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):
(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:
we can understand how it moves between these hosts:
*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. 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.