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Genetic mosaics have zillions of uses besides just facilitating mutant isolation …and geneticists have ways of controlling exactly when and where FLPase is generated …and hence exactly when and where mitotic recombination is induced Sex: --- understanding its biological significance -- appreciating how genetics was used to understand how it is determined. Sex education .. .from a genetical and evolutionary (= genetical) perspective Forces in evolution (and evolution is what genetics -- and life -- is really all about): (1) Natural selection: (this term is in your glossary) (2) Sexual selection: … according to Jacob Bronowski in “The Ascent of Man” (1973) Mendel himself was inspired by the clear-cut difference between males and females and the 1:1 sex ratio human brain = peacock tail ? …but a tail with the capacity to change the way life evolves reproduction of the fittest …remember, responses to changes in biological environment (including parasites) are as important as responses to changes in physical environment. reproduction of the sexiest (this term is NOT in your glossary) runaway sexual selection generally leads to maladaptive ("unfit") sexually selected traits Sex: Sexual reproduction vs. Asexual reproduction Coming together of genetic material from (two) individuals to form progeny that combine genes from all (both) parents 1 symmetrical sex: us equal genetic contribution from each partner qualitative exceptions: mitochondria & Y chromosome asymmetrical sex: Can there be sex without gender? (males & females) unequal genetic contribution from each partner bacteria GAMETES (sex cells) Recall: Isogamous sex systems: Sex: Sexual reproduction vs. Asexual reproduction gametes from each parent are of equal size Anisogamous sex systems: Coming together of genetic material from (two) gametes from each parent are of different sizes individuals to form progeny that combine genes Males: sperm/pollen (smaller) from all (both) parents Females: eggs/ovules (larger) (parents have to be separate to qualify) (hate to disappoint those who think male/female labels are politically incorrect) Sex is ancient and ubiquitous Nevertheless: bdelloid rotifers: no sex for millions of years Evidence: Costs of sex: (1) Males dilute females’ genetic contribution (the couple is the unit of reproduction) no meiosis (genes are missing) (2) Seeking a mate and mating takes time and energy -- and is dangerous unusual distribution of DNA variation suggests no gene mixing for ages (3) Sexual conflicts arise (remember the Haig hypothesis for imprinting) (4) Sex and its consequence, recombination, break up winning gene teams 2 Benefits of sex: (1) Reduces mutational load (escape “Muller’s ratchet” -- irreversible loss of genes) perhaps males particularly useful “Sex determination genes” determine two qualitatively different things (a distinction not often appreciated, even by those who study the genetic programming of sex): population sex ratio sexual dimorphism (developmental differences (rationale for “maladaptations” from sexual selection) including those that determine behavioral differences) (2) Free good mutations from bad genetic backgrounds (3) Help to keep ahead of parasites (there is no “optimal” genotype in the real world) relevant variables for ESD: An extreme example of sexual dimorphism Host (Bonellia) Bonellia viridis Female: 100 mm Male: 1 mm sex is determined by whether: Temperature (turtles, alligators) larva lands on rock Neighbor density (parasitic wasps) larva lands on adult female ESD: environmental sex determination “Presence of male” (tropical fish) vs. GSD: genotypic sex determination Segregation of alleles (genes) determines sex best for generating 1:1 sex ratios apparant paradox: 1:1 for fruit flies: XX females XY males …but what really determines fly sex? Since females are rate-limiting for reproduction, why see 1:1 sex ratio so often? (as usual, Darwin had the answer first) In the aggregate, both sexes contribute equally to the next generation, regardless of population sex ratio (every female needs a male) hence, the minority sex on average will make a disproportionate contribution per individual Natural selection will favor generation of the minority sex. At 1:1, no minority sex! Calvin Bridges (1916): w -/w - (white eyed) expected: “exceptions”: (primary) Females X Males (red eyed) w +/Y w -/w + (red) daughters w -/Y (white) sons white daughers (fertile) red sons (sterile) x red X Y progeny are “secondary” exceptions white daughers (fertile) red sons (fertile!) XX XY XXY X(O) (xxx & o/Y die) XXY XY(±Y) 3 for fruit flies: XX females XY males normal: XX females XY males What about X-chromosome number matters? abnormal: XXY females XO males absolute number: Sex-chromosome difference CAUSES (triggers) different sexual development 1=male, 2 or more = female odd vs. even (paired?) XX X=male? number relative to ploidy (non-sex chromosomes)? Y chromosome does not detemine sex (but is required for male fertility) X AA male, but X A female? X chromosome number determines sex …again, genetic exceptions to the rule provide the answer (autosomal genes) px bw + + bw sp Females X Males For fruit flies: px + sp X(±Y) AA XX AA expected testcross PROGENY phenotypes: XXX AAA Parental types: px + & + sp Nonparental types: (recombinant) ( 6.5 cM) + + & px sp XX(±Y) AAA X:A = 0.5, male X:A = 1, female X:A = 0.67, intersex (phenotypic mosaic) XXX AAA X:A= 1, female (jumbo) XA X:A=1, (dead) female ALSO: one unusually large ++ female X px bw sp Male (1) Three, not two, parental types recovered: px bw + + bw sp px + sp highest freq. &= numbers (2) many intersexual XXY (sterile) progeny AAA (3) normal and jumbo daughters X:A ratio sex-determination mechanism What is it about ploidy changes that affects the way that X-chromosome dose determines sex? (1921 2007, and not what Bridge's thought or what the textbooks say) 4