Sex: --- understanding its biological significance -- appreciating how genetics was used to understand how it is determined. … 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
Costs of sex: (1) Males dilute females’ genetic contribution (the couple is the unit of reproduction) (2) Seeking a mate and mating takes time and energy -- and is dangerous (3) Sexual conflicts arise (remember the Haig hypothesis for imprinting) (4) Sex and its consequence, recombination, break up winning gene teams
Benefits of sex: (1) Reduces mutational load (escape “Muller’s ratchet” -- irreversible loss of genes) perhaps males particularly useful (rationale for “maladaptations” from sexual selection) (2) Free good mutations from bad genetic backgrounds (3) Help to keep ahead of parasites (there is no “optimal” genotype in the real world)
“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)
An extreme example of sexual dimorphism Bonellia viridis Female: 100 mm Male: 1 mm larva lands on rock larva lands on adult female ESD: environmental sex determination
relevant variables for ESD: Host (Bonellia) Temperature (turtles, alligators) Neighbor density (parasitic wasps) “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: 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 (every female needs a male) hence, any 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!
Known for fruit flies: XX females XY males …but what really determines fly sex? Calvin Bridges (1916): w -/w - (white eyed) Females X Males (red eyed) w+/Y expected: “exceptions”: (primary) x w -/w+ (red) daughters XX w -/Y (white) sons XY white daughers (fertile) red sons (sterile) red XY progeny are “secondary” exceptions white daughers (fertile) red sons (fertile!) XXY X(O) (xxx & o/Y die) XXY XY(±Y)
for fruit flies: normal: XX females XY males abnormal: XXY females XO males Sex-chromosome difference CAUSES (triggers) different sexual development Y chromosome does not detemine sex (but is required for male fertility) X chromosome number determines sex
XX females XY males What about X-chromosome number matters? absolute number: 1=male, 2 or more = female odd vs. even (paired? ) XX X=male? number relative to ploidy (non-sex chromosomes)? X AA male, but X A female? …again, genetic exceptions to the rule provide the answer
(autosomal genes) px bw + + bw sp Females X Males px + sp expected PROGENY: px + & + sp Parental types: Nonparental types: (recombinant) ( 6. 5 c. M) + + & px sp XXX AAA ALSO: one unusually large ++ female X px bw sp Male (1) Three, not two, (2) many intersexual XXY (2) parental types recovered: (sterile) progeny AAA px bw + + bw sp (3) normal and jumbo females px + sp
X AA X: A = 0. 5, male XX AA XX(±Y) AAA X: A = 1, female X: A = 0. 67, intersex XXX AAA X: A= 1, female (large) XA X: A=1, (dead) female
GENETIC MOSAICS X-chromosome loss generates “gynandromorphs” XX AA zygote --> XXAA cells / X AA cells (XXAA) (X AA) Female Male XXAA zygote --> XXAA cells/XA cells (“loss” of an entire haploid set) (XXAA) (X A) Female (XA never reaches adult stage but mosaics do)
X AA X: A = 0. 5, male XX AA XX(±Y) AAA X: A = 1, female X: A = 0. 67, intersex XXX AAA X: A= 1, female (large) XA X: A=1, (dead) female GSD by X: A ratio (balance)
The worm: XX self-fertilizing hermaphrodite XO male (heterogametic sex) Origin of males: (1) Spontaneous X-chromosome nondisjunction (rare) to make “O” eggs (+ X self sperm)-> XO male (2) Mating (outcross) of hermaphrodite to male: X eggs join with X or O male sperm -> 50: 50
The worm: XX self-fertilizing hermaphrodite XO male (heterogametic sex) XX AAA X: A= 0. 67 = male XXX AAAA X: A = 0. 75 = hermaphrodite GSD by X: A ratio
HUMANS: XX female XY male XXY Kleinfeler Syndrome sterile male (1: 1000 men) XO Turner Syndrome sterile female (1: 2000 -5000) GSD by Active Y dominant masculinizer
HOUSE FLIES: m/m female M/m male GSD by dominant masculinizing allele M (one of three different GSD systems in the same species!)
Birds, moths and butterflies: ZZ male ZW female is the heterogametic sex (compare: XY males) GSD by feminizing W or Z: A ?
20% of all animals use a very different GSD system: Eggs fertilized --> Queens (females) or workers (sterile) Diploid (± royal jelly) Eggs not fertilized --> Drones (males) Haploid GSD by “haplodiploid” system But is the relevant variable ploidy?
Let’s encourage inbreeding among the honeybees: increased homozygosity suddenly: DIPLOID MALES! a 1/a 2 heterozygotes: females (queens and workers) a 1 or a 1/a 1 hemizygotes and homozygotes: males (fertilization) a 1/a 2 Queen X a 1 Drone --> a 1/a 1 & a 2/a 1 diploid drones GSD by a multiple allele system --- highly “polymorphic” sex gene (many alleles)
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