexual reproduction is of interest to us. But let's not assume that because something is important to us (like sex), that nature gives a damn.


Lab Rat said...

You need sex if you have a nucleus. If you don't, gaining genetic diversity is very, *very* easy you can just pick up random genes from anywhere and try them out. Once you package all the DNA away though you need to have all sorts of random twisted 'crossing over' and specialised sharing-DNA events in order to get any kind of diversity in your genome. And the risks of no genetic diversity at all are far too high.

The Lorax said...

I disagree with the statement "You need sex if you have a nucleus." for two reasons.

First, if true then, its the nuclear membrane that provides a barrier to DNA uptake. However, in a number of eukaryotic organisms that I am familiar with, if you simply permeablize the plasma membrane (done chemically or via electroporation) you can introduce DNA into the cytoplasm and then it doesn't appear to have a problem getting into the nucleus afterwards.

Second, you have to ignore all those eukaryotic organisms that do not have sex.

I agree that diversity is important long-term in a population. However, there are many ways to generate diversity without recombining and randomly assorting alleles. I would even argue that although at least some bacteria readily pick up DNA and horizontal gene transfer has clearly occurred in the bacteria, HGT is not the primary way bacteria generate diversity in the absence of sex. I posit DNA pol IV is a better diversity generator than hoping to find diverse genes in the environment.

Hmm, I see two more posts coming from this discussion...