a tale of several fishes

Well, Marcus & I have just completed a Scholarship preparation session in New Plymouth – it was (I hope!) a useful & enjoyable time for all involved. Marcus & I enjoyed it anyway – we both get a buzz out of working with groups like this (one of the reasons, as far as I’m concerned anyway, is the very perceptive questions & suggestions that students come up with.)

Anyway, my group spent a fair bit of time working through last year’s Schol Bio paper, including this question: Identify  and discuss the patterns and processes of antifreeze glycoprotein (AFGP) evolution in polar fishes. The question also provides quite a bit of contextual information – and it’s really important that you read this carefully! It contains a lot of useful data that you can integrate into your answer (& the markers would have been looking for this).

Antifreeze compounds are, as you might perhaps expect, not uncommon in organisms living in polar regions. Most, but not all, are glycoproteins – proteins with saccharide sidegroups. They function to lower the organism’s freezing point, reducing the risk of ice crystals forming within the tissues (& thus rendering cells leaky,squishy, & generally dead). More precisely, according to the US National Science Foundtation, they ‘plug gaps in existing small ice crystals, … [preventing] the attachment of more ice molecules’, thus stopping the ice crystals from growing large enough to be damaging.

Two groups of animals possessing antifreeze glycoproteins (AFGPs) are the arctic cod (found, as the name suggests, in northern polar waters) and the notothenioid fishes of the Antarctic seas. While these fish groups are poles apart (sorry! couldn’t resist that one!), as the Schol paper says, ‘the AFGPs [they produce] are virtually identical and they both contain the same repeating sequence of three amino acids (threonine-alanine-alanine).’

So, what pattern of evolution are we seeing here, and how might it have evolved?

You’ve probably said, convergent evolution, & you’d be right. But this wouldn’t be enough to keep the examiner happy. You’d also need to explain what you understand by the term ‘convergent evolution’, and the evidence you used to come to this decision. In convergent evolution, organisms which are only distantly related – & whose last common ancestor lacks the trait in question – independently evolve the same or very similar adaptation in response to similar selection pressures. Sharks, dolphins, and the extinct ichthyosaurs all show convergence in body shape, for example. In the case of the polar fishes, there are two lines of evidence that you could cite. The context material included the information that the AFGP genes are found in quite different chromosomal locations in the two groups and have different origins, and yet the proteins are essentially the same, which suggests an independent origin followed by convergent evolution.

There is also the issue of when this particular gene evolved. Again, the contextual info is helpful here. Antarctic waters are to some degree thermally isolated by a circumpolar current, which formed around 30 million years ago after the final separation of Antarctica from South America, Once this happened water temperatures started to drop, getting down to around freezing point about 10-15mya. (Scientists can date these events using oxygen isotope ratios from sea-floor sediment cores – and ‘well done!’ to the student today who knew about that.) Scientists think that at this time there was probably only a single notothenioid species, and that this species acquired the AFGP mutation. (You shouldn’t think that this was the first time this particular mutation occurred, as it almost certainly wasn’t – but it would have been the first time that a fish bearing the mutation was likely to be at a selective advantage from it.) The Arctic Sea, on the other hand, didn’t freeze over until about 4-5 million years ago, and so that’s the earliest point at which an AFGP would be subject to strong selective pressure in its favour- strong evidence for independent origins of the AFGPs. (It also gives us the earliest likely time for polar bear evolution, although many scientists think they evolved much more recently than that.)

As for the part of the question that asks about processes of evolution – here the examiner’s asking you to integrate your knowledge of how natural selection operates with specific information about the ice fish. (That is, you wouldn’t get a lot of credit for talking about natural selection only in very general terms.) You could also talk about speciation within the notothenioid fishes of the Southern Ocean, given that the question identifies two particular species (‘bald notothen’ & ‘threadfin pinhead’) and also tells us that there are more than 100 species of notothens.

Natural selection requires that there be some heritable veriation among the members of a population, so you’d start off by reiterating that some individual(s) in a single species would have to gain the AFGP mutation. (If it had happened independently in more than one notothen species, you’d expect to see some evidence of that, in terms of chromosomal location and origin of the gene(s) concerned.) As the oceans cooled, those individuals possessing the mutation would have been at a selective advantage compared to those without – they’d be more likely to survive & reproduce, and at least some of their offspring would possiess this beneficial mutation and pass it on in turn to their own offspring. Combined with continuing directional selection (progressively cooler waters) this would see the AFGP allele spread throughout the ancestral species population. At the same time, those fish species lacking AFGPs would become locally or globally (if they couldn’t migrate to more sutiable habitat), thus leaving a range of ecological niches. And this in turn could drive further speciation of that ancestral ‘ice-fish’.

An excellent question, & it generated some excellent ideas today. And now I’m off for a walk 🙂

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