Post by John Brittnacher on Feb 27, 2014 2:04:24 GMT -5
So I guess what this comes down to is how different is "different".
Unless I have missed it, there have not been any DNA studies of the Sarracenia purpurea taxa with enough sequence data to be meaningful in this context. You can't use chloroplast or mitochondrial data unless you are looking for evidence of introgression. In a general survey of inter-fertile plants, plastid sequences will bias results because plastids are clonal. You can say you have thousands of "characters" but you really only have one data point because the characters aren't segregating independently. Also because of introgression, even with nuclear sequences you can't assume any one individual can represent a species. You might have inadvertently picked a non-representative member of the species. And you can't assume a small set of sequence locations are sufficient. I hate to sound like a genome sequencing sales person we all know, but it is going to take genome-level sequencing of many many plants and specialized genome analysis to start making sense of the genetics of Sarracenia.
For instance, the recent Ellison et. al PLoS ONE paper shows Sarracenia purpurea subsp. venosa very closely related to Sarracenia purpurea subsp. purpurea but somewhat distantly related to Sarracenia rosea. The result is mostly based on nuclear sequences, which is good. But the Sarracenia rosea result could have been due to introgression which would make it more like some other species. If they had done two or three individuals across the range of each taxon they might have gotten completely different results.
With respect to the massive zone of intermediate plants in the mid Atlantic states, how you view that depends on how focused you are on Sarracenia. The more you study wide ranging plants, the more you find situations like that. I can image if you live in the epicenter it can get confusing. But if you live on the left coast, for instance, it is like what's the big deal? There are many plants that were in multiple refugia during the last and maybe previous ice ages. They changed differently while in the refugia and have now blasted out and intermixed in zones. Taxonomists here can get rather heated over how to deal with this.
In the Ellison papers and others that compare measurements on herbarium specimens, it is true it is hard to tell the Sarracenia purpurea taxa apart. And technically this is the only appropriate way to distinguish taxa. The trouble here is there are differences that you can't measure on dead plants. There is petal color (dark or light red), petal shape (length to width), how the petals fall, as well as pitcher pubescence, how sturdy the pitcher are, how long individual pitchers last, whether the plant could be considered evergreen (or everred) or semi-deciduous. I personally don't know how these characters vary across populations outside the introgression zone and have to rely on Ellison's and Schnell's work. I do notice the pitcher differences between the taxa much more growing them in Oregon than I did growing them in California.
So when you add all this up, do you get species, subspecies, or varieties? So, Mason, how different do things have to be to be considered species versus subspecies versus varieties? And how do you deal with situations like Sarracenia purpurea var montana that is more like Sarracenia purpurea subsp. venosa than Sarracenia purpurea subsp. purpurea?
I look forward to seeing Phil's article in CPN to see what I have missed.
Unless I have missed it, there have not been any DNA studies of the Sarracenia purpurea taxa with enough sequence data to be meaningful in this context. You can't use chloroplast or mitochondrial data unless you are looking for evidence of introgression. In a general survey of inter-fertile plants, plastid sequences will bias results because plastids are clonal. You can say you have thousands of "characters" but you really only have one data point because the characters aren't segregating independently. Also because of introgression, even with nuclear sequences you can't assume any one individual can represent a species. You might have inadvertently picked a non-representative member of the species. And you can't assume a small set of sequence locations are sufficient. I hate to sound like a genome sequencing sales person we all know, but it is going to take genome-level sequencing of many many plants and specialized genome analysis to start making sense of the genetics of Sarracenia.
For instance, the recent Ellison et. al PLoS ONE paper shows Sarracenia purpurea subsp. venosa very closely related to Sarracenia purpurea subsp. purpurea but somewhat distantly related to Sarracenia rosea. The result is mostly based on nuclear sequences, which is good. But the Sarracenia rosea result could have been due to introgression which would make it more like some other species. If they had done two or three individuals across the range of each taxon they might have gotten completely different results.
With respect to the massive zone of intermediate plants in the mid Atlantic states, how you view that depends on how focused you are on Sarracenia. The more you study wide ranging plants, the more you find situations like that. I can image if you live in the epicenter it can get confusing. But if you live on the left coast, for instance, it is like what's the big deal? There are many plants that were in multiple refugia during the last and maybe previous ice ages. They changed differently while in the refugia and have now blasted out and intermixed in zones. Taxonomists here can get rather heated over how to deal with this.
In the Ellison papers and others that compare measurements on herbarium specimens, it is true it is hard to tell the Sarracenia purpurea taxa apart. And technically this is the only appropriate way to distinguish taxa. The trouble here is there are differences that you can't measure on dead plants. There is petal color (dark or light red), petal shape (length to width), how the petals fall, as well as pitcher pubescence, how sturdy the pitcher are, how long individual pitchers last, whether the plant could be considered evergreen (or everred) or semi-deciduous. I personally don't know how these characters vary across populations outside the introgression zone and have to rely on Ellison's and Schnell's work. I do notice the pitcher differences between the taxa much more growing them in Oregon than I did growing them in California.
So when you add all this up, do you get species, subspecies, or varieties? So, Mason, how different do things have to be to be considered species versus subspecies versus varieties? And how do you deal with situations like Sarracenia purpurea var montana that is more like Sarracenia purpurea subsp. venosa than Sarracenia purpurea subsp. purpurea?
I look forward to seeing Phil's article in CPN to see what I have missed.