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January 29, 2010

Rafting

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Apart from sea turtles, the only wild animals I saw on Heron Island were birds. Lots and lots of birds. They made so much noise, I needed earplugs to sleep at night.
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But I can't really complain; they were there first. In addition to birds and bats, many islands were colonized by plants and nonflying animals, often long before humans evolved.

How did they get there? When enough water was tied up in glaciers, sea levels were lower, so some of today's islands were connected to the mainland. Darwin showed that plant seeds can be carried in mud on the feet of birds. Coconuts can migrate by floating in ocean currents, although there are other hypotheses to explain their presence on particular islands. But what about animals that can't fly ? Even if they can swim short distances, what about sharks, like those below (seen from the beach at Heron Island)?
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Small animals may have floated to even distant islands on driftwood "rafts." That only works if currents flow from the mainland to the island, however. CurrentlyAt present, currents flow from Madagascar to the African mainland. In Mammalian biodiversity on Madagascar controlled by ocean currents, published in Nature, Jason Ali and Matthew Huber argue that Madagascar was previously reachable by currents from Africa. Since then, continental drift moved Madagascar into its current position, cutting off the supply of rafting animals from Africa.

Rafts can be made of ice, rather than trees. An earlier paper, Frequent Long-Distance Plant Colonization in the Changing Arctic used DNA analysis to show that plants reached Svalbard from multiple mainland sources. Surviving there seemed to be a bigger problem than getting there. (Thanks to Odd Arne Rognli for calling my attention to this paper.)

But I'm still impressed by aborigines reaching Australia 40,000 or more years ago and by monkeys reaching the New World perhaps 40 million years ago.

January 22, 2010

Evolution threatens Darwin

Dengue ("breakbone") fever was much in the news last month in Australia last month, when I was there for the Applied Evolution Summit. The mosquitoes that spread this deadly disease are currently found in Queensland, where we were, but not further west, in the city of Darwin. They were found in Darwin in the early 1900s, however. Could some combination of climate change, human activity, and evolution put Darwin at risk once again? That was one of the questions discussed by Ary Hoffmann at the summit, largely based on a paper titled "Integrating biophysical models and evolutionary theory to predict climatic impacts on species' ranges: the dengue mosquito Aedes aegypti in Australia", published in Functional Ecology last year.

The mosquitoes typically breed in buckets and rainwater-storage tanks -- reduced use of these tanks may explain the retreat of dengue mosquitoes from Darwin -- so they modeled water-temperature cycles in those containers as a function of climate. They also modeled possible evolutionary changes in two key traits: cold tolerance and drying tolerance of eggs.

Climate change alone was not predicted to allow dengue mosquitoes to reach Darwin anytime soon. Evolution alone could threaten Darwin with dengue within 24 years. A combination of climate change and evolution was predicted to shorten the dengue-free period to 19 years.

Indirect effects of climate change, via changed human behavior, could also be important. In particular, decreases in rainfall, whose direct effects might limit mosquito survival, could trigger increased use of water storage tanks. This would provide renewed opportunities for dengue-transmitting mosquitoes to spread to Darwin and elsewhere.

January 17, 2010

Altruistic punishment by fig trees?

As I was getting ready to write about some of the talks at the Applied Evolution Summit, I received a very interesting paper: Host sanctions and pollinator cheating in the fig tree - fig wasp mutualism, which was recently published in Proceedings of the Royal Society by Charlotte Jander and Allen Herre.

Fig-tree fruits are lined with many little flowers. Female wasps crawl inside to lay their eggs, often carrying pollen from the fig where they, themselves, hatched from an egg. Different fig species host different wasp species. Some wasp species are like many other pollinators, carrying pollen only by accident; fig trees pollinated by these species have to make lots of pollen. Other figs are pollinated by wasps that actively collect pollen and actively pollinate flowers inside fig fruits; these fig species can make less pollen, which frees resources to make more seeds.

But there is presumably some cost to the wasps of transporting pollen. Why not save this cost, travel light, and lay eggs in a fig fruit without pollinating its flowers? This is essentially the same question people in my lab have asked about rhizobia, the bacteria that provide legume plants with nitrogen: once rhizobia have reproduced inside a root nodule, why stick around and invest resources in pulling nitrogen out of the atmosphere and converting it to a form the plant can use?

The questions are similar and so are the answers....
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Wasp inside a fig; photo by Charlotte Jander.

In each case, as shown in our previous work and in this week's paper, the host plant imposes "sanctions" that reduce the reproductive success of partners (wasps or rhizobia) that fail to keep their end of the bargain.

Through painstaking work described in the paper, Jander kept a bunch of wasps from acquiring pollen. (Similarly, Toby Kiers and Bob Rousseau, in my lab, kept rhizobia from acquiring nitrogen, by flushing root nodules with a nitrogen-free, argon-oxygen atmosphere.) She let wasps with or without pollen lay eggs in figs and compared their reproductive success.

When the fig species Ficus citrifolia was pollinated by a pollen-free wasp, a very large fraction of the fruit were aborted by the tree, killing any wasp eggs inside. Even when fruit weren't aborted, very few of the eggs laid by pollen-free wasps hatched into adult wasps. So this species imposed very strong sanctions against what they called "cheaters." (I accept the term, assuming that there is some cost to the wasps from carrying pollen or distributing it, which seems likely but may not have been demonstrated yet.)

Passively pollinated fig species didn't impose such sanctions, whereas other actively pollinated fig species imposed weaker sanctions. In these figs, that is, the reproductive success of wasps without pollen wasn't reduced as much, relative to those with pollen. Across the five actively pollinated fig species compared, there was a strong negative correlation between the frequency of pollen-free "cheater" wasps and fig-tree sanction strength.

So, which was cause and which was effect? I wouldn't expect cheaters being rarer to select for stronger sanctions, so presumably the effect acted in the opposite direction: strong sanctions by figs imposed greater selection against cheating by wasps. For this to be an evolutionary effect, the tendency not to carry pollen would have to be at least somewhat heritable. Under this hypothesis, cheaters arise occasionally in all actively pollinating wasp species, via mutations that change wasp behavior or perhaps physical traits that affect the ability to carry pollen. In species where the host tree imposes strong sanctions, the cheaters quickly die out, while they may persist (at low to moderate levels) in species subject to less-severe sanctions.

But that raises the question hinted at in my title for this week's post: if cheaters are so rare among the pollinators of F. citrifolia, why has natural selection maintained such strong sanctions? Sure, the evolutionary effects of host sanctions benefit this fig species as a whole, by preventing cheaters from becoming more common over generations. But natural selection usually depends on benefits to individuals or close relatives, not to species as a whole. Are F. citrifolia trees more likely to be found near close relatives than other fig species are? If so, then a fig-tree gene for punishing cheaters could, through its effects on the evolution of wasps, benefit other copies of that gene in related fig trees nearby. Of course, a tendency of related trees to cluster together also makes them more likely to compete with each other, which can undermine this effect.

The whole paper is well worth reading, for additional details (e.g., on differences among species in the extent to which cheaters may escape sanctions by sharing a fig with pollinators), for perspective on the challenges of field research (some data were lost when a tree fell down), and for the way in which different statistical tests were compared to see if they led to the same conclusions. All in all, this is one of the most interesting papers I've read in the past year.

Another cool story on dealing with difficult pollinators was discussed recently in Not Exactly Rocket Science.

January 2, 2010

Travel tips: free WiFi etc. in Brisbane

This entry was written in Brisbane, en-route to the Applied Evolution Summit. My train north, The Spirit of the Outback, was stranded in the outback, due to flooding. Fortunately, since I needed to catch the boat to Heron Island tomorrow morning, Rail Australia added a special high-speed train -- made me feel like Professor Moriarty!. Meanwhile, I had most of the day free in beautiful Brisbane. So here are some travel tips for anyone thinking about visiting the area.
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1) For free WiFi, try the State Library (above right). They leave it on even when the library itself is closed, and there's a nice interior courtyard out of the sun and rain -- we've had a good mix of both this week -- with stone benches full of students and visitors using their laptops to access email, etc. The library is near the up-stream end of South Bank park, just across the river from the central business district (CBD). The park itself is wonderful, with several museums, a swimming area near but not in the river, ice cream (we liked New Zealand Natural) and more substantial food, lots of places to sit and relax.

2) Brisbane is very pedestrian-friendly. South Bank park is connected to the CBD by two pedestrian and bike bridges (both with sheltered places to sit, one with drinking-water fountains), plus one shared with cars. We paid $23 each for 1-week transit passes which are valid on ferrys across the river, City Cats boats up and down the river, buses (including the one we took to the great botanic garden, planetarium, and look-out on Mt. Coot-tha), and even on the section of the airport train between South Bank and the Roma Street station where I'll catch my train tonight. The Roma Street Parkland, right by the station, is great, too. Lots of interesting plants (bottle-trees, Banksia), birds (including Australian wood ducks and very colorful and talkative lorikeets), and iguana-like water dragons.

3) Consider staying on Kangaroo Point. In addition to buses and ferrys to the CBD, there were steps down to the river a few blocks from the Paramount Motel, where we stayed (more like a short-term apartment rental, with full kitchen) and then a nice walk to South Bank Air Train station and other South Bank attractions. There was a well-stocked 24-hour store nearby and a full grocery store within about 1 km.

4) Take a trip with Bushwacker Ecotours. Our trip to Springbrook National Park was great. Our guide, Megan, was very knowledgeable and entertaining. She spotted the trap-door spiders and glow-worms in earth banks along the trail, told us how male bush turkeys (which we saw) make compost piles to keep their eggs warm, pointed out tree-scars made by sugar gliders, etc. A high point for me was seeing Casuarina, a nitrogen-fixing tree that looks like a conifer, is more closely related to beeches than to the nitrogen-fixing legumes I study and hosts different nitrogen-fixing symbionts in its root nodules, and has separate male and female plants. For those less-interested in natural history (not readers of this blog, surely, but perhaps your partner?) the walks have lots of beautiful waterfalls and a chance to hand-feed wild (well, unconfined) parrots. Be advised that the climb up out of the valley is equivalent to climbing several flights of stairs with a bit of walking between flights.
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I should probably mention that restaurant meals here cost 1.5-2x what they would in the US. But the botanic gardens and parks are all free, so it hasn't been too expensive, apart from airfare.

I'll write something about the Applied Evolution Summit soon.