Preface: This post is a bit different than a typical post for me (or any of us here at DE!) It relates to an interesting bit of Daphnia biology that I find myself relating a lot when I talk to people more generally about my research. People seem to find it surprising and interesting, so I decided to write a post on it in the hopes that others find it interesting, too.
If I put a bunch of different Daphnia on a microscope in front of you, you’d probably think they all look pretty much the same.* As an example, when keying out the species I’ve done the most work on, Daphnia dentifera**, using the excellent online Haney et al. key, these are two of the first traits you need to focus on:
Those aren’t exactly traits that are overwhelmingly obvious, are they?
I think it is because of their morphological similarity that it is then very surprising to most people when they learn just how old the genus Daphnia is. It’s really old.
How old is it? There is some variation in the estimates, but there is general agreement that the genus arose during the Mesozoic (that’s 252-66 Million years ago [Mya]). One estimate (Colbourne et al. 1996) has the genus as being about 200 million years old; a more recent study (Kotov & Taylor 2011) using fossil remains suggests the genus is at least 145 million years old. In other words, the lineages that led to these two organisms:
diverged about as long ago as the ones that led to these two organisms (based on the mammal phylogeny in Figure 1 of this Murphy et al. paper):
Photo caption: Swamp wallaby (photo by jjron, accessed on wikipedia) and Southern right whale (photo by Michaël Catanzariti, accessed on wikipedia)
(Note added 9/28: I edited the previous sentence in response to a comment that made it clear I hadn’t been precise enough in my wording.)
What does all this have to do with infectious diseases? There are parasites – quite a few, it seems – that are able to move between Daphnia that are very divergent. We even see parasites move between Daphnia and other Cladocera, such as the genus Ceriodaphnia.*** One of the goals of research in my lab is to understand how parasites can do that. What challenges are associated with moving between such divergent hosts, especially when they also differ in important life history traits? What tradeoffs do the parasites face? Why do they sometimes move easily between very divergent species but then not move to a much more closely related species?
In some cases, I suspect we’ll find that what we think are multihost parasites (that is, parasites that can infect multiple host species) actually are examples of cryptic species, where the parasites in one Daphnia species are very different genetically than those in another. But we know in other cases that the parasite can move easily between hosts. As one example, in a new paper that just appeared, we found that a common fungal parasite, Metschnikowia, can easily infect an invasive species, Daphnia lumholtzi, that is spreading across North America. This is notable for several reasons, including that there is a whole lot of genetic divergence between D. lumholtzi and D. dentifera:
Caption: Figure 2 from Colbourne & Hebert 1996**** For any folks who need a reminder on how to read phylogenetic trees, you should read from left to right (given the orientation of this phylogeny). The red dot indicates the most recent common ancestor of D. lumholtzi and D. dentifera. So, even though those two species names appear near each other over on the right side of the tree (that is, at the tips of the tree), their lineages diverged a very long time ago.
So, it seems like it should be possible to use these Daphnia-parasite systems to understand what factors influence how parasites evolve when faced with a diverse suite of hosts. And, while my research is solidly in the realm of basic research, I do hope that what we learn in the Daphnia system will give us general insights into how diseases move from one host to another.
In other words, I hope that, by understanding how a parasite moves between these hosts:
Ceriodaphnia dubia and Daphnia dentifera; photo by Stuart Auld
we can understand how it moves between these hosts:
Photo caption: bat and human silhouettes (bat by Nikita Kozin, human by Edward Boatman for the AIGA collection; both downloaded from The Noun Project)
*Well, unless they had cool helmets like this one, but those are plastic and not reliable taxonomic traits. (See the picture of D. lumholtzi at the beginning of this post; both of these pictures are the same host species.):
D. lumholtzi infected with a bacterial pathogen, Pasteuria ramosa; photo by me
** D. dentifera actually isn’t on their key. What we work on keys out to D. rosea, which used to be considered the same as dentifera, but those groups were later split.
*** There isn’t great resolution on this split, but it seems likely (see, e.g., Stenderup et al. 2006) that Ceriodaphnia isn’t even the sister genus to Daphnia.
**** There is a newer study on the phylogenetics of Daphnia by Adamowicz et al. that focuses on more than just the North American Daphnia and that used more genes in their analysis. Their results supported the relationships shown in this Colbourne & Hebert figure. I’m using the Colbourne & Hebert figure because it is much simpler, thanks to having many fewer species in it.
Dynamic Ecology 
Interesting post Meghan. Can you clarify something? Perhaps I’ve misunderstood but are you saying that there is as much overall genetic difference between those two Daphnia species are there is between Marsupial and Placental mammals? I can see that this might be the case for neutral DNA elements that have accumulated random changes over time, but is the really true overall, given the very different reproductive strategies of these two mammalian clades? I’m happy to be astonished if it is!
Hmmm, your question is different than what I was trying to get at, but is a very interesting one! I think what you’re asking is, if you compare the full genome sequences of a marsupial and a placental mammal, would the # (%?) of differences be the same as or greater than the difference between the genome of Daphnia lumholtzi and Daphnia dentifera. Is that correct?
If so, I don’t think anyone knows the answer to that. The genome of Daphnia pulex has been sequenced, and I think one for Daphnia magna is in the works, but I don’t think it’s been published yet.
The genome of pulex has interesting features, though. The publication is here:
http://science.sciencemag.org/content/331/6017/555
The genome was smaller than they’d expected, but had a high gene count and lots of Daphnia-specific genes.
I just edited the post to try to be more precise in my wording!
Thanks, yes, that was what I was asking, which seemed to be what the original wording was implying. The new wording is clearer.
Nice post, Meg! Even though I don’t work with Daphnia directly anymore, I still find myself getting really excited about zooplankton and sharing all the fun things I learned about their biology and ecology while working in your lab. Thank you for the photo credit 🙂
So… knowing nothing about daphnia… do the different species in daphnia have very different ecologies? That is, do they all need a bit of water and some similar things to eat and they’re fine? Or do they eat different things, live in water of different temps/pH/salinity, or some other ecological difference?
I’ve worked on bovine tuberculosis which has a wide set of mammal hosts. In learning about disease ecology more generally while doing that, it seems that different strains of diseases with wide host ranges tend to specialize by abiotic environment, not necessarily via genetic similarity of the hosts. For example, a bacterium may specialize on diverse mammals that live in arid systems, because it has found a way to survive the transmission between mammals in a dry environment.
The short version is that they (and related genera like Ceriodaphnia and Bosmina) have different ecologies. Body size in particular is a key difference among species, as it affects both competitive ability and predation resistance. Larger species are better competitors; they can eat a wider size range of algae and other particles and filter them from the water at higher rates. Larger species also are more visible to predators. That’s a *very* broad-brush characterization that ignores things like interspecific variation in phenotypic plasticity (defensive spines, etc.), and I have no idea about interspecific variation in disease resistance.
Yes, their ecologies can differ by a lot. To give an example: As an undergraduate, I worked on a project focused on Onondaga Lake in Central New York. As a result of intense industrial pollution, the salinity in that lake increased substantially (to 3 parts per thousand) and the fish died. This made Onondaga essentially a giant pond. The native species of Daphnia disappeared and were replaced by two exotic species (Daphnia curvirostris and Daphnia exilis); those species are normally found in ponds with higher salinities. Onondaga Lake was named a Superfund site and recovered, both in terms of water chemistry and fish communities. The exotic species disappeared (we only know they were there because we can see their resting eggs in sediment cores) and the natives returned. I think that’s such an interesting saga!
On the disease front, one thing that is interesting is that the fungus that readily moves between Daphnia dentifera and Daphnia lumholtzi does not infect Daphnia pulicaria. (Well, almost never. We’ve managed a couple of times, but it’s exceedingly rare.) But it does infect other things in that lineage (meaning: the “Daphnia” subgenus on the Colbourne & Hebert figure), like Daphnia retrocurva. And dentifera, pulicaria, and retrocurva all co-occur, so it doesn’t seem like it’s simply that the fungus can’t tolerate something about the lakes that pulicaria lives in. I would love to know what explains why pulicaria is so resistant!
Cool stuff! Maybe worth looking into the chytrid fungus literature, which I believe has similar stories…
Yes! And another thing that is an interesting comparison with the chytrid is that the fungus has relatively little genetic variation. That’s something else we’re following up on with the fungus that infects our Daphnia.
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