A project undertaken at The University of New South Wales, and supervised by Lee Ann Rollins
Cane toads (Rhinella marina) are notorious invaders causing significant ecological impacts to Australian ecosystems. However, cane toads are also useful models to understand biological invasions and, in particular, how introduced populations with reduced genetic diversity can thrive and rapidly evolve in their new environment. Released as a small, introduced population in coastal North Queensland less than a century years ago, cane toads have shown rapid adaptation allowing them to spread westerly at ever-increasing speeds and they are now approaching the outskirts of Broome. ‘Western toads’ from newly-established areas have evolved a range of dispersal-related traits like longer limbs, higher endurance and increased mobility compared to ‘Eastern toads’ from long-colonised areas. Some of these traits have been shown to be transmitted across generations, yet the molecular changes at the root of this rapid evolution remain largely unknown.
Toad and toad eggs
In comparison to toads from the native range, Australian toads are genetically very similar across the invasion range, which seems at odds with the magnitude of phenotypic variation seen between Western and Eastern toads. By contrast, the role of epigenetics as a potential driver of rapid adaptation, compensating for low genetic diversity, is still an open question. Epigenetic mechanisms, in particular DNA methylation, are promising factors to explain the range of phenotypic changes seen in cane toads as they have spread across Australia. DNA methylation can dynamically change following exposure to environmental factors, and can affect gene activity without requiring any added genetic diversity. Studies in toads suggest that DNA methylation can be responsive to environmental cues and can be directly transmitted from parents to offspring. The key unanswered question is whether these epigenetic modifications following environmental perturbations are still retained in later generations, in absence of the initial environmental trigger.
Our objective in this project is to investigate if environmental influences can induce epigenetic changes that affect gene expression and that are stably transferred across multiple generations in invasive cane toads. We will expose toads to conspecific alarm cues, an environmental stressor known to trigger DNA methylation changes.
We will then:
1) determine if exposed toads’ epigenetic changes are retained in both their offspring and grandoffspring,
2) test if patterns differ between the maternal and paternal line, and
3) ascertain if differential DNA methylation is linked with differences in gene activity in exposed toads and their descendants.
The evolutionary potential of epigenetics has been hotly debated for many years, especially in animals, yet strong evidence has remained elusive beyond unicellular organisms and laboratory models. If our hypothesis proves to be true, our study will provide a first of its kind proof of concept that epigenetics can indeed contribute to rapid adaption and evolutionary change in a wild vertebrate animal.