The New Zealand fresh water mud snail (Potamopyrgus antipodarum) was inadvertently introduced to the United States probably in the mid 1980s and has become seriously invasive. Though its occupation of the new habitats is very recent in evolutionary terms, alterations are already appearing in its phenotype. The rapidity of its occupation and phenotypic change make this a useful animal with which to study adaptation. The shapes and colouration of their coiled shells are important distinguishing features of different snail species. However, individual species have long been known to exhibit variation in their shell shapes, often in response to the forces of water (e.g. river flow or wave action) to which they are exposed, and a study published in 2017 showed that this is the case with the New Zealand snail1.
Image credit: United States Geological Survey
https://commons.wikimedia.org/wiki/File:P_antipodarum.jpg
Examination of these snails in lakes and rivers in the northwestern US showed that the size of the apertures at the bottom of their shells (measured as the ratio of aperture diameter to shell height) correlates with the speed of habitat currents. Shell apertures in snails living in a fast flowing river were larger than those of snails in a slower one, and still smaller in snails located in lakes that have no appreciable flow. This is an adaptive correlation; the larger aperture accommodates a larger foot, which gives the snails better grip in fast currents, making them more effective in their grazing on the micro plant and sedimentary material they eat. Importantly, these habitat associated phenotype variations have emerged since the occupation of the US habitats; they have not come from existing variants among invading snails from New Zealand. (In fact, all the populations studied are clonal; by a particular feature of the species’ reproduction process, a large proportion of populations of it now present in the US, including the four included in the study, are derived from a single invading female.)
Phenotypic variation in these snails has therefore enabled them to occupy niche habitats as part of their progressive invasion of a new continent. Additionally, a special biological feature of New Zealand mud snails makes the occurrence of that variation intriguing. This animal has the capability to reproduce asexually. In this process, females possess progeny embryos at birth, and the progeny are clones, genetically identical to that female parent. This asexual process has an important impact: progeny lack the genetic variation that results from the shuffling effect of sexual reproduction. As already mentioned, the four snail populations used in the study (and indeed others across the western US) are derived from a single invading female. This means that there is virtually no variation between the DNA of snails in these populations because they started as identical clones and the few decades since their clonal origin is far too short a time for many mutations to arise amongst them. So if snails at the four different habitats are genetically identical, how can they differ in their phenotypes? Indeed, the scale and rapidity of their invasion suggest that the species is very adaptable; if there is no genetic variation then there must be an alternative mechanism for that phenotypic versatility. The study went on to reveal a further correlation with the different habitat water flow rates, this time a molecular one. Specifically, the DNA of snails at the different habitats differ in their methylation, their methylation patterns being very specific for the lake and river habitats. Therefore, whereas the coding sequences of the DNA molecules are identical in these snails, their DNA methylation is not, and consequently the different snail populations almost certainly have different patterns in the activity of many of their genes. Epigenetic chemical marking of DNA therefore provides a potential source of phenotypic variation in these animals. The investigators acknowledged that at this stage they have not yet fully proved that the epigenetic changes are the cause of the shell phenotype alterations, but it is likely that they are. There is also reason to believe that the different patterns of gene marking have been induced by the different environments of the snails’ habitats.
”If snails at the four different habitats are genetically identical, how can they differ in their phenotypes? Indeed, the scale and rapidity of their invasion suggest that the species is very adaptable; if there is no genetic variation then there must be an alternative mechanism for that phenotypic versatility”
This example calls into question, in two ways, the exclusivity of random gene mutation as the source of phenotypic variation in adaptive evolution. First, it shows very rapid adaptation in an animal that simply does not have genetic variation. Second, it supports the idea that epigenetic processes can enable phenotypic variation, and consequently adaptation, in wild populations (rather than merely in laboratory situations). Additionally, there is another type of epigenetic inheritance that differs even further from the classical genetics mechanism.
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References
- Smithson, Mark, Jennifer L. M. Thorson, Ingrid Sadler-Riggleman, Daniel Beck, Michael K. Skinner, and Mark Dybdahl. ‘Between-Generation Phenotypic and Epigenetic Stability in a Clonal Snail’. Genome Biology and Evolution 12, no. 9 (1 September 2020): 1604–15. https://doi.org/10.1093/gbe/evaa181 [↩]