sex: --- understanding its biological significance

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 the1:1 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 qualitativelydifferent things (a distinction not often appreciated, even by thosewho study the genetic programming of sex):

population sex ratio

sexual dimorphism (developmental differences)

Bonellia viridis

Female: 100 mmMale: 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 equallyto the next generation (every female needs a male)

hence, any minority sexon 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) sons

XX

XY

XXYX(O)

XXYXY(±Y)

progeny are“secondary” exceptions

x red XY

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, 2or 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 + spFemales MalesX

expected PROGENY:

(autosomal genes)

ALSO: one unusually large ++ female

px bw ++ bw sppx + sp

(1) Three, not two, parental types recovered:

(2) many intersexual(sterile) progeny

X px bw sp Male

XXXAAA

XXYAAA

(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 reachesadult 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 (1:2000-5000)

GSD by Active Ydominant masculinizer

HOUSE FLIES:

m/m female M/m male

GSD by dominant masculinizing allele M

(one of three different GSD systemsin the same species!)

Birds, moths and butterflies:

ZZ male ZW female

female isthe 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!

a1/a2 heterozygotes: females (queens and workers)

a1 or a1/a1 hemizygotes and homozygotes: males

a1/a2 Queen X a1 Drone --> a1/a1 & a2/a1

diploiddrones

(fertilization)

GSD by a multiple allele system--- highly “polymorphic” sex gene (many alleles)