New study asks if monarchs from different populations only "like" the milkweed from their
I have a very cool new paper to talk about today. This one just came off the press, and it has a lot of very interesting results. If you have an interest in milkweeds and how they compare to each other, or if you want to know more about how "our" North American monarchs compare with other monarchs around the world, this is the blog entry for you!
This new paper was coauthored by Micah Freedman, Christopher Jason, Santiago Ramírez, and Sharon Strauss, and was just published in the journal, Evolution. Here is a link to the paper. It's pretty thick and dense, so even if you can download it, it might take a while to go through it and digest all of the information. I'll try my best to boil it down here.
This paper is another from the many research projects of Micah Freedman, who is a graduate student in the lab of Santiago Ramírez at the University of California, Davis. You might recall that Micah has also recently examined if non-migratory monarch populations still have the ability to become migratory (which I blogged about), and he also examined museum specimens of monarchs dating back 100 years to look at how monarch morphology has changed over time (I blogged about that one too). From my read of this new paper, it looks like this project is of a similar theme.
The title of this blog entry shows the general gist of this new paper, although I probably over-simplified it. The goal of this new study was to determine if monarchs show any evidence of "host-plant adaptation" throughout their global populations. That means, they wanted to see if monarchs from a given population "do better" with the milkweed from their region. Recall that the species, "monarch" (Danaus plexippus), doesn't just occur in North America, but there are smaller populations scattered throughout the world. Genetic evidence indicates that the North American population is the original, "ancestral" population, and then in the last 180 years or so, these smaller populations sprung off from the original. These offshoot populations have mostly adapted a non-migratory lifestyle. And importantly, these other populations have mostly just one or two species of milkweed within their range. Contrast this to the North American populations in the east and west, where there are about a hundred different species of milkweeds. Below is a figure from the paper that shows the monarch populations sampled in this study, plus the milkweed species studied.
In this figure, the colors of each dot are important. The dots are where the monarchs came from in this study. Below each population name is an approximate date for when we think that population established. The four-letter codes on the lower left are shorthand for the 6 different types of milkweed used in the study. In order, they are Gomphocarpus physocarpus, Asclepias syriaca, Asclepias speciosa, Asclepias curassavica, Asclepias incarnata and Asclepias fascicularis. These milkweeds are found in the regions where their colors match. The bars next to each milkweed name are a visual depiction of how closely-related these species are. Note also that the milkweed color matches with certain monarch populations. The pictures of the monarch larvae and adults are included in this figure to give a visual depiction of the monarch life cycle, and to show that milkweed species can be really different! That picture on the lower right is Gomphocarpus physocarpus, which has a variety of common names, including balloon plant, or hairy balls milkweed!
The authors collected monarchs from each of these regions and brought them to their lab. It looks like they either did the collecting themselves, or they had collaborators in these regions that did the collecting and shipped them to Davis. Once in the lab, the researchers allowed the monarchs to mate and lay eggs for this experiment. And don't worry, they had enough individuals from each population to ensure there was no inbreeding. They also grew enough of each of the six milkweeds to feed the caterpillars, in their greenhouse. So all of this is really important- it's what we call a "common garden experiment", whereby all of the different populations are evaluated in the same place, at the same time, on milkweed grown at the same place too. This removes any element of environmental or climate variation between the different populations, and it makes the results of these experiments very robust.
Recall that the goal here was to look for evidence of host-plant adaptation by the different populations. That means they wanted to see if monarchs perform better when they are reared on the plant that is in their population range, that is, the plant they're most used to. So the researchers put baby larvae (1 day old) on milkweed either from their region or from outside their region, by placing the potted plants (and larvae) in a mesh sleeve, pictured below. They then allowed the monarch larvae to grow for a while, then they collected data on how well they grew.
The two plants pictured in panel (d) show the differences in milkweed morphology. These are A. fascicularis and A. curassavica. Note that there are 4 larvae on the left plant and 5 on the right. From my read, I think this was a problem with the study. The authors put up to 5 larvae on each plant to start the experiment. Some plants had less than this. But really, I think it would have been better to just put one larva on each plant. Multiple larvae on a plant leads to some competition and even aggression among larvae, so that you end up with some larvae out-competing the others. notice in the right-hand picture, there is one larvae that is really small compared to the rest? That might have happened there. Perhaps the authors here can be forgiven though, because of the sheer amount of work that this project entailed. If you think about it, keeping track of all of the different combinations of monarch populations and milkweed types, recording the data, etc., must have been a monumental task. They write that in all they set up approximately 4000 caterpillars over the course of this experiment!
So now to the data. The authors allowed the larvae to eat for 8 days, and then examined them. They recorded if any larvae died, and then how much the surviving larvae weighed. I'm pasting a figure below from the paper (in the supplemental files), that shows how the larvae mass varied across the different milkweed species (A) and across the different monarch populations (B). I'm going to skip over the bit about the larval survival here for the sake of brevity. Note that these graphs do not directly address the main question about host-plant adaptation. I'll get to that later. But, since I know that a lot of readers here are very interested in how different milkweeds compare, I thought these graphs would be useful.
The monarch populations on the right graph are abbreviated - they are California, Eastern N. America, Hawaii, Guam, Australia, and Puerto Rico. As you can see, the biggest larvae were grown on A. incarnata and A. currasavica, and the population with the biggest larvae was the Eastern N. American one.
After they weighed and counted the larvae, they put them back on their plants and allowed them to finish out their development into adults. Then, once the adults eclosed from their pupal cases, the authors collected and weighed them. Below is the graphs that show the results of this part. Remember this graph is for the adult monarchs. And note that the results here are a little different than the results for larvae.
In terms of the milkweed types, there really was little difference in adult size, except that those monarchs reared on common milkweed, A. syriaca, tended to be a bit bigger. This is consistent with that recent review blog I had done a while back, where I examined all of the various studies where different milkweeds were compared, and it turns out that common milkweed tends to produce larger monarchs.
The graph on the right shows that the monarchs from eastern and western N. America tended to be the biggest, regardless of which milkweed they were reared on. This was a conclusion that was highly-touted in this paper, and the authors offered a few explanations for it. I'll come back to this.
Now, as for the original question - is there milkweed adaptation? Well, the answer appears to be maybe. Below are some more graphs from the paper (last ones, I promise), which show how monarchs did when reared on the milkweed from their population (called sympatric) versus outside of their population (allopatric). I've pooled together the relevant graphs here into one image. The two left graphs show the results for larvae and the right show the adults.
OK, I'll walk you through this. First, larvae reared on milkweed from their range grow bigger. That's what the first chart shows. The second chart shows the data from all larvae from the ancestral range (eastern and western N. America), compared to all "derived" populations (other global populations). Here you can see that N. American larvae tend to grow bigger, regardless of the milkweed they use. This difference is about 50%. That's a pretty big difference! However, from the two graphs on the right (for adults), it looks like this conclusion is no longer valid. By the time those larvae become adults, there is no longer any difference in overall size when you compare sympatric vs allopatric groups. There is a slight (non-significant) tendency for ancestral adult monarchs to be bigger than those from other populations, as shown in the right-most graph.
There were some results I glossed over here that also appear to show a little more support for host-plant adaptation, but not much. When the researchers evaluated larval survival, they found that monarchs reared on their native milkweeds tend to survive better than if they are reared on milkweeds from outside their range. However, this difference was small, like 80% survival on sympatric hostplants compared to 76% survival on allopatric plants.
The researchers concluded that monarchs from the ancestral range (North America) appear to be more adaptable to different milkweeds and out-perform those from other populations. Given the results I'm looking at in this paper, I would mostly agree with this statement. One of the reasons why I would agree is because this is consistent with prior studies that have been done. For example, there was a project that I and Sonia did 10 years ago, where we looked at adult wing morphology from various global populations, to see if migratory monarchs are bigger than non-migratory ones. In fact, we published it in this same journal! The conclusion there was, yes. Below is a figure that shows two N. American monarchs compared to two from Puerto Rico.
In our study there was a very clear difference in overall size between migrant monarch populations and those from non-migrating populations. We inferred this to mean that the migration acts as a natural selection event each year that weeds out small individuals. Now, compare this to the new study - they found that North American monarchs grow bigger than those from other populations...
It's all good. This is the way science works - different studies, different methods, etc. If they all point to the same thing, then that's all good.
Let me see if I can sum up this new study in a simple take-home message. I think in a nutshell, this new study provides more evidence that the ancestral N. American monarchs are generally more robust than those in the derived populations. This could be because North American monarchs are used to a wide variety of milkweeds (and need to be adaptable), or it could be because they are programmed to bulk up as much as possible in order to survive the long migration.
I'm going to leave it here for now. Thanks for reading...
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