Invasive species corrupt DNA, not just ecosystems (Fitzpatrick et al., PNAS 2010)

I rarely think about how invasive species affect genetics.  It’s always in terms of ecosystems or species: invasive brown tree snakes gobbling up birds and lizards in Guam, or zebra mussels overwhelming and altering the environment of the Great Lakes.  How one species outcompetes and replaces another, changing the natural system.  This is partly because many of the common examples are of predator-prey relationships, where the two species are very distantly related and could never breed, thus keeping genetics out of the picture.  But what about situations where the introduced animal and native animal are similar?

This gets us into the muddy waters of what defines a species.  For sexually reproducing organisms, a species is the group of animals with whom one can exchange genetic material via reproduction, or, in other words, can produce fertile offspring.  To distinguish one species from another under this definition, a scientist would need a pretty wide worldview.  How else could he know that a squirrel from England could not mate with a US squirrel if it tried?  And the honest answer is: he can’t.  (Unless he collected squirrels from around the world and tried to mate them all with one another… but that’s a lot of work.)  Thus, species are often also defined based on location or geography, despite the fact that maybe they could mate if they had access to one another.  But what are the chances that a squirrel will swim across the Atlantic for a new girlfriend?

And there’s where invasive species fit in.  In a paper published this week in PNAS out of Knoxville, TN, Lexington, KY, and UC Davis, scientists studied the Salinas Valley in central California, where salamanders from Texas and New Mexico had been introduced in the 1950s for use as bait by fisherman.  These salamanders, the Barred Tiger Salamanders (Ambystoma tigrinum mavortium) had been defined as a separate species from the threatened native California Tiger Salamanders (Ambystoma californiense), as their populations had been living apart for 3-10 million years, and thus it was unlikely that they were still genetically similar enough to mate.  But – alas – this assumption was wrong.  The invasive salamanders have been mating with the native species for the last 50 years, producing hybrids which are able to mate with either species and one another.  The question: is this hybridization significantly changing the DNA of the native species?

To investigate this question, the researchers identified an introduction site at a pond in central California, and took samples of over 200 salamanders (by clipping the end of their tail and immediately releasing them) at this site and others within a region 200 km north.  Using salamanders of each species from non-invaded ranges, they determined the baseline genetic makeup of each species.

They scanned the genomes of the sampled salamanders (say it 10 times fast) for 68 genetic markers to see if any of the invasive genes had “taken over” the native genes.  They saw no real difference in 65 of these species — that is, the salamanders retained their native genes.  However, they saw a drastic increase in 3 of the genes.  In the figure below, taken from their paper (click on image for larger size),  the little “thermometers” measure the DNA differences at different sites, native in white and invasive in black, with the introduction site indicated by the red arrow.  The upper left (A) shows the big picture: of the 68 gene markers studied total, invasive genes are only apparent at the introduction site.  The other 3 boxes (B, C, D) show the three genes that have spread — and as you can see, they have spread far and deep, despite their invisibility overall (A).  The authors were thorough: they tested whether this pattern was due to either sampling error or random genetic drift without natural selection, and neither of these biases accounted for the pattern of these 3 genes.

The function of these genes is unknown.  However, by studying the behavior of the animals, it seems they are related to reproduction.  The hybrids have larger larvae with greater survival and develop more quickly, ever hastening their dispersal.  This raises a few questions:

1.  If these invasive genes are helping survival, then who cares if they invaded?  It is easy to look at this as actually beneficial to the threatened native salamanders.  However, it has unknown impacts on the surrounding ecosystem.  These bigger larvae eat a lot more, impacting the populations of their prey species through indirect effects of the invasion.  A change in the abundance of one species affects all others – what seems to be an immediate benefit can be incredibly harmful in the long run.

2.  How do we define a species?  The native salamanders are a threatened species.  If they have received genes that increase their numbers through hybridization, is this a comeback?  Are they still A. californiense?  Do these 3 genes alone make them A. mavortium?  Are they an entirely new species?  Is it possible to stop the invasion of these genes throughout the state without killing off a threatened species?

I don’t have the answers to these questions.  We human beings are drawn to classification: we want to put all of the animals in neat little piles and call it fin.  But the truth is that species are eternally evolving — as Peter and Rosemary Grant have shown with their Galapagos finches, most recently in November 2009.  The monkeys that live on one side of a jungle can have a different genetic makeup than the ones on the other side even if they can still mate.

Clearly the introduction of these salamanders, which was just an innocent attempt to raise some bait locally, has had unforeseen impacts on the ecosystem.  Humans’ ability to travel has meant that we are bringing animals together that have not evolved to live together, or have evolved apart millions of years ago.  In some ways it feels like what is done is done — and I am not enough of an expert on habitat restoration to tell you otherwise.  But try little things: wash the mud off of your boots before you go hiking in another state or country, don’t release your foreign pets locally (as my roommate Erinrose and I have been tempted to do with our pet turtle, Nicolas Cage), volunteer at your local wildlife refuge.  Biodiversity is important.

How can we save our planet?

Fitzpatrick, B., Johnson, J., Kump, D., Smith, J., Voss, S., & Shaffer, H. (2010). Rapid spread of invasive genes into a threatened native species Proceedings of the National Academy of Sciences, 107 (8), 3606-3610 DOI: 10.1073/pnas.0911802107