The oddities of evolution may have left many marine mammals without defences against some particularly nasty human pollutants, new research in the journal Science reports.
Different environments require organisms to have certain characteristics if they are to thrive there.
Animals that dwell in the light-rich zones, such as the upper ocean or on land, mostly require some sort of light-sensitive apparatus. Hence the eye has evolved independently in humans and cephalopods (octopus, squid and cuttlefish). Similarly, wings evolved separately in birds and bats. These different lineages have converged on the same solution to environmental challenges, a process scientists call “convergent evolution”.
But the process is not always about gain. Sometimes, when an organism transitions from living in one environment to another, physical characteristics that were once critical become redundant. This explains why so many subterranean and cave-dwelling species lose not only their sight but their eyes, too. They no longer have a function and are therefore not “selected for” by evolution.
This process is referred to as “non-adaptive convergent trait loss”. Other examples include the loss of bitterness receptors in many carnivorous species – and the loss of the sense of smell in marine mammals.
This, however, is not the only loss marine mammals have experienced. In the latest research, computational biologist Wynn Meyer of the University of Pittsburgh in the US and a team of American and Australian researchers have identified another such convergent loss.
The team compared the genomes of three marine mammal lineages, or clades, with those of terrestrial mammals, looking for genes that had lost function. What they discovered was that cetaceans (whales and dolphins), sirenians (manatees and dugongs) and pinnipeds (seals) have all lost the function of a gene called Paraoxonase 1 (PON1). By comparison, the 56 terrestrial species the team looked at all had intact and functioning PON1 genes.
The lineages returned to the ocean in separate colonisation events. Pinnipeds split away about 50 million years ago, sirenians diverged from an ancestral lineage of hippopotamus-like organisms around 53 million years ago, and cetaceans split from their ancestral lineage of elephant-like creatures roughly 64 million years ago. Yet despite this, all three have shown the same convergent loss of PON1, with the most conspicuous results found in cetaceans and sirenians, both of which lost PON1 function soon after striking out on their own evolutionary trajectories.
PON1 produces and enzyme involved in the oxidation of fatty acids, and the shift to marine environments is thought to have released it from functional constraints, thus freeing it from evolutionary selection pressures. However, the gene also works to break down a particularly toxic form of organophosphate that are the by-product of oft-used pesticides manufactured by humans. In fact, PON1 is the organisms’ sole defence against them.
Unfortunately, these incredibly powerful neurotoxins are introduced to marine environments in huge volumes through agricultural run-off. The authors identify particular areas of concern, including Florida in the US and Queensland in Australia, as sites with significant marine mammal populations that are showing elevated levels of organophosphate pollution.
Where once PON1 was useless, it may now be crucial. Sadly, the return of selection pressure on the gene may take a terrible toll on marine mammal populations, reminding us once again of what a danger we have become for the other residents of our little blue planet.
Stephen Fleischfresser is a lecturer at the University of Melbourne's Trinity College and holds a PhD in the History and Philosophy of Science.
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