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NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE;Why Sex Is Good;by;Clyde Freeman Herreid;Department of Biological Sciences;University at Buffalo, State University of New York;Birds do it. Bees do it.;Even educated fleas do it.;Lets do it. Lets fall in love.;Cole Porter;Part IIt;Why do so many organisms go through sexual reproduction? It seems like every organism we think about;does it: clams, jellyfish, trees, and elephants. And while were thinking about it: why only two sexes? It;doesnt have to be that way. Some fungi have dozens of sexes, enough to keep a romance novelist and a;scriptwriter of soap operas ecstatic for years.;Sex really isnt necessary for reproduction. Bacteria and many one-celled organisms like amoebae reproduce;quite nicely by simply dividing in half (binary fission). They produce identical copies of themselves, quite an;efficient way of sending ones genes on to the next generation. They do it alone. For them, it doesnt take two;to tango.;Complex organisms can do it too. Some lizard species have only one sexfemales. They reproduce;parthenogeneticallythat is, females produce eggs that spontaneously start development without sperm;being involved at all. They are completely asexual.;Some species have it both ways: they reproduce both sexually and asexually. Queen bees when they produce;females (workers) release sperm out of a storage sac and fertilize the egg in the normal way, but when they;want to produce males (drones) they hold the sperm back and the eggs develop by parthenogenesis.;Water flea (Daphnia) populations seem to switch from asexual to sexual depending on environmental;conditions. And some species of fish actually switch from being one sex to the other depending on which;gender is in short supply. Science fiction writers should love these gender benders.;So, this brings us to a fundamental question that biologists have not solved: If organisms can survive well;without sexin fact, may do better without itwhy has sexual reproduction evolved?;Questions;1. Propose three hypotheses to explain why sexual reproduction has evolved. (At least 20 have been;suggested!);2. Can you propose any way to test your favorite hypothesis?;Why Sex Is Good by Clyde Freeman Herreid;Page 1;NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE;Part IIIs It Always Good?;In a world without sex there would be no males and females. No flowers, no;insects specialized in pollinating them, no extravagant colour and form like the;peacocks tail, and much animal behaviour would not exist. Rolf Hoekstra;All of that is true, but so what? Who needs this stuff that Hoekstra is talking about for survival?;The great German biologist August Weismann proposed an answer to the question of Why sex? He;asserted that sex increases genetic variation. When two different individuals mate by joining their gametes;together, they produce a brand new genetic mixture and this promotes evolutionary adaptation.;This idea held sway for a hundred years until a couple of authors, George Williams and Maynard Smith, said;Hold on. There are a couple of problems with this scenario. Sex is not always good.;1. Mixing of the genes tends to break up favorable combinations. Why break up a good thing?;2. Asexual reproduction is twice as efficient as sexual reproduction at sending ones genes into the;next generation. Every time a sexual mother produces a child, that child only has one-half of the;mothers genes, the other half is from dad. An asexual mother reproducing parthenogenetically;would give her child the complete set. In fact, it is better to have every individual in a population;capable of reproduction (i.e., be a female) than to have individuals who are not (i.e., be a male).;Such populations should rapidly out-reproduce a sexual population. This has been called the twofold cost of sex.;On both of the above counts, it seems clearly disadvantageous for individuals to reproduce sexually! Yet sex;has evolved and seems here to stay.;Many scientists have tried to puzzle their way out of this dilemma by testing some of the assumptions;inherent in the argument.;Questions;1. Can you design a way to test the hypothesis that asexual reproduction leads to a higher;evolutionary fitness (i.e., leads to more progeny) than sexual reproduction?;Why Sex Is Good by Clyde Freeman Herreid;Page 2;NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE;Part IIISex and Stress;There is a snail that lives in New Zealand lakes that has both asexual and sexual individuals. Curtis Lively;(currently at Indiana University) and his colleagues decided that the snails could be used to test the;hypothesis that a changing or stressful environment would favor sexual reproductionthe logic being that if;the environment changes, then variation (sexual reproduction) is a good thing, some of your offspring might;have the right genetic constitution to survive.;Heres the situation the biologists found. The snails live in freshwater habitats and there are over a dozen;worm parasites that attack them. The scientists reasoned that there might be a difference in the fitness of the;asexual and sexual individuals in ponds where there were different degrees of parasitism.;This is what they found: in ponds where there was a high degree of parasitism there was a much higher;percentage (2.5 times more) of sexually reproducing individuals.;Questions;1. Before carrying out the experiment, why did the scientists expect there would be a difference in;fitness between sexual and asexual snails in ponds with different degrees of parasitism?;2. Are the data they obtained consistent with Weismanns hypothesis? Explain your thinking.;Why Sex Is Good by Clyde Freeman Herreid;Page 3;NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE;Part IVAn Experiment;A team of scientists at the Imperial College London tackled the problem and published their results in;Nature magazine (March 25, 2005). They decided to use yeasts, which are single-celled fungi, because they;can reproduce both sexually and asexually, are easy to keep in the lab, and reproduce very rapidly.;Yeasts normally reproduce asexually, but when they are stressed (starved, high temperatures, etc.) they will;reproduce sexually. The scientists did not want this switching to occur. So they genetically manipulated one;asexual strain. They deleted the two genes (spo11 and spo13) required for normal meiosis, so that sexual;reproduction was impossible. Now they had two pure strainsasexual and sexual.;The Imperial College team decided to compare the reproductive rate of the asexual vs. the sexual yeasts;in two different environments: harsh and benign. That is, fitness would be measured by comparing the;growth rate relative to the non-evolved ancestral strain. The benign environment had plenty of nutrients;although glucose was limited so that growth was not uncontrolled. The harsh environment had the same;glucose concentration but was at a higher temperature and had more demanding osmotic conditions.;Question;1. On the graph below plot the results you would expect if Weismanns hypothesis were correct. Plot the;changes in fitness values over time in the populations of sexual yeasts in benign conditions, asexual yeasts;in benign conditions, asexual yeasts in harsh conditions, and sexual yeasts in harsh conditions.;Natural logarithm of relative fitness;1.0;0.8;0.6;0.4;0.2;0.0;-0.2;0;50;100;150;200;250;300;Mitotic generation;Why Sex Is Good by Clyde Freeman Herreid;Page 4;NATIONAL CENTER FOR CASE STUDY TEACHING IN SCIENCE;Part VThe Results;Here are the results of the real experiment.;The change in natural logarithm of fitness of asexual and sexual populations of yeast in benign and harsh;environments. Points show fitness measurements for individual populations with twice log-likelihood error;bars (these approximate 95% confidence limits), the error bars for the benign treatment are plotted but;are mostly too small to be discriminated. The fitted model for the harsh environment is plotted for asexual;(blue) and sexual (red) treatments (parameters: a1 = 0.761, a2(asexual) = -5.287, a2(sexual) = -4.901). Yellow;symbols, asexual strains in the benign environment, green, sexual in the benign environment, blue, asexual;in the harsh environment, red, sexual in the harsh environment. (Reprinted by permission from Macmillan;Publishers Ltd: Nature 434, 636-640, doi:10.1038/nature03405, copyright 2005.);Question;1. What conclusions can you make based upon the data?;Credit: Licensed image in title block Alwyn Cooper | iStockphoto, id #121897. Case copyright held by the National Center for;Case Study Teaching in Science, University at Buffalo, State University of New York. Originally published September 5, 2005.;Please see our usage guidelines, which outline our policy concerning permissible reproduction of this work.;Why Sex Is Good by Clyde Freeman Herreid;Page 5


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