Polyphenism in beetles with insulin signalling Flashcards
What is polyphenism and what causes it?
When an organism can exhibit multiple phenotypes, attributable to phenotypic plasticity
What is phenotypic plasticity?
The ability of a given genotype to produce multiple phenotypes in response to environmental variation.
Phenotypic plasticity demonstrates how genetic programmes can be modulated by the environment, as a result of…
environmental sensitivity
How is phenotypic plasticity adaptive?
It allows organisms to express a phenotype that is best-suited to the environment
Why is phenotypic plasticity default?
Because developmental, physiological and metabolic processes are inherently sensitive to environmental variables.
Give 3 examples of environmental variables that affect phenotypic plasticity.
Temperature, pH, nutrient availability
Is polyphenism common in insect taxa?
Yes
Large horns are advantageous to male beetles, order Coleoptera, but why?
Males face contest competition for females and must physically compete with one another.
Do beetle horns show horn polyphenism?
Yes
What is horn polyphenism dependent on?
Nutrient availability
What is the outcome when a male is raised on an a) nutrient-rich and b) nutrient-poor diet?
a) Large horns develop and they become dominant males
b) Small horns develop and they must employ ‘sneaker male’ tactics to be successful in reproduction
Why don’t females want to mate with small-horned males?
Horns are a fitness-dependent trait, the female perceives them to be indicative of genetic quality/competitive ability
(Refer to Amotz Zahavi 1975, Good Genes etc.)
Give 3 control mechanisms for phenotypic plasticity.
- Developmental pathways
- Gene expression
- Hormonal control
What is responsible for beetle horn polyphenism?
Hormonal control via insulin signalling
Who studied the hormonal control of horn development in the rhinoceros beetle?
What is its latin name?
Emlen et al., 2012
They belong to the subfamily Dynastinae
Emlen et al., 2012;
What did the authors find determined horn length?
Nutrient availability during larval development, with high nutrient content producing disproportionate increases in horn size.
Emlen et al., 2012:
How did the authors find that Dynastinae members were sensitive to insulin?
Peturbations of the insulin/IGF pathway affected horn development more than other structures
Who described the insulin-signalling pathway?
Koyama et al., 2013
Koyama et al., 2013:
Describe how insuling signalling works. Give steps.
- High nutrition stimulates insulin-producing cells in the CNS to increase systemic insulin
- Insulin-like peptides (IPLs) in the blood bind membrane-bound insulin receptors in growing organs
- This initiates cytoplasmic phosphorylation, localisation and inactivation of FOXO
Koyama et al., 2013;
What is FOXO, what does it do?
What happens when it is repressed?
A TSF
Active FOXO induces expression of growth inhibitors.
FOXO repression by insulin signalling means growth is no longer inhibited.
Increased horn size is an example of allometric scaling. What are allometric traits?
What is characteristic of allometric traits?
Those that grow at a different rate (faster) to the rest of the body, thus allometric traits are proportionally larger than the body.
What are isometric traits?
Traits that grow at the same rate as the body, and so are in proportion to the body.
Why do beetle horns show allometric scaling?
Because of their acute insulin sensitivity they grow faster than other tissues, even when exposed to the same concentrations of insulin.
Horns can be described as hyper-allometric traits. Why?
Because the growth rate is so much higher than the rest of the body that the horns are huge in comparison to body size.
Koyama et al., 2013:
How many times more sensitive are horns to insulin than other tissues?
8x more sensitive
Who looked at dung beetles?
What did they describe about how dung beetle larvae gain nutrition?
Simpson et al., 2011
Females oviposit a single egg into a dung ball, this provides the larva with nutrients.
The larva will continue to grow until it has consumed the entire dung ball.
The stress of starvation initiates the next stage of development.
Thus dung ball size determines larva fitness.
