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September 9, 2011

This week's picks

A Gene for an Extended Phenotype "The viral gene that manipulates climbing behavior of the [Gypsy moth] host was identified"

The Foot and Ankle of Australopithecus sediba [hominin fossil from 1.78 and 1.95 million years ago] "may have practiced a unique form of bipedalism and some degree of arboreality"

Assured fitness returns in a social wasp with no worker caste "experimentally orphaned brood... continue to be provisioned by surviving adults... no evidence that naturally orphaned offspring received less food than those that still had mothers in the nest."

The sudden emergence of pathogenicity in insect-fungus symbioses threatens naive forest ecosystems "symbioses between wood-boring insects and fungi... are shifting from non-pathogenic saprotrophy in native ranges to a prolific tree-killing in invaded ranges... when several factors coincide"

Ultra-fast underwater suction traps "this unique trapping mechanism conducts suction in less than a millisecond and therefore ranks among the fastest plant movements known"

The taming of an impossible child - a standardized all-in approach to the phylogeny of Hymenoptera using public database sequences "combines some well-established programs with numerous newly developed software tools"

June 25, 2010

Are scientists smarter than squirrels?

"Monkey-watchers often use the word "aunt" for an adopting female." -- Richard Dawkins, The Selfish Gene
The willingness of animals to adopt and care for orphans has been shaped by past natural selection. Often, Dawkins suggested, adoption represents "misfiring of a built-in rule... a mistake that happens too seldom for natural selection to have 'bothered' to change the rule by making the maternal instinct more selective." This seems a reasonable explanation for the failure of bird parents to kick "brood parasites" out of their nests, a situation I discussed recently.

But this week's paper, by Jamieson Gorrell and colleagues, seems to show that squirrels have a more-sophisticated understanding of selfish-gene theory than I would have expected. "Adopting kin enhances inclusive ļ¬tness in asocial red squirrels" was recently published in the new online journal Nature Communications. The authors analyzed five cases of orphaned squirrels being adopted, all by close relatives, and two cases where they were left to die, even though a relative had a territory nearby. In each case, they asked whether adopting would likely increase or decrease the frequency of the adopter's genes in future generations.

Closely related individuals tend to share gene variants (alleles) even if those alleles are rare in the overall population, so adopting a younger sister or a nephew who would otherwise die could increase one's genetic representation in future generations. On the other hand, adding an orphan to one's litter puts one's own offspring at somewhat greater risk. The authors were able to estimate this risk and compare it to the increased survival chances of the adoptee, weighted by its relatedness to the foster mother. If this benefit exceeds the risk, then Hamilton's rule (the fundamental equation of social evolution) predicts adoption. All of the adoptions that did occur met this criterion -- two cases were right on the line -- whereas the two potential adoptions that didn't occur failed the Hamilton's-rule test. Yet another example of squirrels solving challenging problems.

At least, that's what the data seemed to show. But the "relatedness" term in Hamilton's rule isn't necessarily equal to the relatedness we could calculate from a family tree or from genetic similarity. It would be, if helping an orphan had no negative effect on anyone outside one's current litter. But if there are more red squirrels than red-squirrel territories, then a surviving orphan may end up displacing another squirrel. So the question is, how closely related is that displaced squirrel likely to be to the adoptive mother? In the cases studied, 1/4 to 1/2 of the lactating females nearby were kin to the adopting mother. If that's a representative sample, then a surviving orphan might often end up displacing another squirrel that was as closely related to the mother as the orphan was. In such cases, the mother would have exposed her own litter to increased risk, without doing much to increase her genetic representation in future generations. Even so, the adoptive mothers aren't acting as maladaptively as Dawkins suggested (as if they adopted orphans at random), but their behavior wouldn't be optimal (by Hamilton's rule) unless there were unoccupied territories available nearby. Thanks to Dr. Carin Bondar, whose blog alerted me to this interesting paper.

Meanwhile, over at Science, Jeff Smith and colleagues propose "A generalization of Hamilton's rule for the evolution of microbial cooperation." When one cooperative act (releasing an expensive enzyme, say) benefits all microbes nearby, it's common to assume we can add up all the costs and benefits over a population. But what if twice the enzyme gives three times the benefit? The authors developed some high-powered math to deal with such problems and concluded that certain kinds of cheaters would have a harder time getting established than we would have expected from the simpler version of Hamilton's rule. Scientists are definitely smarter than squirrels, but they can't jump as well.

November 30, 2007

Controlling sex ratios

I formerly thought that when a tendency to produce the two sexes in equal numbers was advantageous to the species, it would follow from natural selection, but I now see that the whole problem is so intricate that it is safer to leave its solution for the future. -- Charles Darwin in Descent of Man
This week, I will discuss recent papers that shed light on the evolution of genes that control sex ratios in insects (fruit flies) and mammals (mice). John Dennehy recently discussed Hamilton's 1967 paper, "Extraordinary Sex Ratios." (Yes, Hamilton, as in Hamilton's r.) See also the last paragraph of this post, on the surprisingly sophisticated adjustment of offspring sex ratios by fig-pollinating moths.

Continue reading "Controlling sex ratios" »

September 27, 2007

Cooperation and cheating in microbes: quorum sensing and persisters

Two papers on cooperation this week. If you were trying to help someone, but end up causing problems for them, were you being cooperative? I have no idea, so I like to study cooperation in microbes. Microbes don't have brains, so "intent" isn't a factor. And the only definition of "benefit" that makes sense is an increase in Darwinian fitness or reproductive success, which is often easy to measure in microbes; just count them.
I like these definitions:

Cooperation: a behaviour which provides a benefit to another individual (recipient), and which is selected for because of its beneficial effect on the recipient. [Exhaling CO2 isn't cooperation; it evolved as a side-effect of breathing oxygen, not to benefit plants.]
Cheaters: individuals who do not cooperate (or cooperate less than their fair share), but are potentially able to gain the benefit of others cooperating. ["Equal share" might be less ambiguous.]

Continue reading "Cooperation and cheating in microbes: quorum sensing and persisters" »

August 31, 2007

Whose genes are these, anyway?

Most of the genome of Wolbachia, a bacterial parasite of fruit flies, has been incorporated into the genome of the fruit-fly itself. Discussion at Not Exactly Rocket Science. Bacteria tend to pass genes around, or (more accurately, perhaps) bacterial genes tend to move themselves around (usually to other bacteria), but this is amazing.

August 29, 2007

Selfish sperm cells

Usually, those alleles (versions of a gene) that become more common over generations are those that are most beneficial to the organisms in whose cells they live. But not always.

The latest issue of PLoS Biology has an open-access article on a particularly selfish gene responsible for Apert syndrome in humans.

Continue reading "Selfish sperm cells" »

March 31, 2007

Can a selfish gene stop malaria?

A bird that risks her life to lead a fox away from her chicks may be influenced by a "selfish gene" (Dawkins, 1976). Genes can't think, of course. However, a gene causing behavior that risks the loss of one copy of itself (in the mother) will become more common over time, if this same behavior often saves more than one copy of itself (in the chicks). The gene can be considered "selfish", in the sense that the welfare of the mother, her species, or the whole ecosystem only indirectly affect the gene's spread. It's as if each gene were at war with rivals (other versions of the gene, or alleles) for its place on the chromosome.

The selfish gene concept is now being used to design new methods to control the spread of disease. Mosquitoes that resist infection by the malaria parasite can be made by genetic engineering. Unfortunately, the small benefit (to a mosquito) of resistance to this parasite is probably not enough for resistant mosquitoes to take over in the wild, because most of the animals they bite aren't infected. (It would be nice if the laws of nature always favored human welfare, but they don't.)

How can we make such beneficial genes spread through mosquito populations? This week's paper, "A Synthetic Maternal-Effect Selfish Genetic Element Drives Population Replacement in Drosophila" by Chun-Hong Chen and colleagues at Cal Tech and UCLA, published on-line in Science, demonstrates one interesting approach.

Continue reading "Can a selfish gene stop malaria?" »