Sex Differences Across Animals

Describe the gynandromorphic bird example.

Half of the rooster/zebra finch has testes and the other half has ovaries; leads to asymmetrical coloring and features

• in zebra finch song nuclei developed on the male side

Chromosomes take priority in gene expression in chickens, cell by cell basis of expressing phenotype; can’t happen in mammals, each cell is guided by gonadal hormones

Birds - morphology is sex-determined; “balkanization” : states separated, don’t pay attention to nearby cells

Mammals - morphology guided mostly by hormones traveling through bloodstream, with some chromosomal influences (i.e. four core genotypes)

What are the modes of sex determination across vertebrates?

Genetic sex determination (GSD)

• XX-XY (mammals, some reptiles)

• WZ-ZZ (birds)

Environmental sex determination (ESD)

• temperature

• Social context

Describe the different types of chromosomes.

Autosomes: all chromosomes, except sex chromosomes

• parts of chromosomes can cross-over and recombine during meiosis in a process called recombination, adds to genetic diversity

Sex chromosomes: one set of chromosomes that determine the sex of the organism

• many more non-homologous regions, very little recombination (don’t exchange same level of DNA like autosomes do)

• Some traits are more adaptive for one sex over the other

Describe the XX-XY mode of genetic sex determination.

Mammalian pattern (and some reptiles like snakes) - XX/XY

• males are the heterogametic sex (XY) → high dosage of SRY

• Females are the homogametic sex (XX) → no dosage of SRY

Depends on timing of SRY, threshold of SRY, and threshold of XY cells (>35% for testes i.e. Y chromosome needs to be present in at least 35% of somatic cells to form testes even if there’s a Y chromosome)

* Chimera: lab constructed half and half animal

What are the exceptions of XX/XY determination in mammals?

Spiny rat and mole vole have lost their Y chromosome - both sexes are XO

• other genes have taken over in activating SOX9 pathway in males

Some grass mice females can be either XX or XY*

African pygmy mouse males can be either X*X or XY

* designates an unknown change in the chromosome, that functionally results in sex-reversal

Describe the WZ-ZZ mode of genetic sex determination.

Bird pattern - ZW/ZZ

• males are the homogametic sex (ZZ) → high dosage of DMRT1

• Females are the heterogametic sex (ZW) → low dosage of DMRT1

• Eggs highly sensitive to estrogens → feminizes body and brain in birds

DMRT1 is SRY equivalent but it’s on the more common chromosome

Estrogen in birds feminizes (opposite to rodents)

ZZ can be sex reversed to female if estrogen is added during critical period of gonad formation

How is the song system organized in zebra finches?

It is not hormonally organized

Male zebra finches sing courtship songs, females do not

Brain regions controlling song in male brains are 5-6 times larger than in female brains

Organizational estradiol does not demasculinize (males still develop the song system)

Sex differentiation of the song system in the zebra finch follows genetic sex not gonadal sex

• although some estradiol exposure will reduce the ability of typical song pattern in most songbirds where males sing - even when taught by tutor

How do activational hormones work in birds?

In male birds, like rodents and primates, the activational hormone is primarily testosterone

However, unlike rodents and primates, there are a few behaviors in birds that require aromatized estradiol to bind to estrogen receptors for activation

• proper song production and learning (in songbirds)

• Territorial aggression in some species (in song sparrows)

• Courtship displays (in Japanese quail)

  • Organizational E2 demasculinizes males, organizational androgens do nothing (as long as we prevent aromatization to E2)

  • Birds are mirror image of mammals

Describe the temperature mode of environmental sex determination.

TSD taxa and climate change caveat: short heatwaves during fluctuating incubation regimes produce females under temperature-dependent sex determination with implications for sex ratios in nature

What are the social context modes of environmental sex determination.

Gonochoristic fish

Sequential hermaphroditism

Simultaneous hermaphroditism

What is a gonochoristic fish?

Gonochoristic fish: an individual remains the same sex throughout their life

• Often XX/XY

• DMY is sex determining region on Y chromosome

• E.g. Medaka fish

What are sequential hermaphrodites?

an individual is born one sex but changes to the other sex at some point during their life

• sex change accompanied by complete changes in gonads and hormones

• Protogyny: beginning as female, switching to male

  • Most common in fish

  • There are primary males and primary females

  • Primary females may turn into terminal males

  • E.g. parrotfish

• Protoandry: beginning as male, switching to female

  • E.g. clownfish

What are simultaneous hermaphrodites?

the sex organ is partitioned into both male and female parts

Ovotestes produce both eggs and sperm

Still practice sexual reproduction (alternate roles)

E.g. black hamlet fish

What are some other environmental sex determiners besides temperature and social factors?

Multi-component, mutually antagonistic pathways: Social status, density, growth rate, pH, oxygen concentration, background color, temperature, sex chromosomes, dominant genes, germ cell numbers

Master regulator or a “parliamentary decision”

• selection has retained antagonism between the male and female pathways across vertebrates

• The specific genes/proteins/enzymes/metabolic pathways involved vary from system to system