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December 18, 2009

Tradeoff-free longevity?

I'm working on my talk for the Applied Evolution Summit, so don't have time to write a detailed post, but here are some papers that looked interesting, with brief comments on some of them:

Amino-acid imbalance explains extension of lifespan by dietary restriction in Drosophila
(published in Nature by Richard Grandison, Matthew Piper & Linda Partridge)
Dietary restriction reduces reproduction and increases longevity in many species. This study, using fruit-flies, showed that adding the amino acid, methionine, to a restricted diet restored total lifetime reproduction to that of fully-fed flies, but with the greater longevity of restricted-diet flies. Extrapolating to humans, the paper suggests that

the benefits of dietary restriction for health and lifespan may be obtained without impaired fecundity

But, if there would be no reproductive cost to doing so, why haven't flies evolved the ability to discard the "extra" food they get when fully-fed -- except for the methionine -- and live longer? I suspect that the restricted-plus-methionine diet affects the timing of reproduction, but data on timing weren't reported. (Instead, they give an "index of lifetime fecundity.") If overall population size is increasing (as fully-fed flies might expect), individuals that reproduce earlier make a disproportionate contribution to the gene pool. So the evolutionary trade-off may be between longevity and earliness of reproduction, not total reproduction. If population is decreasing, however, individuals who delay reproduction make a larger contribution to the gene pool, as laid out in our "shrinking pool" hypothesis. My guess is that flies respond to the restricted-plus-methionine diet as a cue predicting population decline and reproduce later, thereby gaining the observed increase in longevity. Extrapolating to humans again, we might be able to develop diets or other treatments that increase life-span and health, but which cost us teenage pregnancy. Hmmm... might be worth it.

Click "aging" at right for other posts relevant to this topic.

Regulating Alternative Lifestyles in Entomopathogenic Bacteria

Mozambican Grass Seed Consumption During the Middle Stone Age
If our ancestors were eating grass seeds 100,000 years ago, as this paper seems to show, what kind of selection, inadvertent or perhaps deliberate, were they imposing on those grasses?

Phylogeographic reconstruction of a bacterial species with high levels of lateral gene transfer

On the Origin of Species by Natural and Sexual Selection

Coots use hatch order to learn to recognize and reject conspecific brood parasitic chicks
"When experimentally provided with the wrong reference chicks, coots can be induced to discriminate against their own offspring"

Increasing phylogenetic resolution at low taxonomic levels using massively parallel sequencing of chloroplast genomes

Have giant lobelias evolved several times independently? Life form shifts and historical biogeography of the cosmopolitan and highly diverse subfamily Lobelioideae (Campanulaceae)
DNA analysis suggests that giant Lobelias evolved once and then spread, even to remote places like Hawaii, rather than evolving separately in different locations.

December 11, 2009

Delaying reproduction: the "disposable germline" hypothesis

This week's paper sheds new light on trade-offs between longevity and reproduction in Caenorhabditis elegans. This nematode worm is popular with medical researchers because it has a simple nervous system, insulin-like hormones, etc., yet it can be grown in Petri dishes, where it can mature and start reproducing two days after hatching from a tiny egg.

At various stages during this maturation process, C. elegans can essentially put life on hold. Environmental cues, particularly food shortage, switch them off their normal developmental pathway onto a side-track, where they can survive for months without food, but without maturing. Once food returns, they resume development.

This week's paper reports a new kind of developmental delay in individuals on the verge of reproductive maturity. "Starvation Protects Germline Stem Cells and Extends Reproductive Longevity in C. elegans" was published in Science, by Giana Angelo and Marc R. van Glist, working at the Fred Hutchison Cancer Resarch Center, in Seattle.

When worms with developing eggs are starved just before reproducing, some of them "die in childbirth", as the eggs hatch inside their bodies and emerge. Others, however, delay reproduction. Most of the developing germ cells apparently end up getting digested "for fuel", rather than becoming eggs. The adults can then survive for a month or more. Once fed, they resume reproduction, although they then lay fewer eggs than if they hadn't been starved. So an individual who would have started to lay eggs on day 2 can start on day 30 instead, a 15-fold delay in reproductive maturity.

Why do they do this? The authors have identified some genes that help control this process. But evolutionary biologists ask a different kind of "why" question: why have genes for delaying reproduction under starvation displaced genes for reproducing at the usual age?

A common answer is that they are waiting for "better conditions." But better how? Maybe they can produce more eggs if they wait until there's more food. But the relative success of different genes depends on the timing of reproduction, not just the number of offspring. An individual that produces a few eggs early might have lots of great-grandchildren by the time an individual who delayed reproduction started to lay eggs. A key point is that evolutionary changes in the genetic composition of populations depend on the relative performance of individuals with different genes. Maybe there will be more food later, but there will be more food later for everyone: for individuals who delayed reproduction and for the descendants of those who didn't.

It turns out that the one "better condition" that really makes it worthwhile to delay reproduction is a decrease in overall population size. -- not the increased resources per individual that you might get with lower population, but lower population itself. This is because each offspring added to a smaller gene pool will have a disproportionately large effect on the composition of future generations. As we put it in a recent paper, "When Stress Predicts a Shrinking Gene Pool, Trading Early Reproduction for Longevity Can Increase Fitness, Even with Lower Fecundity."

Under our "smaller pond" hypothesis, starvation provides worms with information, specifically, information predicting a decreasing population. That makes delaying reproduction a promising strategy. Even if the worm ends up laying fewer eggs (which isn't necessarily the case, depending on the direct effects of food supply on egg production), they will join a smaller gene pool. Individuals delaying reproduction will therefore be over-represented in future generations.

Food supply isn't the only factor predicting whether population will increase or decrease. If it's crowded, even a large food supply may not last long. Nematodes have previously been shown to detect the degree of crowding, essentially by smelling each other. Crowding can contribute to delays in maturation earlier in life. This week's paper shows that this is also true for adults that delay reproduction. If starved individuals are removed from the crowd, they resume reproduction, even without food. If there are few other worms around -- they probably can't tell the difference between "few" and "none" -- this is their big chance to found a dynasty.

The authors propose a "disposable germline" hypothesis. This is an allusion to Kirkwood's discredited "disposable soma" hypothesis, which attempted to explain trade-offs between reproduction and longevity as the result of limited resources: not enough calories to reproduce and also maintain healthy bodies. Although the "disposable soma" hypothesis has been cited hundreds of times, it hasn't been quite the same since people discovered that starvation increases longevity. To explain this result under the "disposable soma" hypothesis, you would have to assume that starving individuals save so many calories by not reproducing that they actually have more calories available for maintenance than if they had all the food they could eat.

December 1, 2009

Wildlife and plants near Brisbane?

My wife and I will spend a few days in Brisbane between Christmas and New Years, before I go to the Applied Evolution Summit. Any suggestions for places to visit nearby? We like seeing new species of plants and animals.

Better ant fungus farming through chemistry

Leaf-cutter ants feed the leaves to fungi and eat the fungi. Another fungus can parasitize their crop. A few years ago, it was reported that bacteria living on the ants' bodies make antifungal compounds that kill the parasite.

I wondered about this: wouldn't a bacterium that invests resources in antifungal production grow more slowly than a mutant that avoids this costly investment? In the long run, this might hurt ants and bacteria alike, but natural selection has no foresight. So why haven't bacterial "cheaters" that don't make antifungals displaced "altruists" that do? When yeasts (single-cell fungi) were found on the same ants, I suggested that antifungal production might benefit individual bacteria in their war with the yeasts, with activity against the parasitic fungus as a side effect. (Similarly, bacteria that make antibiotics that protect plant roots from fungi have their own selfish reasons.)

Consistent with this hypothesis, it turns out that the antifungal chemicals made by the bacteria aren't active only against the parasitic fungi, and may even harm the fungal crop. But the bacteria presumably benefit the ants more than they harm them, because the ants have specialized structures and secretions whose main function seems to be to support the bacteria. At least, this is true of some fungus-growing ant species. Other species have apparently abandoned use of these bacteria. Instead, they control harmful fungi with antibiotics they make themselves, in special glands. This is an example of a species abandoning one symbiosis (ant/bacteria) when it's no longer beneficial, while retaining a beneficial symbiosis (ant/fungus).
Black lines shows fungus-growing ant lineages that rely on antibiotics they make themselves, rather than those made by symbiotic bacteria, to control parasitic fungi that attack their fungal crop.
Source: Hermógenes Fernández-Marín, Jess K. Zimmerman, David R. Nash, Jacobus J. Boomsma and William T. Wcislo (2009) Reduced biological control and enhanced chemical pest management in the evolution of fungus farming in ants. Proceedings of the Royal Society B 276:2263-2269.