MODULE-9

SEX DETERMINATION

9.1 ANIMALS

G E N E T I C S-I

• Define the term sex chromosome vs. autosomal chromosomes.

• Describe the sex determining mechanisms in the animal kingdom – sex chromosome mediated methods viz: XY-XX; XO-XX; ZW-ZZ; Z0-ZZ .

• Describe the Chromosome Balance Theory of Sex Determination; haplo-diploidy; haplo-diploidy and complementary genes; genic control of sex.

G E N E T I C S-I LEARNING OUTCOMES

• Explain the following mechanisms of sex determination using appropriate examples to illustrate: – (a) Mammalian XY-XX system – (b) Protenor XX-XO system– (c) Birds ZW-ZZ system– (d) ZO-ZZ system– (e) Sex Balance Theory (genic balance)– (f) Yeast (MATa – MATα mating types)– (g) Environmental sex determination

G E N E T I C S-I LEARNING OUTCOMES

X Y X

XX XY½ ½

XY x XX

♂ ♀

Lygaeus systemMost common, e.g. humans, rats, some reptiles-green iguana

Sex determination in the animal kingdom

G E N E T I C S-I

Females: two X-chromosomes, homogametic.

Males: single X-chromosome and Y-chromosome, heterogametic.

Sex determination in the animal kingdom (humans)

G E N E T I C S-INormal humans

Some human abnormalities

SRY is a Y-linked gene and codes for testis-determining factor (maleness).

X O X

XX XO½ ½

XO x XX

♂ ♀

Protenor systemInsects: Hemiptera, Orthoptera

Sex determination in the animal kingdom

G E N E T I C S-Igrasshoppers

Females: two X-chromosomes, homogametic

Males: single X-chromosome, heterogametic

O is not a chromosome.

Z W Z

ZZ ZW½ ½

ZZ x ZW

♂ ♀

Birds, moths, some isopods, some amphibians and some fishes

Sex determination in the animal kingdom

G E N E T I C S-IFemales: one Z and one W chromosome, heterogametic

Males: two Z chromosomes, homogametic

The chromosomes are not shaped like Z or W, just to differentiate from XY system.

Z O Z

ZZ ZO½ ½

ZO x ZZ

♂♀

Butterflies, caddies flies

Sex determination in the animal kingdom

G E N E T I C S-I

Females: one Z chromosome, heterogametic

Males: two Z chromosomes, homogametic

O is not a chromosome.

Lygaeus system (XY) - homogametic sex is female - heterogametic sex is male - Sex ratio is 1:1

Protenor system (XO – XX) - homogametic sex is female - heterogametic sex is male - Sex ratio is 1:1

ZO system - homogametic sex is male - heterogametic sex is female

ZW system - homogametic sex is male - heterogametic sex is female

Sex determination in the animal kingdom

G E N E T I C S-I

Sex determination in the animal kingdom (environment)

G E N E T I C S-IMarine mollusk (Crepidula fornicata)- common slipper limpet.

•Live in stacks- one on top of the other

•First to settle on a solid unoccupied substrate develops into a female.

•Those that settle on top become males and serve as mates for the female.

•After a while the males turn into females and attract more male larvae on top of them.

•This is referred to as sequential hermaphroditism- each animal can be both male and female, although not at the same time.

Sex determination in the animal kingdom (environment)

G E N E T I C S-I• Some reptiles- temperature during embryonic development determine

sexual phenotype.

• Turtles- warm temperatures produces females; cool temperatures produce males.

• Alligators- warm temperature produces males (>34oC); cool temperatures produce females (<30oC).

Effect of climate change?

Chromosome balance theory of sex determination (Genic balance) in Drosophila melanogaster (fruit fly)

- Sex is determined by the balance between sex chromosomes and autosomal complement of chromosomes

- Lets say that ‘A’ represents a haploid set of autosomal chromosomes X/A ratio = 1 or higher = female X/A ratio = 0.5 or lower = male X/A ratio = >0.5 - <1.0 = intersex

The genome of ♂ fly = AAXY The genome of ♀fly = AAXX

-Y chromosome is involved in male fertility but not in the determination of sex

G E N E T I C S-I

Evidence for the Genic balance theory in Drosophila melanogaster Bridges (1922) observed that Drosophila occasionally produces a triploid

due to non-disjunction.

AX AY AX

AAXX AAXY½ ½

AAXY x AAXX

♂ ♀

AX AY AAXX

AAAXXX AAAXXYX/A = 1 X/A = 0.66

AAXY x AAXX

♂ ♀

G E N E T I C S-I

AX AY AAX

AAXY x AAAXXX

♂ ♀

AXX

AAAXX AAXXX AAAXY AAXXY

X/A =0.6 X/A =1.5 X/A = 0.33 X/A = 1

INTERSEX

SUPERFEMALE

SUPERMALE

FEMALE

Male not fertile Male fertile

Evidence for the Genic balance theory in Drosophila melanogaster

G E N E T I C S-I

Intersex (Gynandromorphs) in Drosophila melanogaster:

- Have male and female organs - Highly influenced by envt - high To – more female - low To – more male - Modifying factors also influence

- more male or female

G E N E T I C S-I

Superimposed on the genic balance system is another level of control of the determination of sex - transformer gene (autosomal gene)

This gene in the homozygous recessive state (tra. tra) converts a diploid female into a sterile male.

AAXX Sterile Male

AAXX Female

tra.tra

Tra.__

Transformer gene sex determination in Drosophila melanogaster

G E N E T I C S-I

Transformer gene

Genic Balance

Overall sex determination in Drosophila melanogaster

G E N E T I C S-I

Autosomes alone are involved.

The number of autosomal complements determine sex.

e.g. Hymenopteran insects such as bees, wasps, antsIn Bees - eggs haploid males (A)

Pathenogenesis (arrhenotoky)

FertilisationDiploid females (AA)

Quantity of food

Queen Female worker

(worker)

The queen controls sex ratio

Haplo-diploidy sex determination

G E N E T I C S-I

e.g. Hymenopteran insects such as bees, wasps, ants

In Wasps - eggsPathenogenesis (arrhenotoky)

Fertilisation

Diploid female worker

haploid males (A)(worker)

Diploid male workerHomozyous gene ‘S’

Heterozygous gene ‘S’

Haplo-diploidy + complementary gene method of sex determination

G E N E T I C S-I

The ‘S’ gene has 9 multiple alleles Sa, Sb, Sc, Sd….Si

Homozygous states

SaSa, SbSb, ScSc, SdSd…… SiSi = diploid worker males

Heterozygous states

SaSb, SaSc, SaSd etc = diploid worker females

There are more heterozygous combinations than homozygous and hence more worker females than worker males

G E N E T I C S-IHaplo-diploidy + complementary gene method of sex determination

Lower plants such as chlamydomonas and fungi- one or a few autosomal genes control sex.

e.g. Chlamydomonas - ‘A’ = + mating type = - mating type

e.g Basidiomycete fungi – two genes, A & BAB – mating type-1Ab – mating type-2aB - mating type-3ab – mating type-4

Sex determination- one or few autosomal genes (genic sex determination)

G E N E T I C S-I

Males and females look similar.

Fungi , lower plantsSingle or few autosomal genes

InsectsWasps Entire autosomal complement + single geneBees Entire autosomal complementDrosophila Autosomes + sex chromosomes

Higher animals Sex chromosome mediated methods- XY, XO, ZW, ZO- environment

G E N E T I C S-I

SUMMARY