Developmental Plasticity

Before starting, a small note: I currently have 4 time-consuming projects running in parallel, so updates will be sporadic. Sorry.

In this post, we’ll look at how the environment can influence development, including phenotypic plasticity (what is an ecomorph?) and epigenetics. Also, for the first time ever, I will put in a reference and further reading list at the end.

An organism’s development is a finely-tuned and highly complex process. Cells communicate with each other, changing each other’s gene expressions; all this happens in perfect unison and with impeccable timing. Cells secrete various chemical signals, collectively called paracrine factors, that are responsible for instructing what they and their neighbours should do.

But is that all there is to development? If we decipher every single event that happens in an embryo as it develops, will we then be able to predict how an organism will look like just by looking at its genetic sequence? No. The same genotype can produce different phenotypes – the environment in which an embryo grows can have subtle to radical effects on the final organism.

The most common environmental factors include temperature (responsible for the coat colour of Himalayan rabbits), food supply (important for making ant queens), gravity (one of the obstacles behind human space colonisation is that our muscles atrophy in microgravity), light (plants!), presence of predators (Thais snails develop a toothed aperture when predators are there) or even presence of other members of the same species (responsible for desert locust migrations).

The question of whether this plasticity is adaptive cannot be answered. A Himalayan rabbit’s coat colour depends on melanin: more melanin makes black fur. However, Himalayan rabbits have a mutation in one of the enzymes controlling melanin production which causes melanin to only be produced in cold temperatures (i.e. the extremities, ears and nose – see linked picture above). The body remains white, which is excellent camouflage for snow – making it an adaptation (albeit not an ideal one; I imagine the ears stick out like a sore thumb). But there is the flipside, where this plasticity can be deadly – many diseases can be traced back to the environment affecting our development.

One generalisation that is often made is that the environment is secondary to genetics. This is also a pretty loose statement: while the genome has to be there to decide if a structure ultimately gets made, it is often the case that the environment gives the instructions. The genome is not a blueprint.

One genome can code for a whole range of phenotypes, and the final product is often decided by the environment. For example, dung beetle horns are coded for by the genome, but their length depends solely on the amount of food the beetles receive as larvae (it is slightly more complicated, as a certain threshold must be reached before a male even grows horns, but I do not want to complicate matters). This kind of developmental plasticity, where there are a wide range of opportunities, is called a reaction norm.

There is a second, arguably more extreme type: polyphenism. This is when the environment specifically dictates the phenotype of the organism. Think of turtles: at one range of temperatures, females will be produced, at another range, males. That doesn’t mean that in the gap between the two ranges, hermaphrodites are produced. It’s either one or the other (in the intermediate range, different proportions of male/female develop). Ecomorphs are examples of polyphenism – think of butterflies that change colour in summer and winter.

Epigenetics

Before concluding, it is important to mention epigenetics. It’s one of the hotspots for research these days and, as is often the case, gets horribly abused by the mass media. Before defining it, let’s get one thing out of the way: epigenetics is not neo-Lamarckism.

Okay. Epigenetics refers to a phenomenon whereby different phenotypes are produced not by the alteration of genes themselves, but by changing gene expression. The link to our topic here is glaringly obvious: it’s a mechanism which enables the environment to influence development. As I said, it has nothing to do with Lamarckism (Refresher: Lamarck proposed that an organism can pass on traits that it acquired during its life; as an example, this would mean that Arnold Schwarzenegger would automatically give birth to a ‘roided up douchebag with an Austrian accent).

However, there is something called epigenetic inheritance, where successful phenotypes get passed on to the next generation. This can happen only at the cellular level. Chromatin (the DNA – protein combination that makes up chromosomes) can get modified, and these changes are acquired by the daughter cells. Now, if this chromatin modification occurs in a germ cell, then it will get passed on to the offspring.

The extent to which this affects or contributes to evolution is still a matter of debate, so I cannot comment on it or I risk brainwashing you.

Well, that does it as an introduction to the field of ecological developmental biology (eco-devo). What you’re supposed to get out of this is the need for interdisciplinary research: the developmental biologists do wonderful work elucidating gene cascades in the lab. But the only way to go forward is to integrate fields, even if they’re as seemingly disparate as ecology and developmental biology, because nature is not modular; it’s a complex web with interactions both obvious and obscure, and we’re only scratching the surface in beginning to understand them.

And yes, I do realise I mixed up at least 3 metaphors in that last sentence, but I’m not Richard Dawkins.

Further Reading

E. Avital, E. Jablonka, Animal Traditions: Behavioural Inheritance in Evolution (Cambridge Univ. Press, Cambridge, 2000).

S. F. Gilbert, Ecological developmental biology: Developmental biology meets the real world. Developmental Biology 233, 1-12 (2001).

E. Jablonka, M. J. Lamb, Evolution in Four Dimensions: Genetic, Epigenetic, Behavioral, and Symbolic Variation in the History of Life (MIT Press, Cambridge, MA, 2005).

E. Jablonka, G. Raz, Transgenerational epigenetic inheritance: Prevalence, mechanisms, and implications for the study of heridity. The Quarterly Review of Biology 84, 131-176 (2009).

S. E. Sultan, Development in context: The timely emergence of eco-devo. Trends in Ecology & Evolution 12, 575-582 (2007).

L. Van Speybroeck, From epigenesis to epigenetics: The genome in context. Annals of the New York Academy of Sciences 981 (2002).

M. J. West-Eberhard, Developmental Plasticity and Evolution (Oxford Univ. Press, Oxford, 2001).

http://www.youtube.com/watch?v=M6UsKJUmq2A

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