The cost and benefits of eukaryotic sex o Asexual reproduction sperm mitosis only generation of new individuals without fusion of egg and Fission Budding Fragmentation separation of parent into two individuals of same size new organisms arise from outgrowths of existing ones reforming regrowing lost body parts through regeneration haploid or diploid breaking of body down into several pieces and then egg develops without being fertilized progeny can be Parthenogenesis fusion of haploid gametes to make a zygote generally egg and o Sexual reproduction sperm meiosis o Sex does not require sexual organs o Sex does not require multicellularity o Needs for sex Diploid phase of life cycle Meiosis makes cells haploid Union of haploid meiotic products fertilization or syngamy general term for 2 gametes coming together o Eukaryotic sex was invented in unicellular organisms o Eukaryotic sexual life cycles Haplontic life cycle Most time as haploid organism Haploid cells fuse and become zygote syngamy Zygote is diploid for brief time Meiosis produces spores which produce more haploid cells Ex Chlamydomonas alga Diplontic life cycle Most time as diploid organism Germ cells undergo meiosis and produce haploid cells Syngamy forms zygote Zygotes undergo meiosis Gametes are haploid for brief time Ex humans Haplodiplontic life cycle Half time spent as diploid and half time spent as haploid Depending on environment and time of year Sporophyte Gametophyte Ex green alga Ex Paramecium multicellular diploid cell produces spores multicellular haploid cell produces gametes o Macronucleus many copies of active genes only rebuilt every generation o Micronucleus contains complete genome in one or two copies swap copies of micronuclei during meiosis o Micronucleus merge into one large diploid micronucleus New macronuclei are made during meiosis o Costs of anisogamous sex Sex can have evolutionary costs Isogamy vs anisogamy Isogamy 2 mating type gametes that are the same size o Unicellular organisms Anisogamy 2 gamete types that are different sizes o Sperm is small and egg is large Oocyte large egg that is immobile o Defines biological sex male female o Multicellular organisms o Evolved many times just like multicellularity Choices for single celled organisms hypothetical situation Fuse with each other do meiosis and release recombinant haploid sexual progeny make 4 haploid cells Stay separate and divide directly into two clone asexual progeny mitosis make 2 diploid haploid cells depending on original form Seems that anisogamous sex should be quickly out competed by asexuality for two reasons 1 Once extreme anisogamy evolves eggs provide nearly all material for the zygote male cannot reproduce without female o Cost of males reproduction is egg female limited because sperm are more abundant than necessary and males don t make eggs o N 2 g growth of population g number of generations N population size half are male and half are female Shows the reproductive output of sexuals vs parthenogenic females 2 Meiosis and outcrossing are costly even without a dedicated male sex o Sex is disadvantageous even when organism is hermaphrodite o Plant can reproduce pathogenically without pollen placed in with wild type frequency of new allele increases almost by o Cost of meiosis half which frequency of wild type allele decreases slightly continues until no wild type plants left vulnerable to those that donate sperm but refuse to use that of others organisms that always outcross are Some organisms do not have sex and are asexual ex dandelions Flowering plants are angiosperms Sex outcrossing was ancestral feature Asexuality is a derived trait by means of convergent evolution o Benefits of sex immediate benefits that outweigh the costs 1 Increasing genetic diversity of progeny allows them to adapt to an ever changing environment which increases fitness of parents if offspring survive ex parasites pathogens 2 Allows for faster adaptation as beneficial alleles can be combined quickly 3 Sex eliminates deleterious mutations Muller s Ratchet if you cannot have sex then you have as many deleterious mutations as your mom these mutations increase overtime in asexual species ratchet only turns one way C elegans nematodes can reproduce by self fertility of hermaphrodites or by outcrossing through males rare Under stressful conditions mutagen and pathogen influence progeny of crosses increases in frequency and do better than those that come from self fertility so there are benefits of outcrossing o Seeing value of sex in real time Sex and Life History o Spectrum of sexual variation Gonochorism 2 sexes male and female Development is regulated to prevent intersexual forms caused by mutations in sex chromosomes Sexes can have different characteristics ex male beetle is large and has pinchers Origins o Evolved many separate times as ancestor was hermaphroditic Hermaphroditism 1 sex that makes both sperm and egg outcrossing Normal process in angiosperms Self incompatibility all have same flowers but cannot use own pollen Insects and wind help with outcrossing by carrying pollen Also exists in animals ex sea slug 1 sex hermaphrodite with separate male and female flowers Monoecy plants only Androdioecy Gynodioecy Selfing Parthenogenesis 2 sexes hermaphrodite and male 2 sexes hermaphrodite and female 1 sex hermaphrodite no mating involved 1 sex female and no males o Evolution of mixed strategies in plants Ancestor of angiosperms was hermaphroditic Pure male female sexes evolve by loss of floral organs for opposite sex Androdioecy is rare regain of male function by dioecious females o Sexual systems can change very quickly ways organisms have evolved selfing Self fertility can exist from pure outcrossing in plants Nematodes both gametes exist in organism at once Most of the worm is dedicated to making eggs as a female XX chromosomes but it can also make some sperm self fertilizing hermaphrodite Can only use sperm for self limited spermatogenesis perform selfing first sperm then oocytes Brassicales break down barriers of selfing Adaptation that allows for use of own pollen for fertilization Hermaphrodite breaks down barriers for selfing Recent development in history Stigma usually rejects pollen from own plant o SCR protein o SRK pollen SCR receptor if matches SCR kills pollen o Mutation in code makes SKR inactive so plant can self fertilize itself Even though outcrossing works well selfing evolved because it is helpful when mates are scarce o
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