Aaron Millar, from the University of Canterbury discusses his article: Altitudinal Differentiation Occurs Alongside High Plasticity in a General-Purpose Genotype Invasive Plant
In 1965, Herbert Baker imagined a super-weed, a plant that could invade anywhere and deal with anything. He called it the “general-purpose genotype”, a plant which could adapt to any environment with massive levels of phenotypic plasticity. Phenotypic plasticity is the ability of a single plant to change how it grows to match its environment (as opposed to evolution, which would take multiple generations). If you forget to water your garden and your beans grow longer roots to compensate, that is plasticity. A true general-purpose genotype would be overwhelmingly plastic to everything. We have never found Baker’s super-weed. It turns out that if you try to do everything, you end up doing nothing well enough – but some plants manage to get close. We still call them “general-purpose genotypes”, and many of them are weeds on a global scale.
However, all that plasticity could cost them in the long run, because it can limit their ability to evolve. If a plant has all the tools to solve its problems with plasticity, then there might not be the selective pressure needed to cause evolution elsewhere. This is hugely important. Even for a very threatening weed, evolution is needed to make it more successful and help it spread further, because it will reach the limits of what plasticity can do. Evolution is also needed for plants to remain viable under climate change – plasticity alone is unlikely to be enough. The question is whether these general-purpose genotype weeds can genetically differentiate while staying highly plastic.
To answer this, we used yellow monkeyflower (Erythranthe guttata), a widespread invasive plant in the UK as well as New Zealand. In the Canterbury region of New Zealand, monkeyflower is spreading through two very different areas: warm, dry lowland plains and upland valleys with much lower temperatures and greater precipitation. We wanted to know if monkeyflower is handling the differences entirely through plasticity, or if there are genetic differences emerging between these two groups.
We took cuttings of 378 plants from 38 populations and ten different water systems and grew them in shared gardens in upland and lowland Canterbury. This allows us to distinguish genetic and plastic differences. Any differences between the plants within a garden must be genetic, because their environment is the same. How cuttings from the same plants respond to the different garden conditions shows their plastic differences.
Remarkably, we saw significant genetic differences between the upland and lowland plants developing alongside high plasticity. The upland plants were oriented more toward securing reproduction. They flowered two weeks earlier than the lowland plants and for a week longer, producing 26% more flowers. They were also more resilient to the cold. By contrast, the lowland plants had larger leaves, grew taller, and had higher photosynthetic rates – so they may have been geared more toward rapid growth. This could reflect evolution, with upland plants selected to reproduce faster because of the harsher climate, while the good conditions in the lowlands favour plants that grow more to compete better with other plants.
Plants from all populations showed large and similar plastic differences between the two environments as well. Plasticity effects were still generally stronger than genetic differences. This makes sense, because these plants are still highly plastic and have only been evolving in New Zealand for 150 years – but the key is that they can have both. This is important because it confirms that even general-purpose genotype weeds have the capacity to genetically change and boost their future success, while keeping the benefits of their strong plastic responses. Understanding the ways weeds continue to threaten our environment is vital to responding to them effectively, both now and in the future.
