Most stickleback researchers catch their fish in two ways: setting minnow traps and seining. For a third (and rather creative) way, you’ll want to check out this clip from a British fishing show:
[youtube=http://www.youtube.com/watch?v=Nfp0gDGdTGA&hl=en&fs=1]Month: May 2009
If you’re quick you might be able to catch Peter on National Geographic TV tonight on the show “Hooked On Fish”. It’s a great show, using stories of anglers catching huge fish as a jumping-off point to talk about some fascinating natural history and conservation issues.
If you’re not fast enough to see it on the air tonight, you can at least get a sample of it here.
This week, I’m going to discuss a cool paper that came out of Dolph Schluter’s lab in 2008. The paper zooms in on a particularly interesting part of stickleback evolution, the transition between an ancestral marine form that breeds in fresh water to a population that lives in freshwater year-round.
Usually, (and this is one of the “color-coded for your convenience” things that make stickleback a fantastic model system) you can get a good idea where a stickleback is from by looking at its armor plates. Stickleback from marine habitats tend to have a full complement of plates, whereas sticklebacks from freshwater habitats will have few to no plates:
Stickleback armor plate phenotypes: fully plated (top), partially plated(middle), low plated(bottom)
The authors sorted through hundreds of marine stickleback to find fish that had intermediate numbers of plates, which signified that they were heterozygotes for the gene that governs plate number, Eda. These fish were placed in experimental ponds and allowed to breed. Because the fish were heterozygotes for Eda, they produced offspring with high, medium, and low plates, which gave the authors a chance to observe if natural selection favored the low-plated form in freshwater.
In each pond, the frequency of the low allele increased over time, and in a similar way. There was a slight dip when fish were very young, but then frequency increased until the fish reached breeding condition. Interestingly, fish carrying the low allele grew faster and reached breeding condition sooner than fish carrying the high allele, probably because building armor plates takes energy that could be spent on growing more quickly.
The story is more complicated than that, though – not only is there a period early in life where the high allele appears to be favored, but there is also a point where fish with intermediate plates have the highest fitness, which is difficult to explain. The authors raise the possibility that the Eda gene that controls plates in stickleback may affect other traits (pleiotropy). Either way, it looks like even the most well-understood stickleback phenotype has more to tell us.
Barrett, R., Rogers, S., & Schluter, D. (2008). Natural Selection on a Major Armor Gene in Threespine Stickleback Science, 322 (5899), 255-257 DOI: 10.1126/science.1159978
There are millions of sticklebacks across the globe, but you can also find sticklebacks in fossil form. The scientific name for most fossil sticklebacks is Gasterosteus doryssus, but morphologically this fossil “species” belongs within the threespine stickleback complex.
One Miocene fossil site has offered up some fascinating insights into the pace of evolution in threespine stickleback. Today I’ll be focusing on a paper that examines evolution in diet type in this unique stickleback “population”.
A few weeks ago, I mentioned “limnetic” and “benthic” stickleback – two different morphs of freshwater stickleback that live in different places within a lake and eat different things. Limnetic stickleback generally swim in the open areas of the lake and feed on zooplankton like calanoid copepods. Benthic stickleback stay close to the lakebed and feed on insect larva and small crustaceans like gammarids and ostracods.
In an earlier paper, it was shown that you can identify whether a stickleback is benthic or limnetic just from tiny scratches on the teeth. That technique was applied to fossil sticklebacks, with some striking results: at different periods in time, the population changed from limnetic to benthic and back again to limnetic.
Most stickleback in this lake were limnetic, which makes a lot of sense – in order for the stickleback to be preserved in anoxic sediment, the lake had to be fairly deep, which opens up a lot of potential habitat for limnetic stickleback. In addition, the substrate the sticklebacks are buried in is called diatomaceous earth – basically, millions and millions of dead diatoms, a type of phytoplankton. Lots of phytoplankton swimming around suggests there was zooplankton that ate them, which would provide a perfect source of food for limnetic stickleback.
Fossil sticklebacks (photo courtesy of Michael Bell)
So what about the point in time where the population changed from limnetic to benthic? The authors suggest that because of the speed of the change – and because there are few sticklebacks from these rocks that are halfway between benthic and limnetic – it might be the case that the limnetic sticklebacks went extinct and were replaced by a new population of invading benthic stickleback.
Still, even if we can’t say for sure whether the limnetics were replaced by benthics or whether they evolved into benthics, we can say that the benthic population evolved into a limnetic population over a few thousand years, because the pattern of tooth wear changes from the heavy markings typical of a benthic to the lighter markings typical of a limnetic.
It’s rare that we can use fossils to examine how a specific population changes over time, but because we can take our understanding of modern stickleback and apply it to the fossils, we can learn a lot about the dynamics of evolutionary change.
Purnell, M., Bell, M., Baines, D., Hart, P., & Travis, M. (2007). Correlated Evolution and Dietary Change in Fossil Stickleback Science, 317 (5846), 1887-1887 DOI: 10.1126/science.1147337
Some weeks ago, I discussed a large phylogenetic study that separated sticklebacks from the seahorses and pipefishes – today I’m going to discuss a phylogenetics paper that zooms in on the relationships between different sticklebacks(and their very closest relatives).
Many of the same scientists from the earlier stickleback phylogeny were involved in this paper, though there is one new face, Yale’s Tom Near, a longtime Wainwright Lab collaborator and former CPB Postdoc.
The group sequenced the mitochondrial genomes of all nine sticklebacks and stickleback relatives, and they also sequenced 11 nuclear genes. They used both maximum-likelihood and Bayesian methods to estimate a phylogenetic tree of sticklebacks.
Here’s what they found:
The mitogenome and nuclear gene data dovetail beautifully, as do the maximum-likelihood and Bayesian methods for each dataset, so there’s every reason to feel confidant about this arrangement of species.
There are a number of interesting results here: Aulorhynchidae, the family that includes the tubesnout, turns out to be paraphyletic – perhaps the Aulorhynchidae should be folded into the family Gasterosteidae and considered proper sticklebacks?
The thing I find the most interesting is the phylogenetic position of Spinachia spinachia, an elongated stickleback similar in appearance to the tubesnout. The paper suggests that perhaps Spinachia‘s elongate form is the result of convergent evolution.
It’s also worth thinking about the geographical distribution of stickleback in the context of this phylogeny: Spinachia and Apeltes, two Atlantic Ocean-only species, are grouped together, while the most basal stickleback relatives are all found in the North Pacific.
There are some interesting future directions possible here as well. One of Tom’s specialties is using fossil data to calibrate phylogenies, so it’s likely we’ll see a phylogeny in the near future that gives us an idea of the timescales of major stickleback divergence events.
KAWAHARA, R., MIYA, M., MABUCHI, K., NEAR, T., & NISHIDA, M. (2009). Stickleback phylogenies resolved: Evidence from mitochondrial genomes and 11 nuclear genes Molecular Phylogenetics and Evolution, 50 (2), 401-404 DOI: 10.1016/j.ympev.2008.10.014
Today’s Stickleblog deals with a recent paper in the journal Nature by Luke Harmon(a contributor on the blog Dechronization – check it out!), Dolph Schluter, and a number of other folks.
The paper features the threespine stickleback species pairs, which have become a famous evolutionary model system in the last several decades. In a few British Columbia lakes, you can find not one but two different kinds of stickleback – a small slim “limnetic” form that eats zooplankton in open areas of the lake, and a large deep-bodied “benthic” form that eats small invertebrates on the lake bottom.
A lot of work has already been done on the stickleback species pairs, but Harmon and the others took things in a new direction and examined whether these two specialized sticklebacks could affect the lake environment itself — in other words, are sticklebacks ecosystem engineers?
To answer the question, the researchers set up large outdoor tanks using sediment and small invertebrates from an actual stickleback lake. Then, they added fish: one set of tanks received only the limnetic, another received only the benthic, another received both limnetic and benthic, and the last set received generalist sticklebacks from a single-species lake.
The type of sticklebacks added to the tank had an effect on the invertebrate community – if the tank had limnetic sticklebacks(or limnetic+benthic), there were far fewer calanoid copepods. There were also large differences between the generalist stickleback tanks and the species-pair tanks in primary production.
The most striking finding was that sticklebacks had an effect on the clarity of water; generalist sticklebacks had significant more transparent water than any of the other treatments, and species-pair treatments had the least clear water.
A lot more work will be required to uncover exactly how the sticklebacks are producing these effects, but it seems that the difference between one generalist stickleback and an adaptive radiation of two specialist sticklebacks can have important consequences for the habitats they live in.
Harmon, L., Matthews, B., Des Roches, S., Chase, J., Shurin, J., & Schluter, D. (2009). Evolutionary diversification in stickleback affects ecosystem functioning Nature, 458 (7242), 1167-1170 DOI: 10.1038/nature07974