Polyphenism in beetles with insulin signalling

Question 1

What is polyphenism and what causes it?

Answer 1

When an organism can exhibit multiple phenotypes, attributable to phenotypic plasticity

Question 2

What is phenotypic plasticity?

Answer 2

The ability of a given genotype to produce multiple phenotypes in response to environmental variation.

Question 3

Phenotypic plasticity demonstrates how genetic programmes can be modulated by the environment, as a result of…

Answer 3

environmental sensitivity

Question 4

How is phenotypic plasticity adaptive?

Answer 4

It allows organisms to express a phenotype that is best-suited to the environment

Question 5

Why is phenotypic plasticity default?

Answer 5

Because developmental, physiological and metabolic processes are inherently sensitive to environmental variables.

Question 6

Give 3 examples of environmental variables that affect phenotypic plasticity.

Answer 6

Temperature, pH, nutrient availability

Question 7

Is polyphenism common in insect taxa?

Answer 7

Yes

Question 8

Large horns are advantageous to male beetles, order Coleoptera, but why?

Answer 8

Males face contest competition for females and must physically compete with one another.

Question 9

Do beetle horns show horn polyphenism?

Answer 9

Yes

Question 10

What is horn polyphenism dependent on?

Answer 10

Nutrient availability

Question 11

What is the outcome when a male is raised on an a) nutrient-rich and b) nutrient-poor diet?

Answer 11

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

Question 12

Why don’t females want to mate with small-horned males?

Answer 12

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.)

Question 13

Give 3 control mechanisms for phenotypic plasticity.

Answer 13

1. Developmental pathways
2. Gene expression
3. Hormonal control

Question 14

What is responsible for beetle horn polyphenism?

Answer 14

Hormonal control via insulin signalling

Question 15

Who studied the hormonal control of horn development in the rhinoceros beetle?

What is its latin name?

Answer 15

Emlen et al., 2012

They belong to the subfamily Dynastinae

Question 16

Emlen et al., 2012;

What did the authors find determined horn length?

Answer 16

Nutrient availability during larval development, with high nutrient content producing disproportionate increases in horn size.

Question 17

Emlen et al., 2012:

How did the authors find that Dynastinae members were sensitive to insulin?

Answer 17

Peturbations of the insulin/IGF pathway affected horn development more than other structures

Question 18

Who described the insulin-signalling pathway?

Answer 18

Koyama et al., 2013

Question 19

Koyama et al., 2013:

Describe how insuling signalling works. Give steps.

Answer 19

1. High nutrition stimulates insulin-producing cells in the CNS to increase systemic insulin
2. Insulin-like peptides (IPLs) in the blood bind membrane-bound insulin receptors in growing organs
3. This initiates cytoplasmic phosphorylation, localisation and inactivation of FOXO

Question 20

Koyama et al., 2013;

What is FOXO, what does it do?

What happens when it is repressed?

Answer 20

A TSF

Active FOXO induces expression of growth inhibitors.

FOXO repression by insulin signalling means growth is no longer inhibited.

Question 21

Increased horn size is an example of allometric scaling. What are allometric traits?

What is characteristic of allometric traits?

Answer 21

Those that grow at a different rate (faster) to the rest of the body, thus allometric traits are proportionally larger than the body.

Question 22

What are isometric traits?

Answer 22

Traits that grow at the same rate as the body, and so are in proportion to the body.

Question 23

Why do beetle horns show allometric scaling?

Answer 23

Because of their acute insulin sensitivity they grow faster than other tissues, even when exposed to the same concentrations of insulin.

Question 24

Horns can be described as hyper-allometric traits. Why?

Answer 24

Because the growth rate is so much higher than the rest of the body that the horns are huge in comparison to body size.

Question 25

Koyama et al., 2013:

How many times more sensitive are horns to insulin than other tissues?

Answer 25

8x more sensitive

Question 26

Who looked at dung beetles?

What did they describe about how dung beetle larvae gain nutrition?

Answer 26

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.