Plants and animals that are seemingly harmless in their native habitats can become quite aggressive or even destructive in a new location.
Think of the rats that have been a source of human and animal diseases for generations and the kudzu, purple loostrife and other plants that wreak havoc in otherwise natural habitats.
These “invasive” organisms have potentially devastating biological and economical ramifications. Mosquitos introduced to Hawaii carried avian malaria, which decimated native birds, and millions of dollars goes annually to eliminating or controlling nuisance plants such as the Brazilian pepper.
Recent studies suggest epigenetics – changes to the packaging of DNA that affect how genes get expressed – may play a role in why some plants and animals cause so much trouble outside their native ranges.
Invasion alert
When a species invades a new area, only a few plants or animals establish a new population. Biological theory predicts that this limited number of organisms limits a species’ variation and therefore the potential for evolution by natural selection.
Despite these expected limitations, many invasive species appear to thrive even with low levels of genetic variation.
In a recent study of the highly invasive Japanese knotweed (Fallopia japonica) published in Ecology Letters, plants across the 160km of Long Island, New York had almost no genetic variation.
Instead their response to new habitats (such as sand dunes and marshes) was related to DNA methylation, a type of epigenetic mechanism that alters gene expression.
Epigenetics: the making of an invader?
Gene expression is the process whereby the DNA of an organism is translated to a particular function or behaviour of that organism.
The knotweed findings indicate that epigenetic effects such as DNA methylation alters gene expression and could influence how the plants manage to thrive in diverse habitats. Even though they have basically the same DNA, the genes expressed by each plant vary based on the environment they’re placed in.
Similarly, in house sparrows, one of the world’s most common birds, invasive populations have adjusted quickly to novel conditions in Africa.
Over the short span of 60 years, house sparrows have spread across Kenya, changing dramatically as they’ve moved into new areas.
In Kenya they fight parasites differently, respond to stress differently, and are much more exploratory in their behaviour.
With so little genetic variation to work with, it’s very likely that epigenetics is helping this invader expand its range. Although genetic variation in these populations is low compared to most other parts of the world, epigenetic variation is so high that no two individuals are similar epigenetically.
This indicates that while the genomes may be similar at the DNA sequence level, the expression of the DNA might vary quite a lot among individuals.
Epigenetics and the future of invasions
So, even though two individuals may have the same DNA sequence, their behaviour, health, size and many other traits can differ quite a bit just because different portions of the genome are activated or deactivated.
Some of these environmentally induced changes can be inherited, thus having a more persistent effect on traits within the species. Inheritance of environmentally induced epigenetic effects essentially provides a mechanism for inheritance of acquired traits, which has long been disregarded in biological circles.
Historically, novel variation was thought to emerge only via mutations in DNA sequence, which means that changes in populations would happen relatively slowly - over thousands if not millions of years in most cases.
In light of epigenetics though, the environment has two roles: environment still “selects” the individuals that get to breed, or in other words it sorts among the mutants that arise in DNA sequences.
But epigenetic changes induced by the environment also reveal trait variation through gene expression. If such epigenetic variation is heritable, trait change can show up faster in populations than would occur by traditional sequence-based evolution.
For those reasons, it’s not surprising that invasive species have been the basis of many ecological epigenetics studies.
Because epigenetics research reveals how the genome translates into important traits in response to environmental factors, this work is having a broad impact, on our understanding of issues as diverse as obesity, cancer rates, and how species may react to global climate change.