Hey All,

Here's a paper of mine in the most recent issue of Copeia where we identified patterns of hybridization in darters based on records compiled from museum records and literature. There are already a few new specimens I know about that were collected after this was accepted for publication. Teaser: over 1/4 of darter species are represented in hybrid specimens!

Have a good finish to 2009! Ben

 Keck&Near2009.pdf   461.61KB   77 downloads

Ben,

Nice paper. It leaves me with a couple questions...

Is it possible that the hybridization is a result of anthropogenic disturbance across this "relatively narrow evolutionary time frame"? How would you tease that apart? I know many (most?) of the caeruleum x spectabile sites personally, and they're channelized, homogenized with marginal riffle habitat, heavily eutrophied, with amazing temperature and oxygen swings.

Perhaps it's a successful evolutionary response to environmental stress, and our species' success is without doubt, a great environmental stressor... But I'm having a little trouble stomaching this chicken n egg kind of thing. Perhaps you can clear that up a bit?

I'm always willing to hear genetic evidence on hypotheses, but I think it's an absolute mistake to overlook and then especially to extrapolate without knowing WHAT is shaping and selecting the genes.

An interesting topic, any way you slice it

Thanks for posting!

Todd

Thanks Todd! It leaves me with lots of questions!

This is just the beginning; it was the first chapter in my dissertation and sets up most of my other questions in my dissertation and ones I'm working on now, some of which you listed. I see it as: 1) hybridization happens when reproductive isolating barriers fail, 2) reproductive isolating barriers most often fail when animals are exposed to new environments, perhaps from dispersal or disturbance, and 3) genetic exchange between parental groups, populations or species, or the formation of a 'hybrid species', is dependent on much more than the hybridization event and these can be determined through quantification of environmental variables and experimentation. I don't think we have evidence that would suggest there has been selection on groups to hybridize under environmental stress, it does happen more in stressed conditions, but saying it is a response to stress that has been selected for is really reaching.

No one has looked at rates of introgression through time to see if there has been an increase that could be attributed to anthropogenic disturbance. This is something we are studying in darters, but we also need some well resolved phylogenies to base this on and those are at least a week away (okay probably a bit longer than that). Additionally, hybridization does not always result in gene flow, or introgression, between parental groups. So, phylogenetic methods can estimate rates of introgression, but not necessarily rates of hybridization. Previous papers on the caeruleum X spectabile issue have shown that introgression has occurred recently and in the past (likely before people were around), but again this doesn't really say anything about the rates of hybridization in the past. As we said in the paper, the causes of us observing more hybrid records from the past several decades may be due to lots of things, including more ichthyologists out collecting.

There are a lot of factors, such as behavior, genetic compatibility, and environment, that influence the likelihood of gene flow after hybridization and those can be determined through mate choice, hybrid viability, and common garden experiments. Mitochondrial introgression has been shown to confer an advantage in trout and pupfish, so it'll be interesting to see if the same holds for darters.

Ben

Got it, neat stuff. I'll read the discussion more carefully and look forward to future papers on the topic. I did really enjoy your talk on chlorobranchium showing evidence of range expansion and contraction. I would imagine those are some of the processes that you were examining with regard to these questions.

I also noticed that you cite Nothonotus at the generic level in this paper, but then refer to it as a subclade (which is easier to get behind). Are there any other authorities using Nothonotus at the generic level?

I'm not disputing it's a real category... I find even environmental evidence that shows that elevating these classifications (including Ulocentra, Doration, Catonotus, Boleichthys, perhaps others) are descriptive when interpreting darter community. It's just that I'm not qualified to make that determination, and I can't just go on "Well Ben and Tom said so"

I wondered if that had moved along at all?

Todd

The chlorobranchius stuff is a part of the bigger introgression picture I'm working on, and has implications for the observed mitochondrial introgression in redlines that I present in an Evolution paper that is available in the 'early view' section of the journal's website. For darters I'm moving two ways from my dissertation work, larger scale phylogenetic work on all darters, and population level methods for Nothonotus camurus, N. chlorobranchius, and N. rufilineatus. The chlorobranchius study, partially funded by a NANFA grant, is awaiting the addition of specimens from a few populations that will hopefully be collected in late spring. I hope to have that out fairly quickly, both in American Currents and another journal.

As for Nothonotus, we refer to Percina, Ammocrypta, Crystallaria, Nothonotus, and Etheostoma as subclades of darters, Etheostomatinae. I agree that some other current 'subgenera' are also good groups and should probably recognized as well, but unlike Nothonotus we didn't have complete sampling or resolution in the phylogenies, so we are collecting more data. We elevated Nothonotus because the evidence and phylogenetic support was there and there was no reason to wait to make the change. Your statement about what nomenclature is descriptive is an important statement about the currently recognized genera. Nomenclature should be evolutionarily informative and stable, but the ICZN code makes a point of stating that stability is second to information. If you consider almost all recent publications on darters, species of Percina and Etheostoma are almost always referred to with their 'subgenus' included, but not so with Ammocrypta or Crystallaria (or Nothonotus if one accepts our elevation). That suggests to me that Percina and Etheostoma are not adequately describing key evolutionary information for these organisms. No, most darterologists are not using Nothonotus, but we don't expect nomenclatural changes for darters to happen overnight (or four years as the case may be). Afterall, there are still fish people using Etheostoma for Ammocrypta. Although, we were surprised that our use of Nothonotus was never mentioned in the review process, whether or not that means the winds are shifting I don't know. I would go on about darter nomenclature but some people would probably call it a 'tirade', again.

Ben

Who are the ICZN Reps for perches? Is there resistance there? I know this is the case for a lot of reassignments in Unionid mussels, and the younger folks are just having to sit patiently.

It's also ecologically informative. After SFC, I went into the Etowah headwaters and had hoped to finally see brevirostrum. I was given a site, went there, knowing what was implicit in "Ulocentra" about habitat, walked straight to them, got them on the first haul, like I knew right where they were without ever having been there (helps that they live in such a distinct habitat among the continuum in a mountain headwater lol).

This isn't the first time this has happened, and when I get undergrads who haven't been fish dudes making these kinds of judgments, these groupings are very real to me (ie we're not just quantifying my brain ).

Well I'll just keep using "this may suggest" in my manuscripts I'm writing

Todd

Very interesting but for some reason I'm not surprised. I tend to be one that says hybrid quite quickly. I recall seeing many redline darters in central and eastern Tennessee that I was insistent upon them being hybridized with bluebreast. Assuming/Knowing that redlines seem to almost be a habitat invader, at least recently, in the group it would not surprise me that this was actually the case (hybridization) especially given some of the disturbances in the habitat and the often rediculous densities of redline darteres. With so many nesting events going on a hybridization event occurring seems likely with a disturbance via flow or silt. For example, with the apparent disappearance of bloodfin from Blackburn Fork/Roaring River system could a surging population of redlines and their genes swamp a marginal population of bloodfins already reeling from habitat disturbance (dam, bad ag pracitces, impoundment of Cumberland River, and urban Cookeville). The same for microlepidus in the Stones? I've become even more of a believer now being in Maryland where a majority of our Etheostma were introduced to waters of the state within the last 5 or so decades either through piracy or the bucket. I've physcially been able to observe the ecological differences and the genetic evidence was also recently presented. I'm begining to wonder what finer shifts, like dietary have been taking place. I'll definately need to give a detailed read to thisbecuase at ifrst glance it seems like a happy head scratcher.

Edited by ashtonmj, 28 December 2009 - 05:02 PM.

I've been reading the book Darwin's Armada by Iain McCalman, about the development of Darwin's ideas including the work of Joseph Hooker, Alfred Wallace and Thomas Huxley. The whole lumper/splitter question has been going on since at least the 1830s, originally with plants. Hooker was driven crazy by "splitters" during his work describing the plants of the southern hemisphere. But Darwin was of two minds about it, because if evolutionary processes are ongoing you would expect to find intermediate forms with some confusion as to what is a "real" species. Ben's work with darters touches on this both with what genera we should recognize, and also the whole hybrid question. Even though I'm often a sceptic about hybrids, I know that many taxa are imperfectly separated and a variety of factors can weaken reproductive isolating barriers within groups like the Etheostomatinae. So I'm glad that Ben (and Tom) are able to put some number on the frequency of this process which sets up considering what specific environmental factors facilitate hybridization.

I think one thing you mention Bruce is of particular importance. We have been trying to classify things into a binomial system based on our perception (whether it was morphology, behavior, genetics, etc.). Things might not always fit into that static mold of Genera species in whichever species concept context and I'm perfectly okay with that. I think I came to that acceptance pretty quickly because of my work with mussels, which show incredible variation in practically every aspect of their being and have the added bonus of far too much lumping and an apparent rush to split.