This week in science blogging

Posted on by

I’d like to try something new and see if it sticks — a weekly post briefly noting a handful of items from the scientific blogosphere I’ve noticed in the past week. Here goes:


Photo by ehpien.
  • NeuroDojo: Chickadees grow new brain cells while caching food — but they grow more in the wild than they do in captivity.
  • Dechronization: Mitochondrial DNA is not as good for evolutionary and ecological genetics as was once thought. It doesn’t even evolve in a nice, clocklike fashion!
  • The EEB & flow: Plant communities have a lot going on underground.
  • Conservation Maven: Exotic giant tortoises seem to make pretty good replacements for extinct native giant tortoises. (Of course, it’s hard to imagine introduced giant tortoises ever running amok in a new habitat.)
  • EcoTone: Loss of top predatory fish can spur algae blooms.

Evolving antibacterial therapies

Posted on by

On Slate, Brian Palmer says we need better tactics, not better antibiotics, to combat drug-resistant bacteria. But the new “tactics” he describes are, basically, new drugs:

In vitro studies have shown that chemicals like ascorbic acid shut down the movement of antibiotic resistance between cells. (Right now it’s effective only at concentrations that a person couldn’t tolerate, but it’s a start.) Because almost all antibiotic resistance relies on genetic transfer, this technique might be the solution we’ve been seeking since the very first colony of bacteria solved penicillin in 1944.

Drugs that combat gene transfer between bacteria probably would slow the spread of new antibiotic-resistance genes. Until bacteria evolve ways to transfer genes in spite of anti-transfer drugs, that is.

A genuinely new approach to circumvent antibiotic resistance will require actually thinking about the evolutionary consequences of therapy — and creating natural selection that eliminates the damage done by bacteria without also creating a fitness advantage for resistance to the therapy. That’s tricky, to say the least, but it’s not impossible. Such an approach has been outlined to control disease-carrying mosquitoes, for instance.

For yucca moths, does (flower) size matter?

Posted on by

ResearchBlogging.orgIn a paper just released online at Molecuar Ecology ahead of publication, genetic tests on moth larvae provide the latest piece to the puzzle of why there are two kinds of Joshua tree — because the tree’s pollinators need to match its flowers [PDF].

I’ve written extensively about the interaction between Joshua tree and its pollinators. Like all yuccas, Joshua tree is pollinated only by yucca moths. Female yucca moths collect pollen in special mouthparts and deliberately apply it to a yucca flower after laying eggs inside it. When the eggs hatch, the moth larvae eat some of the seeds inside the developing fruit. Yuccas prevent their pollinators from laying too many eggs by selectively killing flowers too badly damaged by egg-laying [$-a].



TOP: The two forms of Joshua tree (western type on left, eastern on right). BOTTOM: Scaled comparison of moth body sizes and tree pistils. To lay eggs in a flower, moths must drill from near the top of the pistil to the positions marked by dotted lines. Photo by jby, Illustration from Smith et al.(2010), figure 1.

This last element of the interaction may have had significant consequences for Joshua trees’ evolutionary history. Joshua trees are pollinated by two different species of moths, which occur in different parts of the tree’s range: the larger Tegeticula synthetica in the west, and the smaller T. antithetica in the east. Joshua trees pollinated by the two different moth species are themselves different, both in their overall shape, and in the shape of their flowers’ pistils — specifically, the length of the route that a moth must drill to lay her eggs [PDF].

How does this difference in flower shape affect Joshua tree pollination? If a larger moth attempts to lay eggs in a smaller flower, it may be do more damage to the flower than the “native” pollinator would, triggering the tree to kill the flower. On the other hand, smaller T. antithetica might be able to lay eggs in a larger western-type flower without this risk. If this is the case, moths probably can’t pollinate western trees with eastern pollen, but they might be able to do the reverse.

Such one-way pollen transfer between the two Joshua tree types could produce a population genetic pattern called “chloroplast capture.” Joshua tree pollen doesn’t contain the full genetic code of the tree that produces it — it lacks the genes contained in the chloroplast, the cellular structure that conducts photosynthesis, because pollen grains typically don’t have chloroplasts. The DNA in the cellular nuclei of newly-formed seeds is a mixture of nuclear DNA (nucDNA) from a pollen grain and from one of their “maternal” parent’s ovules, but they get all their chloroplasts, and chloroplast DNA (cpDNA), from the ovule. If moths carry pollen from eastern trees to western trees, then the seeds produced would contain western cpDNA, but also some eastern nucDNA.


Asymmetric pollen transfer can lead to eastern-type trees with western-type chloroplasts. Figure 2 from Smith et al.(2010).

This is what we’ve found in Joshua tree populations near the region where the two tree types and their pollinators come into contact. At these sites, trees look like the eastern type (meaning they likely have eastern nucDNA, though we haven’t tested that yet) but have cpDNA that matches nearby populations of western-type trees [PDF].

The genetic pattern is only suggestive of one-way pollen transfer between the two Joshua tree types, though. We haven’t yet tracked the movement of moths directly, or estimated whether they actually are less successful when laying eggs on the wrong tree type. The newly-published study provides exactly these data. My colleague Chris Smith placed glue traps on Joshua tree flowers at the contact zone to estimate how often adult moths of each pollinator species visited each type of tree in the mixed population. Adult moths were more likely to be trapped on their “native” trees, though they did show up on the other type sometimes.


A yucca moth larva emerges from a Joshua tree fruit in the lab. Photo by jby.

Chris and I then collected fresh fruit from trees in the contact zone, and caught yucca moth larvae as they chewed their way out. Chris and another coauthor, Chris Drummond, then identified the species of each larva based on their genetics (the two pollinators look very similar at that stage) — and in our sample, the pattern of specificity was even stronger than that in the adults. The larger moth species, T. synthetica, never emerged from fruits of the small-flowered eastern trees. The vast majority of larvae of the smaller T. antithetica were also found inside their “native” tree’s fruit — but a handful did emerge from large-flowered western trees.

This mechanism could create the genetic pattern we see in Joshua tree populations. Larger T. synthetica doesn’t seem to lay eggs in (or pollinate) small-flowered eastern trees, but smaller T. antithetica can occasionally lay eggs in (and pollinate) large-flowered western trees. This should create asymmetric gene flow, with pollen moving from eastern trees to western trees, but not the reverse. The two Joshua tree types may not yet be reproductively isolated, separate species — but we won’t know for sure without looking at the plants’ nuclear DNA. As it happens, I’m working on that right now.

References

Godsoe, W.K.W., Yoder, J.B., Smith, C.I., & Pellmyr, O. (2008). Coevolution and divergence in the Joshua tree/yucca moth mutualism The American Naturalist, 171 (6), 816-823 DOI: 10.1086/587757

Marr, D., & Pellmyr, O. (2003). Effect of pollinator-inflicted ovule damage on floral abscission in the yucca-yucca moth mutualism: the role of mechanical and chemical factors Oecologia, 136 (2), 236-243 DOI: 10.1007/s00442-003-1279-3

Smith, C.I., Godsoe, W.K.W., Tank, S., Yoder, J.B., & Pellmyr, O. (2008). Distinguishing coevolution from covicariance in an obligate pollination mutualism: Asynchronous divergence in Joshua tree and its pollinators. Evolution, 62 (10), 2676-87 DOI: 10.1111/j.1558-5646.2008.00500.x

Smith, C.I., Drummond, C., Godsoe, W.K.W., Yoder, J.B., & Pellmyr, O. (2010). Host specificity and reproductive success of yucca moths (Tegeticula spp. Lepidoptera: Prodoxidae) mirror patterns of gene flow between host plant varieties of the Joshua tree (Yucca brevifolia: Agavaceae). Molecular Ecology DOI: 10.1111/j.1365-294X.2009.04428.x

Sins of omission

Posted on by

I can’t help but think this is going to strain Rick Warren’s warm, personal friendship with Melissa Ethridge.

Via Slog. Yesterday’s Fresh Air also had a great (by which I mean alternately appalling and rage-inducing) interview with Jeff Sharlet. My reaction: more or less as before. Also, thank God (or whoever) for Fred Clark.

“The Origin,” 150 years old today

Posted on by

Charles Darwin’s groundbreaking work, The Origin of Species, was published 24 November, 1859, 150 years ago today. This makes a rather neat bookend to the Darwin Bicentenary, the year of events commemorating the 200th anniversary of Darwin’s birth on 12 February, 1809. I’m going to be lazy and simply link to everything I wrote back concerning that earlier anniversary.

Oh, and serendipitously, today is also the anniversary of the discovery of Lucy in 1974. I saw her in person (behind glass) on a trip to Seattle during last year’s fall break, which was pretty cool.


Photo by CharlesFred.

Aphid-tending ants cull the sick from the herd

Posted on by

ResearchBlogging.orgJust released online at Biology Letters: aphid-tending ants have been observed to selectively remove sick members of their “herd” [$-a].

Most aphid species produce some sort of sweet honeydew as waste while feeding on their host plants; ant-attended aphid species use this honeydew to attract ants. In many cases, the ants “milk” the aphids by stroking them to prompt release of the honeydew. While exploiting a colony of aphids, ants defend it as a food resource, protecting the aphids from predators. Aphid species that commonly rely on ant protection often lack defensive adaptations [$-a] found on species that don’t interact with ants.


Ants tend aphids on a milkweed plant. Photo by dmills727.

Niesen et al. report the results of experiments performed ants attending colonies of milkweed aphids, Aphis asclepiadis, which are susceptible to a fungal pathogen that can wipe out aphid colonies in a matter of days. In two experiments, they introduced aphids into the ant-attended colonies, and tracked what the ants did to them. They found that

  • Ants were more likely to remove the corpses of fungus-killed aphids than either the corpses of aphids killed by freezing or introduced live aphids; and
  • Ants were more likely to remove live aphids contaminated with fungal spores (conidia) than live aphids without spores.

The authors speculate that this behavior is a re-application of ants’ treatment of their own sick and dead within the colony. It seems clear that it should have benefits to both ants and aphids in this new context, slowing or preventing the spread of the fungus within an aphid colony. This benefit isn’t directly tested by Nielsen et al., but such an experiment is a logical next step.

Reference

Nielsen, C., Agrawal, A., & Hajek, A. (2009). Ants defend aphids against lethal disease Biology Letters DOI: 10.1098/rsbl.2009.0743

Way, M. (1963). Mutualism between ants and honeydew-producing Homoptera. Ann. Rev. Entomology, 8 (1), 307-44 DOI: 10.1146/annurev.en.08.010163.001515

“Sheeeeit!”

Posted on by

In the spirit of Kh(ax)n:


Google search results “Sh(ex)it!” for x = 1 to 30. Graph by jby.

I was inspired by that highlight reel from The Wire that’s been going around the internet. (Caution: here be multiple and baroque uses of the f- and n-words, brief violence, and deep cynicism.)

And now, back to work.

The brood of vipers

Posted on by

NY Times:

Citing the Rev. Dr. Martin Luther King Jr.’s call to civil disobedience, 145 evangelical, Roman Catholic and Orthodox Christian leaders have signed a declaration saying they will not cooperate with laws that they say could be used to compel their institutions to participate in abortions, or to bless or in any way recognize same-sex couples.
…
They say they also want to speak to younger Christians who have become engaged in issues like climate change and global poverty, and who are more accepting of homosexuality than their elders. They say they want to remind them that abortion, homosexuality and religious freedom are still paramount issues.

This, of course, is on the heels of the threat by the Roman Catholic Archdiocese of Washington, D.C. to discontinue charitable work if the city council passes a law prohibiting discrimination on the basis of sexual orientation.


Photo by jikido-san.

Christianity was founded on stories of a man who preached sacrifice of self to the needs of others, went drinking with prostitutes and other social outcasts, and was executed as a common criminal by the government. The figure of Jesus as described in the Christian scriptures is, to me, an ideal I can only hope to emulate. Yet many of the people who take his name for their moral identity — and the loudest, most vehement and visible of those who do — would evidently react with disgust if they met their Lord on the street, and condemn his teachings as un-American if they actually understood them.

I wanted to write something more scathing than that. But I’m just tired. I’m going to hand the mic over to that guy they keep claiming to follow.

Woe to you, teachers of the law and Pharisees, you hypocrites! You shut the kingdom of heaven in men’s faces. You yourselves do not enter, nor will you let those enter who are trying to.
…
Woe to you, teachers of the law and Pharisees, you hypocrites! You give a tenth of your spices — mint, dill and cummin. But you have neglected the more important matters of the law — justice, mercy and faithfulness. You should have practiced the latter, without neglecting the former.
…
Woe to you, teachers of the law and Pharisees, you hypocrites! You are like whitewashed tombs, which look beautiful on the outside but on the inside are full of dead men’s bones and everything unclean. In the same way, on the outside you appear to people as righteous but on the inside you are full of hypocrisy and wickedness.
…
You snakes! You brood of vipers! How will you escape being condemned to hell?
(Matthew, chaper 23, New International Version.)

Cost of killing nest-mates offset by benefits of killing nest-mates

Posted on by

ResearchBlogging.orgAmong birds, brood parasites are the ultimate freeloaders — species like the common cuckoo and the brown-headed cowbird lay their eggs in other birds’ nests, leaving the host to raise the parasite chicks at the expense of its own. But while brood parasitism is easy on the parents, it isn’t so easy on their chicks, as a study recently published in PLoS ONE suggests.


A reed warbler feeds a common cuckoo chick. Photo from WikiMedia Commons.

A brood parasitic chick faces two challenges. The first is to avoid being recognized by its adoptive parents and ejected from the nest; the second is to win parental attention in competition with their adoptive nest-mates. The first challenge may be partially met by the evolution of eggshells that match host eggshells; and brood parasite parents may also help by keeping watch on the host nest so they can punsish hosts who eject introduced eggs. (This punishment behavior has been described as an “avian mafia [$-a].”)

In competition with their adoptive nest-mates, though, parasitic chicks are on their own. If the host’s own eggs hatch, the host has more mouths to feed and less time to spend on the parasitic chick. On the other hand, a brood parasitic mother can’t kick out the host’s eggs at the time she leaves her own egg with the host, because the host may abandon a nest that contains only a single unfamiliar-looking egg. This leaves it to freshly-hatched brood parasite chicks to do the heavy lifting involved in ejecting their host’s eggs themselves.


A common cuckoo chick pushes one of its host’s eggs out of the nest. Detail of figure 1 from Anderson et al. (2009).

Egg eviction looks like hard work — the chicks attempt it while they’re not much bigger than the eggs. Anderson et al. investigated the cost of all this adoptive-siblicidal effort by manipulating reed warbler nests that had been parasitized by common cuckoos,* taking away the hosts’ eggs in experimental nests, and comparing the growth of cuckoo chicks in those nests to that of chicks in unmanipulated nests, who had to do the evicting themselves.

They found that there is a cost to eviction effort: during the period of development when they would be doing all they could to push eggs out of the nest, cuckoo chicks grew faster when they didn’t have eggs to push. But they didn’t grow much faster, and by the time they were ready to leave the nest, the advantage had disappeared. Anderson et al. take this to mean that the cost of eviction is “recoverable” through the benefits of increased parental attention later on. I would add that it points out how important your choice of time frame can be when investigating how traits or behaviors affect organisms’ evolutionary fitness — sometimes a cost paid at one point in development is an investment toward later benefits.

——–
*The common cuckoo is the species first known to parasitize other birds’ nests, and its name is the linguistic source of the term “cuckold.”

References

Anderson, M., Moskát, C., Bán, M., Grim, T., Cassey, P., & Hauber, M. (2009). Egg eviction imposes a recoverable cost of virulence in chicks of a brood parasite. PLoS ONE, 4 (11) DOI: 10.1371/journal.pone.0007725

Hoover, J., & Robinson, S. (2007). Retaliatory mafia behavior by a parasitic cowbird favors host acceptance of parasitic eggs. Proc. Nat. Acad. Sci. USA, 104 (11), 4479-83 DOI: 10.1073/pnas.0609710104

Lahti, D. (2005). Evolution of bird eggs in the absence of cuckoo parasitism. Proceedings of the National Academy of Sciences, 102 (50), 18057-62 DOI: 10.1073/pnas.0508930102

Soler, M., Soler, J., Martinez, J., & Moller, A. (1995). Magpie host manipulation by great spotted cuckoos: Evidence for an avian mafia? Evolution, 49 (4), 770-5 DOI: 10.2307/2410329

Hey, PNAS?

Posted on by

Hey, Proceedings of the National Academy of Sciences? (Can I call you PNAS? Thanks.) You know what really drives me crazy, as a scientist and a blogger, PNAS? Internationally-recognized journals that release scientific studies to the press before they make them available online.

Why does this make me crazy? I’m glad you asked, PNAS. It makes me crazy because sometimes I read something as batshit absurd as this gem from Wired Science:

No exact rule exists for deciding when a group of animals constitutes a separate species. That question “is rarely if ever asked,” as speciation isn’t something that scientists have been fortunate enough to watch at the precise moment of divergence … [emphasis mine]

(Which statement is like claiming that physicists rarely ask about gravity.) And when I read something like this I’d really like to be able to go and see whether that’s actually in the peer-reviewed article in question, or if it just materialized in the course of the sausage-making that is popular science journalism.

But, apparently, PNAS, you’re more interested in having your results butchered by people who think biologists don’t ask questions about speciation than you are in having them read by, you know, biologists. You’ll forgive me, I hope, if I take that a little bit personally.