Posts Tagged ‘speciation’

I suppose it would have made more sense to have written this before explaining stickleback parental care, but it took me a while to get a reasonably good video of courtship behaviour.

Stickleback courtship has been studied since the 1930s, when Niko Tinbergen began investigating it. That is, the male’s role has been studied since then; it wasn’t til the 80s or so that people properly characterized female behaviour.

Here’s a quick overview: The males, as I’ve explained before, build nests. They also develop a red throat and belly, a bright blue iris, and, depending on the population, a blue body.

When a male spots a gravid female, he initiates courtship by either approaching her directly or zigzagging towards her. This zigzagging is pretty obvious in the video I’ve included below.

The female will respond with a “head-up”, a posture that shows off her egg-filled belly. The male may then nip at the female or swim below her and prick at her underside with his dorsal spines. (Interestingly, limnetic males do more zigzagging, while benthic males do more biting.) Then he’ll try to lead her back to the nest, following a meandering path. He pokes at the entrance to the nest with his snout, and then the female, assuming she’s still interested, will check out the nest too. If she decides to spawn, she swims into the nest and deposits her eggs, which takes about a minute. After she swims out the male swims through to fertilize them.

I made this video of a limnetic male courting a female in a jar (that’s how I get them to nest), which for obvious reasons can only show a few aspects of courtship behaviour. The nest is right in the middle of the frame (there are bits of plants around it).

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(For a refresher on stickleback and what I’m doing to them, read this.)

Here is what stickleback nesting behaviour looks like:

This is a benthic male (in the “wrong” habitat, you’ll notice) tending a nest that has eggs in it. He fans them to circulate oxygenated water over them, he touches up the outside of the nest, and he rubs his body along it to deposit spiggin, which holds it together. Sometimes he’ll also swim through the nest, which gets spiggin on the inside as well. I haven’t managed to get a video of this yet.

The eggs in this nest were freshly laid, so this male is still in courtship mode, and would be a for a few days—he will try to mate with multiple females until the eggs are old enough and he switches into parental mode.

Here’s a closer look at the nest, which, yes, I destroyed after the experimental trial. The top matches the pond bottom well:

If I flip it over, it’s apparent that it’s composed of all sorts of odds and ends—a stray bit of Chara, many twigs and pine needles, soggy straw:

The eggs are laid in a tunnel through the middle of this mess. Each is maybe two millimetres across, and there could be a few dozen to a hundred in a clutch.

In the wild, after they hatch, stickleback fry sit on the bottom for a week with their father guarding them. Then they’ll fill their swim bladders with air and become able to control their buoyancy. They’ll stay in a little shoal around the nest for up to a few weeks under the watchful eyes of stickledad. Later they’ll mingle with juveniles from other families as they feed and grow, while their father, if he’s survived, will try to nest again.

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It’s high time I explained what exactly I’m doing chest-deep in a pond every morning.

My life’s focus these days is a little fish called the threespine stickleback. They’ve been studied systematically for many years, starting with Niko Tinbergen’s work on their territorial behaviour. Today they are considered an emerging model system: there are extensive, well-established lab resources (including a sequenced genome) for studying them, and research on these fish is used to answer more general questions in biology, particularly evolution.

I am tempted to say that the stickleback is more than just a fish. This is not just me being flippant; they are actually more than one species. The large saltwater stickleback population that stretches around the northern hemisphere is subdivided into clusters that don’t interbreed (i.e. species), and marine stickleback have repeatedly colonized lakes and streams, evolving into new species as they did so. It is these repeated, parallel, and most importantly recent—often they occurred less than 12,000 years ago as the last ice sheets retreated—speciation events, and their accompanying changes in morphology and behaviour, that have made sticklebacks into such an important study organism for evolutionary biology.

It’s two of the lake species that my research at the ponds focusses on. In five watersheds that we know of, the sticklebacks that colonized freshwater became not one but two species. One, the benthic species, lives near the lakebed and is larger; the other, the limnetic species, lives in open water and is much smaller. Since these species are so young, they can hybridize, though in the wild they rarely do. My research focuses on why they don’t; in other words, why they remain separate species.

Sticklebacks are odd fish. The males build nests, for one, and guard their eggs and later their fry. As the water warms in spring, each males stakes out a territory in the littoral zone. His throat turns bright red and his body a bright blue-green. He gathers bits of vegetation, sand, anything he can find, like a bird collecting twigs for its nest, and glues them together with a protein made in his kidneys. The nest has a tunnel through the middle; sometimes he swims through it to shape it. Here, he hopes, a female will lay her eggs.

Many people have noticed that in at least some of the “species pairs”, the benthics hide their nests under dense vegetation while the limnetics nest in plain sight on bare sand, rock, or sunken logs. My experiment this spring and summer aims to test whether this difference in habitat actually affects a female’s choice of mate. In other words, if I put a male of the “right” species on the “wrong” habitat, will a female still mate with him?

This is why I’ve been moving algae around for the past few weeks. I’ve built 1×1 metre pens in the shallow ends of two ponds. Half of them are left open while the other half has been covered with plants. I collected fish from one of the species pair lakes, and they’re being held in aquaria until I’m ready to experiment on them. I’ve now put the first two males into enclosures, and they’ve both built nests. As soon as I have some females that look like they’re ready to lay eggs, I’ll put one in each enclosure and see whether they spawn. If being on the “wrong” habitat decreases the chance that a pair spawns, then this difference in habitat plays a role in reproductive isolation, the lack of hybridization between two related species.

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ResearchBlogging.orgThe study of speciation—the formation of new species—has had a long history in evolutionary biology, but the past few decades have seen leaps in how we think about the process that creates biodiversity. We now know that natural selection is almost always heavily involved in the process, and that new species can form even when there is some ongoing hybridization between the evolving lineages.

One way to approach a recently evolved pair of species is to ask (1) why did they split in the first place and (2) how do they keep separate. (more…)

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ResearchBlogging.orgAnd now for some more research blogging! Here’s a paper comparing the roles of geography and ecology in the early stages of speciation. I’m incredibly excited about this study because it begins to get around some of the major difficulties inherent in studying reinforcement and ecological speciation. But first let me back up and explain some background. (more…)

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My first creationism-bashing post. Yippee. (more…)

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