HB Lecture 6

In mammals, which sex is heterogametic, and what are its sex chromosomes?

Male (XY)

In birds, which sex is heterogametic, and what are its sex chromosomes?

Female (ZW); males are ZZ

What is temperature-dependent sex determination (TSD)?

Sex of the offspring is set by egg incubation temperature rather than by sex chromosomes

What is genetic (chromosomal) sex determination (GSD), and what does its sex ratio do across incubation temperatures?

Sex is set by sex chromosomes; ~50% males regardless of temperature

Do species exhibiting TSD have heteromorphic sex chromosomes?

No (e.g. alligator karyotype 2N = 32 has no heteromorphic sex chromosomes and no microchromosomes)

In turtles showing TSD, which incubation temperatures produce males?

Low temperatures (100% males at low, 100% females at high)

In alligators, which incubation temperatures produce males?

High temperatures

Pattern of sex ratio vs temperature in some lizards (including the leopard gecko)?

Males only at an intermediate temperature range; females at both low and high extremes (two thresholds, "intermediate notch")

How narrow is the temperature range over which the TSD sex ratio flips from 0% to 100% males?

Only about 1–2°C (dynamic range), i.e. very high temperature dependence

Alligator eggs incubated below 30°C hatch as which sex?

Female (abrupt switch to male above 30°C)

During which days of incubation is the alligator egg sensitive to temperature?

Days 21–28 (developmental window)

What is meant by "thermo-temporal regulation" of sex in alligators?

Both the temperature and the developmental time window of exposure determine sex

In all TSD species, what can a temperature change at the mid-development sensitive period induce?

Sex reversal

What gonad forms in TSD species at intermediate temperatures, and how does it develop?

Ovotestis, which gradually develops as a testis later

What does a transient temperature switch at the critical developmental phase programme, beyond sex?

The brain and subsequent growth (alligators switched to male temperature grow larger than those switched to female temperature)

Which sex builds alligator nests, and where are favoured nest sites?

Female; nearest to water (so slightly cooler)

Why can TSD influence alligator population dynamics?

Nest temperature (set by nest site) shifts sex ratio, so resource use and population size can be self-regulated (resource management and population control)

What is oestrogen, in terms of hormone class?

A sex steroid

Which enzyme converts testosterone to oestrogen (oestradiol-17β)?

Aromatase

Which enzyme converts testosterone to 5α-dihydrotestosterone?

5α-reductase

In red-eared slider turtle studies (Crews), what happens when oestrogen or an aromatisable androgen is applied to eggs at a male-producing temperature?

Offspring hatch as females

In red-eared slider turtle studies (Crews), what happens when an aromatase inhibitor is applied to eggs at a female-producing temperature?

Offspring hatch as males

Take-home: what does blockade of oestrogen do in TSD species?

Gives a male phenotype and overcomes female-inducing temperature effects (oestrogen activates the female pattern of development)

In the European pond turtle, which temperatures favour females?

High temperatures

In the European pond turtle, what drives aromatase activity up in the gonad?

A high (female-producing) incubation temperature, but only during the thermosensitive period

In pond turtle eggs moved from a male-producing to a female-producing temperature after the thermosensitive period has ended, what happens to gonadal aromatase activity?

It stays at baseline (no rise); the switch is ineffective outside the period

Which stages of freshwater turtle development make up the thermosensitive period?

Stages 16–22

In turtles, what is the state of the gonad before the thermosensitive period?

Indifferent (bipotential), containing germinal epithelium and medullary epithelial cords

In turtles, is early gonadal sex differentiation reversible or irreversible?

Reversible initially; becomes irreversible later in development

In TSD, what does high aromatase expression do to gonad fate?

High oestrogen activates ovarian differentiation genes and represses testicular genes, giving an ovary

In TSD, what does low aromatase expression do to gonad fate?

Low oestrogen fails to activate ovarian genes and fails to repress testicular genes, giving a testis

What is the default gonadal state in mammals, and in reptiles?

Mammals: female; reptiles: male

In TSD turtles, how is SOX9 expression regulated at a male-producing temperature, and at a female-producing temperature?

Upregulated at male-producing temperature; downregulated at female-producing temperature

What is the sex-determining trigger in mammals, birds and reptiles?

Mammals: SRY; birds: DMRT1 dosage (two copies, ZZ); reptiles: temperature

In the mammalian XY gonad, how does SRY establish testis fate?

SRY upregulates SOX9, forming a feed-forward loop that upregulates FGF9 and silences WNT4

In the mammalian XX gonad, which gene dominates, and what does it silence?

WNT4; silences FGF9 and SOX9

Which two genes are activated in the testis of all of mammal, chicken, turtle and alligator?

SOX9 and DMRT1

Which came first in vertebrate evolution, TSD or GSD?

TSD preceded GSD

What is not known about TSD at the molecular level?

How temperature activates the pathway

Which type of gonadal sex-determination influence is lesser in birds than in other vertebrates?

Gonadal steroids (bird sex determination relies more on gene dosage)

What sex-determining system do marsupials use?

Chromosomal: an SRY switch plus an X-based dosage mechanism

In a normal male macropodid (kangaroo/wallaby), which structures are present and absent?

Scrotum and testis present; no pouch and no mammary glands

In a normal female macropodid, which structures are present?

Pouch and mammary glands

Phenotype of an XO (Turner-like) intersex marsupial?

Ovaries or streak gonads, Müllerian ducts present, Wolffian ducts absent, no penis, scrotum present, no pouch or mammary gland

Phenotype of an XXY (Klinefelter-like) intersex marsupial?

Intra-abdominal testis, Müllerian ducts absent, Wolffian ducts present, penis developed, pouch and mammary gland present, no scrotum

What is the conclusion from XO and XXY marsupials about pouch and mammary gland development?

It is X dosage dependent: genes on the X drive pouch and mammary tissue; a second X suppresses the scrotum

How is mammary development triggered in eutherian mammals, versus marsupials?

Eutherians: oestrogen is the switch (glands in both sexes); marsupials: chromosomally mediated, and no male marsupial is known to produce breast tissue

How does X inactivation differ between eutherians and marsupials?

Eutherians: random (maternal or paternal X); marsupials: paternally derived X is inactive, though some genes escape (explaining dose dependence)

Which hormone is essential for maturation and development of male germ cells in mammals?

Testosterone

Which hormone is likely essential for differentiation of the brain in mammals?

Oestrogen

What is the structure of the penis in marsupials and monotremes?

Bifurcated (two-pronged)

In the female marsupial tract, how do sperm and the joey travel?

Sperm go up the lateral vaginae; the joey goes down the middle (pseudovaginal birth canal)

What is embryonic diapause in macropodids, and what induces it?

Arrested development of the embryo (blastocyst); induced by a lactation-induced negative feedback

Why is diapause advantageous in macropodids?

Only four teats (limited space), so it freezes development of the next embryo until the current joey leaves and stops suckling

Is macropodid embryonic diapause dependent on cold?

No; it occurs at body temperature

How does macropodid gestation length compare with the ovarian cycle, and does the mother "know" she is pregnant?

Gestation equals the ovarian cycle length; she does not "know" she is pregnant

Which eutherian phenomenon is analogous (but different) to embryonic diapause?

Lactational infertility

How long does pregnancy last in the tammar wallaby?

28–30 days

When does organogenesis occur in marsupial pregnancy?

Late pregnancy

Approximate weight of a newborn Bennett's wallaby?

750 mg

What structure of the newborn marsupial is analogous to the umbilicus?

The teat

Why do newborn tammar wallabies have claws on their forelimbs?

To tear the amniotic sac and climb to the pouch

What do newborn joeys exhibit to find the pouch, and what does it depend on?

Negative geotropism (move away from the centre of the earth); depends on maternal posture (mother leans back)

Where does the majority of marsupial development take place?

In the pouch

Which pesticide spill in the 1980s affected Lake Apopka alligators, and what is it classed as?

DDT; a weak oestrogen

Which Florida lake served as the non-DDT comparison to Lake Apopka?

Lake Woodruff

List effects of environmental oestrogens in alligators (e.g. at Lake Apopka).

Ovotestis formation, reduced testis growth rate and size, abnormal external genitalia (small phallus), altered neural pathways controlling GnRH release, increased liver size, raised liver vitellogenin (egg protein)

Name two environmental threats to survival of TSD species discussed.

Global warming and environmental oestrogens (endocrine disruptors)

Which parameters beyond sex does temperature programme in the leopard gecko?

Brain structures, behaviour and general physiology (growth, sex hormone levels, sexual and aggressive behaviour)

Leopard gecko males from the more male-favouring incubation temperature show which traits versus other males?

Dominance in access to females, higher growth rates, higher frequency of agonistic displays

[APPLICATION] A clutch of alligator eggs is laid in a mound far from water in a warm, sun-exposed site. What sex ratio is expected, and why?

Mostly males: high incubation temperature during days 21–28 favours males in alligators

[APPLICATION] Rising global temperatures warm the nesting sites of an alligator population. Predict the sex ratio and the population consequence.

Male-biased hatchlings; fewer females limits reproduction and threatens survival (can cause a population crash)

[APPLICATION] Turtle eggs are incubated at a male-producing temperature, but oestradiol is applied during the thermosensitive period. What sex do they hatch as, and why?

Female; oestrogen activates ovarian genes, overriding male-producing temperature

[APPLICATION] Turtle eggs incubated at a female-producing temperature are treated with an aromatase inhibitor during the thermosensitive period. Predict the sex and reason.

Male; no aromatase means low oestrogen, so ovarian genes are not activated and testicular genes are not repressed

[APPLICATION] Pond turtle eggs at 25°C are moved to 30°C only after the thermosensitive period has ended. Predict aromatase activity and sex.

No aromatase induction; eggs remain male-producing, because temperature only acts during the thermosensitive period

[APPLICATION] During the thermosensitive period a turtle gonad shows rising aromatase activity and falling SOX9. What does it become?

An ovary (female-producing temperature; high oestrogen downregulates SOX9)

[APPLICATION] A newly found lizard gives all females at 26°C and 34°C but males only between 30 and 32°C. Which TSD pattern is this and which species is an example?

Males only at intermediate temperatures (two thresholds); e.g. leopard gecko

[APPLICATION] An unknown reptile has no heteromorphic sex chromosomes and sex ratio varies between nests. What is the likely sex-determining mechanism?

Temperature-dependent sex determination (TSD)

[APPLICATION] A juvenile male alligator from a DDT-contaminated lake has a very small phallus and an ovotestis. Explain the likely cause.

DDT acts as a weak oestrogen (endocrine disruptor), causing intersex gonad formation and abnormal genitalia

[APPLICATION] A wallaby has an XXY karyotype. Predict its scrotum, pouch, mammary glands and testis position.

No scrotum; pouch and mammary glands present (two X doses); intra-abdominal testis; penis developed

[APPLICATION] A wallaby has an XO karyotype. Predict its scrotum, pouch and mammary glands.

Scrotum present; no pouch and no mammary glands (one X dose); ovaries or streak gonads, no penis

[APPLICATION] A female kangaroo is lactating for a joey in the pouch and has a fertilised embryo that is not developing. Explain why.

Embryonic diapause: lactation (suckling) induces negative feedback that arrests embryo development until the joey leaves the teat

[APPLICATION] A newborn wallaby joey crawls from the urogenital sinus to the pouch after the mother adopts a leaning-back posture. Which behaviour guides it, and what happens if she does not lean back?

Negative geotropism (moves away from earth's centre); without the correct posture the joey cannot navigate the right way to the pouch