Environmental Factors Affecting Growth and Development

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Phenotypic characters are not only an expression of inherited genes, rather they are influenced by environmental factors as well. Environmental factors that influence phenotypic expression can be temperature, diet, and presence of predators.

I - Phenotypic plasticity

Phenotypic plasticity is the ability of an organism to react to an environmental input with a change in form, state, movement, or rate of activity. There are two main types of phenotypic plasticity: reaction norm and polyphemism.

A. Reaction norm
In reaction norm, the genome encodes a range of potential phenotypes, and the environment that the individual is exposed to determines what phenotype should be expressed. 

B. Polyphemism
The genome only encodes for the phenotype that would best suit the organism in the environment it is in. The environmental factors that usually influences the organism's phenotype are temperature, diet, and presence of predator.

1. Temperature-induced polyphemism
Temperature can influence the sex of the organism as well as pigmentation.

In turtles, sex is determined by temperature. At the embryonic stage, a certain range of temperature induces female development while another temperature range induces male development. This results to different proportions of males and females. 

Pigmentation in the wings of Malawian butterfly, Bicyclus anynana, is also determined by temperature. Morphing during during the dry season results into a mottled brown butterfly. This enables them to escape predation by hiding in dead leaves. When morphing happens during the wet season, adult butterflies display prominent eyespots on their wings that deflect attack from predators such as birds and lizards.  

2. Diet-enduced polyphemism
In dung beetles, eggs are laid in balls of dung. When the larvae hatches, they feed on the dung. Metamorphosis happens after the larva finishes off the whole dung ball. The amount and quality of the dung ball will determine the size of the horn and the behavior of the adult male dung beetle. Adult dung beetles with larger horns will have a higher chance of reproductive success as they are able to fight off other males guarding the females.

In honeybees, diet determines if an individual is going to be queen or worker. Larvae fed adequately by the workers become queens. The queen-forming activities is induced by the protein royalactin. Royalactin reacts with the EGF receptor in the fat body of the honeybee larvae and stimulates production of juvenile hormone which elevates yolk sac protein level. Yolk sac proteins are necessary for egg production.

3. Predator-induced polyphemism
Rotifers that develop in pond waters where their predators are also present changes their morphology to counter predation. They develop into adults with larger body and spines than normal making them more difficult to eat. The same is observed in the snail species Thais lamellosa. When they are exposed to their predator species like crabs, they develop thickened shells  and a "tooth" on its aperture. Crabs do not prefer to attack snails with thickened shells.


II - Developmental symbiosis

Developmental symbiosis happens when one organism is involved or influences the physiological events of another organism. A good example of this is the bacteria Wolbachia.

Reproduction in Drosophila species can be affected by the bacteria Wolbachia. Females infected with Wolbachia are able to produce four times more offsprings than females that are not infected. Aside from an improved reproduction, Wolbachia also provides resistance against viruses.

In the wasp species Asobara tabida, the bacteria Wolbachia enable the wasp to complete yolk sac development and egg maturation. Without the presence of Wolbachia, the ovaries of the wasp will undergo apoptosis and no egg production will happen.

Wolbachia can also influence sex change. In pill bugs such as Armadillidium vulgare, genetically male individuals infected with Wolbachia are transformed into females. This is essential for the life cycle of Wolbachia as they can then be transmitted to the next generation.
Reference:
[1]Scott G.F. 2016. Developmental Biology 11th Ed. Sunderland, Mass: Sinauer Associates.

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