As in many reptiles, the sex of leopard geckos is determined not by chromosomes, but by the environment. Hatchlings will develop into either males or females depending on the temperature they experience as incubating eggs. Sex isn’t hard-wired because the key genes involved in gonad differentiation are up- and down-regulated by temperature, so a fairly warm nest will produce mostly males, a cool nest mostly females, and a nest close to the temperature threshold, an evenly mixed brood.
In other organisms as well, all kinds of environmental factors – from light to nutrients to social interactions – can be important players in gene regulatory pathways. In 1953, when Watson, Crick, and Franklin discovered that the double-helix DNA molecule carried genetic information, the DNA gene code (or genotype) of an animal or plant was viewed as a ‘blueprint’ for its features. It turns out, however, that an organism’s development is often strongly influenced by its environment. As a result of the many ways that environmental factors can affect gene expression (via mechanisms such as epigenetic silencing, hormone levels, or metabolic feedbacks), we can now view the genotype as a more or less flexible repertoire of possible outcomes, rather than a rigid blueprint.
A closer examination of the leopard gecko reveals a second key point. Sex determination isn’t entirely environmental, because the precise temperature threshold for developing as a male rather than a female is influenced by an animal’s genotype. As a result, the sex of a particular hatchling depends on both its genes and its environment. In fact, the vast majority of traits in all kinds of organisms reflect this kind of developmental interaction between ‘nature’ (the individual’s genotype) and ‘nurture’ (its environmental conditions).
Eco-Devo (ecological developmental biology) is the field of biology that investigates how a given plant, animal, or microbial genotype can produce different outcomes in different natural environments. There are countless examples of this kind of developmental plasticity. In cichlid fish, features such as jaw anatomy and tooth shape develop differently if a juvenile fish’s early diet consists of soft foods versus harder prey items such as larval snails and shrimp. These developmental responses in turn shape the fish’s lifelong food preferences and thus its ecological impact as a predator.
Juvenile diet also influences development and feeding behavior in certain birds and insects. For instance, Melanoplus grasshoppers that are fed fibrous, low-nutrient food develop larger, more muscular mouthparts and a larger gut compared with individuals given food that is easier to chew and more nutritious. In mammals, low quality food can likewise result in suitable changes to the digestive tract such as increased gut capacity and larger intestinal surface area to maximize nutrient uptake. Plants also develop differently when resources are scarce: in low light, they produce large, thin leaves that catch as many photons as possible, and in dry or nutrient-poor soil, they allocate more of their body mass to roots to generate large uptake surfaces.
Plastic responses to the environment can even extend to the next generation. Yellow monkeyflower plants subjected to simulated herbivory produce offspring with dense defensive hairs, even though the seedlings themselves have not been damaged. In this way, parent plants living in a site where herbivores are present can pre-adapt their progeny to repel potential attackers. This adaptive effect results from inherited modifications to gene expression without any changes in the offspring DNA sequence per se.
Clearly, in many cases, plastic eco-devo adjustments to different ecological conditions result in a plant or animal body that is functionally fine-tuned to the environment (or parent’s environment) that brought it about. For this reason, eco-devo studies provide new insights to how adaptation works. Because precise cues and developmental pathways are influenced by genes, these studies also show how genetically distinct individuals or populations may differ in response.
Eco-devo information is especially important for understanding the potential for organisms to succeed in the changed environments being created by human activities. This potential depends in part on each species’ environmental response patterns to aspects of its habitat. In tropical anemonefish, parents raised in seawater with predicted future levels of carbon dioxide produce progeny that show none of the usual negative effects of high CO2, due to adaptive effects on enzyme function that are inherited from the pre-exposed parents. Individuals in certain bird species respond to warm days by speeding up their developmental timing, allowing them to raise their young in synch with the earlier onset of Spring due to climate change (and gobble the peak supply of caterpillars). Yet other species lack these beneficial types of plasticity, putting them at greater risk of extinction in altered conditions.
In some cases, new patterns of environmental response may evolve which will maintain adaptation under changed conditions. For this to occur, populations must contain genetic variation for eco-devo responses, so that natural selection can promote the increased frequency of genotypes with the most beneficial response patterns. In current populations of the leopard gecko, for instance, only males would be produced at the higher nest temperatures that are expected under global warming. The presence of genotypes with different sex-determining temperature thresholds would provide the raw material for populations to evolve higher thresholds that would allow both males and females to be produced. Fortunately, this kind of genetic variation has been found in this species, but such in-depth information about eco-devo variation is seldom available. Another worrying fact is that changes to environmental cues may derail existing adaptive pathways, requiring the evolution of new cues to guide development. We need to learn a lot more about eco-devo responses to predicted future environments, and about genetic variation for these responses, to understand how various species are likely to fare in a changing world.
For more information, read Sonia’s book:
Organism and environment: Ecological development, niche construction and adaptation
Sultan SE 2015. Oxford University Press.
This post was originally published on OUP blog, 5 June 2016
