Random Natural History: Valley Oaks and their Galls.

Ok, time for a short bit of natural history. I live in the Sacramento Valley in northern California. The dominant tree species (outside of urban areas) seems to be the Valley Oak (Quercus lobata). Now, there aren’t a whole lot of trees in the valley, so it’s pretty lucky that Valley Oaks are fairly spectacular.

Valley oak (Quercus lobata) on Joseph D Grant County Park Canada de Pala Trail

They are also little ecosystems unto themselves. The first thing most people notice about them are oak apple galls, so called because they bear a disturbing resemblance to (rotting) apples.

"Oak apple" galls of California Gall Wasps (Andricus quercuscalifornicus, Cynipidae, Hymenoptera) on Valley Oak (Quercus lobata, Fagaceae)

Trees can often be so laden with them that they actually look like cultivated apple trees. The galls are woody, though, not squishy like actual apples. What is a gall, you ask? Good question. A gall is essentially a plant tumor. In many cases (as here) galls are caused by insect parasites. An adult insect lays eggs in the tissue of a plant, and those eggs release hormones that induce the plant to form the gall. Galls can provide food and shelter for their hosts until they are ready to mate and lay new eggs. Galls can be quite complicated structures, the result of parasites evolving very refined control over their hosts over time. As a result, galling insects can frequently be identified by their galls alone. Oak apple galls are caused by a wasp, Andricus quercuscalifornicus, but are exploited by a constellation of at least 20 other arthropods that feed on the galls, A. quercuscalifornicus, and each other.

These aren’t the only galls associated with the Valley Oaks. There are at least two more. One of which is fairly bizarre and the original inspiration for this post: the California Jumping Gall. This gall is also caused by a wasp, Neuroterus saltatorius. In contrast to the oak apple galls, these galls are tiny, only about a millimeter across. What they lack in size, however, they make up for in quantity. These galls form on the undersides of oak leaves by the hundreds of thousands. When they mature, they drop off the leaves, wasp larva still inside. Once on the ground, they start “jumping”. The larvae violently fling themselves around inside the gall, presumably to try to move it into a sheltered spot where they can finish out their life cycle and emerge the following spring to lay new eggs.

The galls are dropping now in my neighborhood, and the result is that sidewalks and gutters under valley oaks appear to be full of jumping grains of sand. It’s a pretty weird sight:

Here’s a link to another video:

ARKive video - California jumping gall wasp - overview

Well, that’s all I’ve got for now. I’ll end on another photo of an amazing Valley Oak.

Quercus lobata VALLEY OAK/ROBLE

Scientists at work among the Joshua trees

When he’s not dismantling racist pseudoscience, Chris Smith studies the evolutionary ecology of species interactions. Willamette University sent along a videographer on Chris’s last field trip to study Joshua trees and the moths that pollinate them in central Nevada, and the result is now posted on Vimeo. It’s mainly geared toward showcasing how Willamette undergraduate students participate in the fieldwork, but I’d say it makes the desert look mighty good, too.

When a bad bird goes good … and then bad again.

cuckoos

Brood parasites are definitely the bullies of the avian world.  They lay their eggs in the nests of other birds, sometimes destroying the host’s own eggs or just waiting for their nestlings to do the dirty work after they hatch.  They then outcompete any surviving host nestlings for food, while the poor host parents are worked to the bone to feed the monstrous nest invader.

In spite of the steep costs of nest parasitism, most avian host species do not have effective mechanisms for detecting and removing brood parasites from their nests.  So, why don’t mama birds notice they have a GIANT intruder in their nest and carry out some infanticide of their own?  One hypothesis is that the cost of a mother bird making a mistake and pushing the wrong baby out (i.e. her own) outweighs the benefit of developing such a behavior.

This week in Science, Canestrari et al. published evidence for another hypothesis – that sometimes, it might actually be good to have your nest parasitized.

Continue reading

Living at the edge, range expansion is a losing battle with mutations

Environments can vary substantially in habitat quality, local population abundance, or carrying capacity. Under some climate change scenarios, new, higher quality habitats become available along the margin of a species’ range (e.g. higher latitudes or altitudes) (Thomas et al 2001). These new habitats may be able to support larger population sizes. Factors of demography, evolution, and qualities of the abiotic and biotic communities all interact to determine where a species is found and may influence the ability of a species to expand its range. New research is building genetically explicit models in order to understand how the interplay of these different factors influence evolutionary changes,

Wordle of Peischl et al 2013

The authors of a recent study focus on how the interaction of the demographic process of range expansion changes the way that natural selection favors beneficial and deleterious mutations (Peischl et al 2013). Using both computer simulations as well as mathematical approximations, the authors find that at the range margins, individuals carry a substantial load of deleterious mutations.

Continue reading

The cost of attracting pollinators is … attracting everyone else

Flowers of Dalechampia scan dens, with key measurements indicated. Figure 1 of Perez-Barrales et al. (2013).

Flowers of Dalechampia scandens, with key measurements indicated. Figure 1 of Perez-Barrales et al. (2013).

Flowers that rely on animal pollinators to remix their genetic material have evolved a tremendous diversity of strategies for attracting those pollinators—from beguiling scents to elaborate visual displays to pretending to be a lady pollinator.

But there’s a downside to making a big, showy display to attract pollinators—you might also attract visitors who have less helpful intentions than gathering up some pollen and moving on to the next flower. Showy flowers might attract animals that steal the rewards offered to pollinators—or they might attract animals that eat the flowers themselves, or the developing seeds created by pollination. So the evolution of attractive floral displays might very well be a compromise between attracting the right visitors, and attracting the wrong ones.

Continue reading

Pssst. Your holobiont is showing.

Here’s a sad story: Species A mates with Species B. They succeed in making a Hybrid Baby but their Hybrid Baby dies before it can fully develop. (I warned you it was sad.) Why did that happen? Sure, sometimes two genomes are just too different to successfully coexist – both the stars and the chromosomes must align to make a baby. Other times, as recently reported by Brucker and Bordenstein, the Hybrid Baby’s microbiota is the problem.

I think (or rather Google thinks) this is a Nasonia wasp.

In Nasonia wasps, there are three closely related species that all diverged less than one million years ago: Nasonia vitripennis (who I’m going to refer to as the V wasp), N. giraulti (the G wasp) and N. longicornis (the L wasp). When L and G mate and their LG offspring are mated to other LG offspring, 8% of the males die. When V and G mate and their VG offspring are mated to other VG offspring, 90% of the males die.

The phylogeny of the three Nasonia wasps (left) and the crosses that result in hybrid male lethality.

The phylogeny of the three Nasonia wasps (left) and the crosses that result in hybrid male lethality.

Brucker and Bordenstein hypothesized that microbes were responsible for the hybrid lethality of the the VG hybrids. Through DNA sequencing, they found that the gut microbes of the VGxVG wasps were unlike either parental type (in abundance or diversity), whereas the LGxLG wasps were. So, when a hybrid’s gut microbiota is like one of the parental species, the hybrid males live. When the gut microbiota is unlike a parent, the hybrid males die. They further found this could be boiled down to a change in the single dominant species: whereas a Providencia bacterium was most abundant in both V and G parents, a Proteus bacterium was most abundant in VGxVG wasps.

But that doesn’t conclusively show that microbes are responsible for the hybrid lethality. Brucker and Bordenstein then compare germ-free hyrbids to conventional hybrids – in other words, if we remove the germs (the microbiota, that is), do the hybrids still die? The short answer is no. Under normal conditions, about 80% of the pure Vs and pure Gs survive, whereas only 10% of the VGxVGs survive. Under the germ-free conditions, about 70% of the pure Vs and pure Gs survive and 60% of the VGxVGs survive. That’s a pretty significant increase in living hybrids! And to strengthen the case even more – when the germ-free wasps were fed a mixture of Providencia and Proteus bacteria, the hybrid survival rates went down to about 30%.

The authors perform other experiments for this study that include analysis of wasp genomic loci that were previously linked to hybrid lethality and a transcriptomic analysis, where they find immune genes to be a significant player. However, I’m going to switch gears a little bit and talk about the context the authors frame their discoveries in: the HOLOGENOME concept.

Most evolutionary biologists probably consider the individual as the fundamental unit of natural selection. We think about the genes of one mother or one father being passed on to one descendant. But is this view too constrained? The “hologenome” is all the genomes that belong to the “holobiont” – an organism and all its microbes. The Hologenome Theory of Evolution posits that the holobiont is the fundamental unit of natural selection, not just “the big organism”. Generally speaking, this makes a lot of intuitive sense, I think: we macros are pretty dependent on micros to get our genes to the next generation. But is the reverse true? To be THE fundamental unit of selection, the holobiont must pass its hologenome to its offspring – and I’m not sure this assumption universally holds. Certainly some macro-organisms always pass specific micro-organisms to their offspring (coprophagy in mammals might be a good example). But in most cases, where our microorganisms come from is a mix of vertical transmission (from our parents) and horizontal transmission (from the environment). I just can’t make this distinction make sense with what I think I know about heredity and selection. Natural selection depends on traits that make an organism more fit being passed on to its offspring and if some – or most? – of our microbiota is randomly acquired from the environment, natural selection can’t act on it. On the other hand, it’s very possible reality doesn’t abide by our definitions: perhaps only a few microbial taxa need to be passed directly from parent to offspring and these “founders” get microbial communities off on the right track and the rest of the communities fall into place from the environment.

Regardless – Brucker and Bordenstein pretty conclusively turned that sad story into a science story by showing that in Nasonia wasps, gut microbes play an integral role in hybrid survival. And if the Hologenome Theory of Evolution applies anywhere, I’d say it does here!

A healthy viable Nasonia holobiont (top) and an unhealthy, inviable Nasonia holobiont (bottom). From Brucker and Bordenstein (2013), figure 1B.

The sad story told in pictures: A healthy, viable Nasonia holobiont (top) and an unhealthy, inviable Nasonia holobiont (bottom). From Brucker and Bordenstein (2013), figure 1B.

Brucker, R. M. & Bordenstein, S. R. 2013. The hologenomic basis of speciation: gut bacteria cause hybrid lethality in the genus Nasonia. Science 341: 667-669.

When the going gets tough, mutualism gets going

C. alliodora III

Cordia alliodora. Photo by Karen Blix.

Evolution by natural selection is not usually considered very peaceful—the “survival of the fittest” is usually assumed to come at the expense of competitors for food or shelter or other resources. But the “fittest” can also be those who recruit assistance from other individuals, or other species—and who provide assistance in return.

This was the perspective of Peter Kropotkin, a Russian prince and political anarchist who studied the wildlife of Siberia while working as an agent of the Czar’s government. In the harsh conditions of the Siberian winter, Kropotkin reported finding not a bitter struggle over scarce resources, but what he called “Mutual Aid” among species, as well as in the human settlements that managed to eke out a living.

Something like what Kropotkin described is documented in a new paper by Elizabeth Pringle and colleagues. Examining a protection mutualism between ants and the tropical Central American tree Cordia alliodora, Pringle et al. found that drier, more stressful environments supported more investment in the mutualism.

Continue reading