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) (2) Free good mutations from bad genetic backgrounds (3) Help to keep ahead of parasites (there is no “optimal” genotype in the real world) perhaps males particularly useful (rationale for “maladaptations” from sexual selection)

“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)

Bonellia viridis Female: 100 mm Male: 1 mm larva lands on rock larva lands on adult female An extreme example of sexual dimorphism 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? 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! (as usual, Darwin had the answer first)

Calvin Bridges (1916): Known for fruit flies: XX females XY males white daughers (fertile) red sons (sterile) (primary) white daughers (fertile) red sons (fertile!) expected: w - /w + (red) daughters w - /Y (white) sonsX XY XXY X(O) XXY XY(±Y) progeny are “secondary” exceptions x red X Y w - /w - (white eyed) Females X Males (red eyed) w + /Y “exceptions”: …but what really determines fly sex? (xxx & o/Y die)

for fruit flies: normal: XX females XY males abnormal: XXY females XO males X chromosome number determines sex Y chromosome does not detemine sex (but is required for male fertility) Sex-chromosome difference CAUSES (triggers) different sexual development

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

Parental types: px + & + sp Nonparental types: (recombinant) ( 6.5 cM) + + & px sp px bw + + bw sp px + spFemalesMales X expected PROGENY: (autosomal genes) ALSO: one unusually large ++ female px bw + + bw sp px + sp (1)Three, not two, parental types recovered: (2) many intersexual (sterile) progeny X px bw sp Male XXX AAA XXY AAA (3) normal and jumbo females

XX AA X:A = 1, female X AA X:A = 0.5, male XX(±Y) AAA X:A = 0.67, intersex XXX AAA X:A= 1, female (large) X A X:A=1, (dead) female

XX AA zygote --> XXAA cells / X AA cells X-chromosome loss generates “gynandromorphs” GENETIC MOSAICS (XXAA) Female (X AA) Male (XXAA) Female (X A) Female XXAA zygote --> XXAA cells/XA cells (“loss” of an entire haploid set) (XA never reaches adult stage but mosaics do)

XX AA X:A = 1, female X AA X:A = 0.5, male XX(±Y) AAA X:A = 0.67, intersex XXX AAA X:A= 1, female (large) X A 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: ) 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 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) Eggs not fertilized --> Drones (males) Diploid (± royal jelly) 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 a 1 /a 2 Queen X a 1 Drone --> a 1 /a 1 & a 2 /a 1 diploid drones (fertilization) GSD by a multiple allele system --- highly “polymorphic” sex gene (many alleles)