Asexual Reproduction

The Evolution of SexAsexual Reproductionhttp://www.youtube.com/watch?v=ak-f-WKr0fg• Binary fission - The method bywhich bacteria reproduce. Thecircular DNA molecule is replicated;then the cell splits into two identicalcells, each containing an exact copyof the original cell's DNA.• Vegetative propagation - Type ofasexual reproduction in plants thatrelies on multicellular structuresformed by the parent plant.• Parthenogenesis - stimulation ofmulticelled divsion from haploidgamete (unfertilized egg).Asexual v. Sexual ReproductionHamilton’s computer model of reproduction:In a game between asexual and sexual reproduction, asexual reproductionalways wins -- other things being equal.1

“male” gametes have flagellato increase their ability to find“female” gametes…Evolution of two sexes• Dioecious (individuals male or female)• Essential difference: size of gametes– Medium-sized gametes lose out…• 50/50 sex ratio common in nearly all obligatorysexually reproducing species.Different gamete size requires different organarchitectureDifferent gamete size requires differentbehavioral strategiesFemale = the form of the organism thatcarries the most provisioning for theoffspring (i.e., the egg). All other "sexes" areby definition males.The paradox of sex - It’s complicated,costly and dangerous!1) ENERGY and TIME2) RISK of DEATH– Predation– Disease3

Advantage of asexual femalesThe 50% cost of sex - illustrated by John Maynard Smith (1978)Generation1SexualsF x MAsexualsFFraction ofindividuals that areasexual1/32FxMFxMF F F F1/23FxMFxMFxMFxMF F F FF F F FF F F FF F F F2/3clone of offspring multiplies at twice the rate of the progeny descended from a sexualfemale - ultimately asexuals should completely take over, but they don’t...tested by Dunbrack et al. (1995) asexual black beetles v. sexual red beetlesthe black asexual strain with thereproductive advantage loses outunder experimental conditions -eventually the sexual (evolving)strain eliminates the black beetleselimination rate dependentupon environmentalconditionsHypotheses on the evolution of Sex1) Unpredictable environment (ecological hypothesis)– sexresults in more genetically varied offspring, thus sexualreproduction can increase fitness by producing an offspring thatwill survive in an unpredictable and changing environment. Red Queen Hypothesis The Red Queen: “It takes all the running youcan do, to keep in the same place.”--Lewis Carroll, “Through the LookingGlass”• The Red Queen Hypothesis: “For an evolutionary system, continuingdevelopment is needed just in order to maintain its fitness relative to thesystems it is co-evolving with.” --Leigh Van Valen (1973)• This never-ending evolutionary cycle describes many natural interactions betweenhosts and disease, or between predators and prey: As species that live at eachother's expense co-evolve, they are engaged in a constant evolutionary struggle fora survival advantage. They need "all the running they can do" because thelandscape around them is constantly changing.–sexual populations can respond toselection faster than asexual populations.Another way to put it is that sex andrecombination allows allocatingindependently arising favorable mutantsto one offspring much more efficiently.Asexuals have to wait for theseindependent mutations to occur one afteranother in the same lineage in order tofind them combined in a single offspring.4

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Parthenogenesis - relies on fertilization of thereduced or unreduced polar nuclei (pseudogamy)• most asexual lineages of eukaryotes havearisen quite recently from sexual ancestors– In Plants:• Diplospory - meiosis is modified• Apospory -meiosis is completely bypassed• Adventitious embryony• Vegetative reproductionParthenogenic lizardsWhen females reproduce without theinvolvement of males or spermOffspring not completely “clones” oftheir motherOccurs in 7 lizard clades & 1 snakecladeC. inornatus C. tigrisC. neomexicanusasexual species6

Hedging bets - Aspen strategysee online article:http://www.nature.com/scitable/knowledge/library/case-study-the-glorious-golden-and-gigantic-13261308?ref=nf7