Temperature Dependent Sex Determination Flashcards
Describe TSD in alligators
- High temps favour males
- Abrupt switch above 30 – get many more males – but females produced again at higher temps, suggests window of optimum temp
- Developmental window where egg is sensitive to temp (21-28)
• Site of nest production seems to be important – nests nearer the water are cooler, and are favoured • Low alligator population: − Females can get their nests near the cooler areas − More females produced − Population will increase • High population: − cant get nests near cool areas − More males produced − Population density decreases
Describe general features of TSD
- In all species exhibiting TSD, there is a sensitive period at mid-development where temp change can induce sex reversal
- Transient switch at the critical development phase programmes the brain and subsequent growth
- Intermediate temperature can lead to ‘ovo-testis’ which gradually develop as testis later
- Range of temp resulting in 100% females or males can be as little as 1C
- There are no sex chromosomes
Describe the role of estrogen in TSD
Red Slider Turtle:
- Eggs incubated at low male-inducing temps → offspring hatch as males
- Switch to high female inducting temps → offspring hatch as males
- Apply estrogen or aromatisable androgens to eggs → offspring hatch as females
- Apply aromatase inhibitor → offspring hatch as males
→ estrogen is the core component of the temperature regulated switch
European Pond Turtle (High Temp Favours Females)
Charles Pieau
• Thermosensitive period between 16 and 22 days
• Eggs incubated at female inducing temps have increasing levels of aromatase
• Temp switch from male inducing (25) to female inducing (30) during the thermosensitive period doesn’t make much difference in activity – just caused the aromatase activity to start later
• Temp switch from 25 to 30 outside this period doesn’t result in activation of aromatase
• Aromatase gene is under a developmental clock
→ mirror image of what we see in mammals
• Mammals – female default
• Reptiles – males default in the absence of the aromatase switch mechanism
• Birds – males defeault plumage in the absence of estrogen
What effects can environmental estrogens have on reptiles?
- Lake Apopka had massive DDT spill in 1980 – weak estrogen
- Alligator penises smaller in Lake Apopka than nearby Lake Woodruff
Effect of environmental estrogens:
• Small testis
• Formation of ovo-testis
• Abnormal external genitalia
• Altered neural pathways controlling GnRH release
• Increased liver size
• Increased levels of vitellogenin from liver
Summarise similarties/differences in events in the sex neutral early embryo in mammals and TSD
Mammals:
• SRY causes sertoli cells to differentiate
• This relies on expression of SOX-9
1. SRY upregulates SOX-9
2. This activates a feed-forward loop that upregulates FGF9 and silences Wnt4
3. Leads to development of the testis
- Too little SOX-9 leads to male to female reversal.
• In females
• No SRY, Wnt4 dominants, ovary formed
- SOX-9 production leads to XX males.
TSD animals:
• Male producing temps → high SOX-9
• Female producing temps → SOX-9 shut down
Describe the effects of TSD and behaviour
Leopard Gecko:
• Mostly female at low and high temps
• Male at mid temps
Males from incubation at 32C:
• Dominant in access to females
• Higher growth rates
• Higher frequency of agonistic displays
Males from incubation at 34C:
• Subdominant
• Lower growth rates
• Lower frequency of agonistic displays
- Females from incubation at 32C (normally produces males) more aggressive than females from incubation at 34C
- Phenotypic differences persist upon gonadectomy → suggests a non-reversible brain remodeling
- Differences in neural metabolism (cytochrome oxidase activity) in limbic areas thought to underlie the behavioural variation
- Males from female biased IncT have elevated metabolic capacity in the POA, an area critical for courtship behavior
- This metabolic increase might prime these males to be more sexually vigorous
