Category Archives: science

Why aren’t there more sickle-cell anemics in the Mediterranean?

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This post was chosen as an Editor's Selection for ResearchBlogging.orgThe story of sickle-cell anemia and its malaria-protective effects is a textbook case how environmental context determines the fitness of a given genetic profile. However, the evolution of human blood disorders in response to selection from malaria parasites might be more complicated than that textbook story.



Malaria-causing parasites (dark-stained) among human red blood cells (top), and “sickled” red blood cells (bottom). Photos via WikiMedia Commons.

Malaria is caused by mosquito-spread parasites that attack their hosts’ oxygen-bearing red blood cells. A particular mutation in the gene that codes for part of the hemoglobin molecule – the molecule that actually stores oxygen inside red blood cells – leads to deformed, sickle-shaped, blood cells. People who carry two copies of the sickle cell gene develop sickle-cell disease, in which the sickle-shaped cells reduce oxygen transport efficiency and interfere with blood circulation. People with only one copy of the sickle-cell gene are healthy, and better able to resist malaria infection than those with no copies. The textbook story is that, in regions where malaria is common, such as sub-Saharan Africa, the advantage of malaria resistance is enough to offset the fitness risk of carrying the sickle-cell gene – that one-fourth of children born to parents who each have one copy of the gene will themselves have two copies and develop sickle-cell disease.

However, there are regions like the Mediterranean where malaria has historically been prevalent, but in which the human population hasn’t evolved the higher frequency of sickle-cell genes that you’d expect from the scenario outlined above. A new paper in PNAS demonstrates that this may be because of interactions between the sickle-cell gene and another genetic blood disorder, thalassemia [$a].

Thalassemia is a class of genetic disorders affecting the protein subunits that comprise hemoglobin. Each hemoglobin molecule is formed by binding together two “alpha”-type subunits, and two “beta”-type subunits. If there is a shortage of correctly-formed subunits of either type, then hemoglobin formation is impaired, resulting in anemia or (if the mutation stops subunit production altogether) death. However, like sickle-cell genes, thalassemic mutations can confer resistance to malaria; and if alpha-thalassemia is paired with beta-thalassemia, the reduced production of both subunits can balance out.

As it happens, in combination with alpha-thalassemia, the sickle-cell gene’s malaria protection is neutralized. Using population genetic models, the new study’s authors show that this effect may have actively prevented the sickle-cell gene from establishing in the Mediterranean, where alpha- and beta-thalassemias are more common than in Africa. In the Mediterranean, the presence of beta-thalassemia genes reduces the fitness cost of (mild) alpha-thalassemia genes; and in the presence of alpha-thalassemia genes, the sickle-cell gene confers no protection to people with one copy but still induces sickle-cell disease in people with two copies.

These interactions between genes are called epistasis, and they can have dramatic impacts on evolution. Although I haven’t seen many cases as well-characterized as this one, epistasis is probably widespread in the complex systems of genomes, where thousands of regulatory and protein-coding genes interact to build living things.

References

Penman, B., Pybus, O., Weatherall, D., & Gupta, S. (2009). Epistatic interactions between genetic disorders of hemoglobin can explain why the sickle-cell gene is uncommon in the Mediterranean Proc. Nat. Acad. Sci. USA, 106 (50), 21242-6 DOI: 10.1073/pnas.0910840106

A quantum leap in ethical-eating nonsense

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I don’t think this piece in the New York Times is meant to be sarcastic. If it isn’t, it’s the most ridiculous thing I’ve ever read w/r/t the ethics of vegetarianism v. omnivory:

But before we cede the entire moral penthouse to “committed vegetarians” and “strong ethical vegans,” we might consider that plants no more aspire to being stir-fried in a wok than a hog aspires to being peppercorn-studded in my Christmas clay pot. This is not meant as a trite argument or a chuckled aside. Plants are lively and seek to keep it that way. The more that scientists learn about the complexity of plants — their keen sensitivity to the environment, the speed with which they react to changes in the environment, and the extraordinary number of tricks that plants will rally to fight off attackers and solicit help from afar — the more impressed researchers become, and the less easily we can dismiss plants as so much fiberfill backdrop, passive sunlight collectors on which deer, antelope and vegans can conveniently graze. It’s time for a green revolution, a reseeding of our stubborn animal minds. [Emphases mine.]

It’s the pathetic fallacy masquerading as a serious argument.

Science blogging, elephantine brains edition

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Caution: convergent evolution. Photo by Adam Foster | Codefor.

For the week before your midwinter holiday of choice.

  • Bacteria can make plastics. (Lab Rat)
  • Genes regulating brain development show signs of convergent evolution in humans and elephants. (Ecographica) Which discovery matches nicely with
  • A type of neuron once thought unique to great apes has evolved independently in cetaceans and elephants. (Neurocritic)
  • Hollow artificial red blood cells could be used in transfusions, or to deliver drugs. (io9)
  • Good news: lobsters can check the climate-change-driven spread of the long-spined sea urchin. Bad news: lobsters are delicious. (Conservation Maven)
  • Two competing strains of parasites are less damaging to a host than one. (The EEB & flow)

Cuckholding crows don’t necessarily have healthier chicks

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ResearchBlogging.orgBirds are bad at monogamy. There are a number of good evolutionary reasons to cheat on your mate, and it’s not clear which one is the most likely explanation. A new study of American crows, however, suggests that, for females, cheating isn’t necessarily the best choice [$-a].

Avian infidelity isn’t obvious, because many birds are socially monogamous, forming couples for one or more breeding seasons to raise chicks. However, DNA-based paternity testing has overturned this intuition — a 2002 review of such studies [PDF] estimated that “cheating” occurs in 90% of bird species, and an average of 11% of chicks are “illegitimate.”

The biological term for this non-monogamy is “extrapair copulation,” often abbreviated to EPC. Evolutionary reasons for EPC behavior break down by which parent benefits from the cuckoldry: Females benefit if EPC means their chicks will be less inbred, which can make them less prone to disease or recessive genetic disorders. Males benefit if EPC means they will have more chicks than they would otherwise. Perhaps more importantly, EPC might impose real costs on females, if it leads mated males to invest less in caring for the chicks in their nests because they can’t be sure the chicks are theirs [PDF].


Crows in flight. Photo by wolfpix.

In the new study, Townsend et al. evaluated the costs and benefits of EPC for female American crows, which have a social structure that adds a twist to the cost-benefit analysis. Mated pairs of crows live in larger family groups, which include “auxiliary,” unmated males who may help feed and protect chicks — perhaps especially if those chicks are the result of their own EPC. Females also engaged in EPC with males from outside the family group, who should be less closely related than within-group males, and whose chicks would be more genetically healthy than those sired by any within-group male, mated or not.

Townsend et al. observed several such family groups over four years, using DNA fingerprinting methods to identify the parents of chicks as they were born, and tracking the chicks’ health and survival as well as how frequently mated crows and auxiliary males tended them. Contrary to what might have been expected, chicks produced by EPC were more, not less, inbred; they didn’t grow faster or have a higher probability of survival than chicks produced by mated parents. On the other hand, cuckholded males tended chicks sired by others as often as they did their own.

The most telling result is that broods containing chicks produced by EPC were more frequently tended by auxiliary males — but only when the EPC was with a within-group male. This suggests that EPC mainly benefits male crows, not females. From a mated female’s perspective, EPC produces chicks that are less genetically fit, and no more or less likely to survive, than chicks sired by her mate. On the other hand, an unmated male can only have offspring through EPC, and if he does, it makes sense for him to give them extra assistance. Males from outside the family group don’t stick around to offer that help, but auxiliary males from within the group can, and do.

References

Arnqvist, G., & Kirkpatrick, M. (2005). The evolution of infidelity in socially monogamous passerines: The strength of direct and indirect selection on extrapair copulation behavior in females. The American Naturalist, 165 (s5) DOI: 10.1086/429350

Griffith, S.C., Owens, I.P.F., & Thuman, K.A. (2002). Extrapair paternity in birds: A review of interspecific variation and adaptive function. Molecular Ecology (11), 2195-212 : 10.1046/j.1365-294X.2002.01613.x

Townsend, A., Clark, A., & McGowan, K. (2010). Direct benefits and genetic costs of extrapair paternity for female American Crows (Corvus brachyrhynchos). The American Naturalist, 175 (1) DOI: 10.1086/648553

This week in science blogging

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Photo by MonsieurJC.

Science blog reading from the interstices of Dead Week:

  • The EEB & flow: “Green” product labels break out into categories that parallel competing schools of thought among conservationists.
  • Conservation Maven: Birds provide approximately $310/hectare in pest-removal services on coffee plantations.
  • Conservation Magazine: A hunting ban in Ontario’s Algonquin Provincial Park has allowed the local wolf population to re-establish its former pack structure.
  • Neuroskeptic: Women dosed with testosterone are more likely to treat others fairly than those given a placebo, but less likely to play fair if they think they’ve received testosterone.
  • Cognitive Daily: University students’ “gaydar” is as much as 70% accurate.

Picky eating, not genetics, splits leaf beetles

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This post was chosen as an Editor's Selection for ResearchBlogging.orgMany different factors can conspire to create reproductive isolation between populations and, ultimately, separate species. Disentangling them is often tricky, but a study recently published in PNAS takes a crack, and demonstrates that two populations of leaf beetles are divided by food preferences, not genetics [$-a]




Neochlamisus larva, and two possible food plants, red maple and willow. Photos by Scott Justis/BugGuide.net, Mary Keim, and John Tann.

Some populations of the leaf beetle Neochlamisus bebbianae eat red maple, and others eat willow; each type grows better on their native host plant. Hybrids between the two species are possible, and they don’t grow as rapidly when raised on either host. This might mean that ecology — adaptation to the different host plants — is creating reproductive isolation between the two forms of Neochlamisus. But it might also mean that the two forms are genetically incompatible.

Many species are separated by intrinsic genetic incompatibility. In these cases, hybrids have reduced fitness, or die outright, because the two species have evolved separately in such a way that mixed genomes cannot produce important proteins correctly. One example was recently found in two lines of the wildflower Arabidopsis thaliana — both lines had duplicate copies of an important gene, and in each line a different copy mutated into non-functionality, so some hybrids between the two lacked any functional copies [$-a].

To differentiate between this kind of genetic incompatibility and ecological isolation, coauthors Egan and Funk conducted not one but two generations of hybridization between maple and willow Neochlamisus populations. In the first (F1) generation, they bred parents from each host-specialized type; but in the second they performed a “backcross,” breeding the F1 hybrids with mates from one or the other of the parental populations.

This produced a population of backcrossed hybrids with 3/4 of their genes from one parental type, and 1/4 from the other. If intrinsic incompatibility separated the types, then these backcrossed hybrids would grow poorly no matter what their host plant. However, if adaptation to separate host plants isolates the types, then backcrossed hybrids would perform better on the host plant of the type with which they shared more genes. This is what Egan and Funk found — backcrossed hybrid larvae grew faster on maple if they shared more genes with maple-type Neochlamisus, and similarly for willow.

References

Bikard, D., Patel, D., Le Mette, C., Giorgi, V., Camilleri, C., Bennett, M., & Loudet, O. (2009). Divergent evolution of duplicate genes leads to genetic incompatibilities within A. thaliana Science, 323 (5914), 623-6 DOI: 10.1126/science.1165917

Egan, S., & Funk, D. (2009). Ecologically dependent postmating isolation between sympatric host forms of Neochlamisus bebbianae leaf beetles Proc. Nat. Acad. Sci. USA, 106 (46), 19426-31 DOI: 10.1073/pnas.0909424106

This week in science blogging

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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

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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?

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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

“The Origin,” 150 years old today

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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.