Flag for Taxon: Smooth Rockcress (Boechera laevigata)
| Flagger | Content Author | Content | Reason | Flag Created | Resolved by | Resolution |
|---|---|---|---|---|---|---|
| bouteloua | Smooth Rockcress (Boechera laevigata) |
treated as Borodinia laevigata in POWO, please discuss before swapping |
Sep. 3, 2018 02:07:12 +0000 | bouteloua |
committed taxon changes |
Comments
great, thanks for doing the legwork! sounds like we can go ahead with the swaps and split
Looks like the genus split would be:
Boechera sensu lato ---split into--->
Boechera sensu stricto
Borodinia
Yosemitea
Hi -- well, it never gets real until it gets real... I've heard from multiple folks with concern about this since the taxon swap was effected yesterday.
Mike Windham (one of the authors) told me about a year after the publication of the 2013 paper that additional data undermined the proposed taxonomic conclusion and advised me not to use Borodinia. I will email him and see his current thoughts. It does appear that there has been no explicit rebuttal of the proposal published -- but it is hard to interpret what that means.
I was under the same impression; that the 2013 article was found to be problematic with subsequent data and analyses.
Well, shoot. Sorry for the false confidence, everyone. The elapsed time, plus the acceptance by POWO, made me think that this was the new standard. Are those subsequent analyses published?
Regardless, if the general sense in the field is that Boechera is the appropriate place for these species, then maybe we ought to revert and explicitly disagree with POWO. I don't feel strongly about this either way. (and I'm less in the know)
My recollection is that Mike Windham was second-guessing the generic taxonomy based on the observation of hybrids between Boechera and Borodinia. Personally, I have no problem with intergeneric hybrids, and we had reason to believe there was at least one intergeneric hybrid floating around in there when the 2013 paper was written. So I don't really agree with Mike's second-guessing here and I don't think the situation has changed appreciably since the 2013 paper. Boechera sensu stricto is almost certainly monophyletic; Borodinia is probably monophyletic; short of expanding Boechera to include all of Boechereae, there is no broader circumscription of Boechera that we would have any reason to believe is monophyletic. No one wants Boechera = Boechereae, so the alternatives are Boechera sensu stricto or, I guess, being agnostic about the circumscription of Boechera. Agnosticism is certainly a defensible empirical position, but doesn't give us useful guidance on what to call the plants.
Thanks Patrick! Appreciate the clarification. This helps tremendously! I briefly revisited the 2013 article last night just to refresh my memory. Like you mentioned, Borodinia appears most likely to be monophyletic, although the support statistics for the clade might be considered somewhat low possibly (I believe the reported values represent parsimony jackknife support and maximum likelihood bootstrap support). It would be interesting to see how the 2013 methods might compare with Bayesian approaches. Thanks again for all your help...
Cheers,
Howard
Patrick -- Interesting.
Can you email me (or send refs) for anything later than the 2013 paper that is pertinent? Interested in especially:
Any more recent phylogenies or analyses.
Best morphological distinctions between Boechera, Borodinia, and other Boechereae.
I'm about to issue the 2020 version of my Southeastern Flora and would like to make an informed choice on this!
I don't have access to Phytokeys, so if the 2018 paper on the Boechera suffrutescens complex has any higher rank implications, I would need that sent.
@whiteoak Sending you the Phytokeys article....
I went back to 2013 (didn't take a DeLorean) and see the trees there and the key, which presumably presents the morpho distinctions pretty well.
@whiteoak - The suffrutescens paper does not have any higher rank implications. There was a presentation at the annual Botany meeting - 2015? 2016? - in which the Boechera x Borodinia hybrids were discussed, but I'm not aware of any more recent phylogenetic analyses or other relevant published work since 2013. I haven't been working on Boechera in the last few years, though, so I may have missed something.
I haven't had a particular reason to try to track down morphological characters that would separate these genera. Subjectively, they are obviously distinct groups, but not in a way that is easily translated into a couple of straightforward or easily quantifiable characters. If I were writing a key to genera, I would be looking at what the possible species are in the region covered by the key. In the southeast, you only have Borodinia, so in that sense it is trivial. In the areas where Boechera and Borodinia overlap, the possible Boechera are: Boechera stricta (trichomes mostly malpighiaceous, some simple; erect fruits); Boechera collinsii, Boechera retrofracta, and possibly some other member of the holboellii group that I'm not thinking of (fruits strongly reflexed, the pedicels abruptly reflexed at the base); and hybrids involving Boechera stricta as one parent (trichomes mostly subsessile, 2- or 3-rayed, the rays parallel to the leaf surface, appressed or nearly so. The characters in parentheses are not found in any of the members of Borodinia. Under identical conditions there would also be an obvious difference in leaf size (leaves are larger in Borodinia), but the variation within taxa induced by differences in shading, soil moisture, and so forth would be too large to allow a generally-applicable size threshold.
Oh, I'd forgotten about the key in the 2013 paper. Looking at it now, it's useful as a general summary, but there are enough exceptions that without geography you wouldn't want to lean on it too heavily. For instance, looking for branched trichomes should work 100% of the time in the areas where Boechera and Borodinia co-occur. In, say, Indiana... not so much... but in Indiana why would you be trying to key between Boechera and Borodinia?
Okay, thanks. I'll go with Borodinia in the East.
Best wishes!
I found the abstract for the presentation at Botany - in 2014. Since this issue has been floating around in "pers. comm." land without being aired publicly in any clear form, I think it's worth trying to explain what exactly I think the situation is... and I've probably gone rather overboard below. I've hit a character limit in comments, if nothing else. Ignore if you like, read on if you enjoy minutiae!
"Newly documented hybrids in the tribe Boechereae (Brassicaceae) challenge current generic circumscriptions in the group.
As circumscribed by Al-Shehbaz in 2012, the largely North American mustard tribe Boechereae included 126 accepted species divided among eight genera: Anelsonia (1), Boechera (110), Borodinia (1), Cusickiella (2), Nevada (1), Phoenicaulis (1), Polyctenium (1), and Sandbergia (1). A subsequent publication by Alexander et al. in 2013 transferred seven species from Boechera to Borodinia and added the monospecific genus Yosemitea, also segregated from Boechera, based on a combination of molecular and morphological evidence. Boechera, by far the most diverse genus in the tribe, is notorious for harboring a network of interspecific hybrids constrained only by geographic isolation or deep phylogenetic divergence. Now, microsatellite-based studies of Boechera and related genera reveal the existence of three previously undocumented hybrids that challenge the efficacy of even phylogenetic distance in preventing hybridization within Boechereae. One of the new hybrids is an apomictic triploid combining genomes from two core Boechera species (B. arcuata and B. perennans) with a genome from B. davidsonii, a distinctive species robustly resolved as sister to all other Boechera s.s.. The species currently called B. tiehmii is shown to have arisen through hybridization between B. lemmonii and the monospecific genus Nevada. Finally, all eastern North American populations previously assigned to B. stricta prove to be allotetraploid hybrids between that species and Borodinia laevigata. Whereas the Boechera/Nevada hybrids do not appear to backcross with either parental lineage and could be assigned to a new nothogenus, this is not true for tetraploid Boechera stricta × Borodinia laevigata. This taxon hybridizes with several species of Boechera s.s., in addition to forming rare triploid backcrosses with B. laevigata."
Boechera stricta × Borodinia laevigata would fall into Boechera using the morphological characters mentioned above and in the 2013 paper. Since gene flow between Boechera and Borodinia is one of the obvious arguments for merging the two genera, I think it's worth mentioning that hybrids between Boechera stricta × Borodinia laevigata and Boechera s.s. do not imply gene flow between the two genera. The usual scenario in Boechera is that a digenomic hybrid is diploid and apomictic, and can produce triploid, di- or trigenomic hybrids through diploid sperm from unreduced spores in the digenomic hybrid uniting with a haploid egg from normal meiosis in a sexual diploid species. Basically, hybridization is rampant and complex but doesn't result in gene flow back into the parents. The scenario described in the abstract for Boechera × Borodinia hybrids is an allotetraploid, digenomic hybrid that can backcross with sexual diploid Borodinia and with Boechera s.s. The backcrosses to Borodinia produce triploid hybrids, implying diploid gametes produced through normal meiosis in the allotetraploid uniting with haploid gametes through normal meiosis Borodinia. There isn't enough detail in the abstract or the emails that I've tracked down to specify a mechanism for the backcrosses involving Boechera s.s., but no mechanism for gene flow to get from an allopolyploid back into sexual diploid Boechera is known, leaving a couple of different ways of producing allopolyploid apomicts in play. The general Boechera rule that backcrossing creates new, reproductively independent lineages rather than gene flow between existing lineages is still in effect here, so far as I know.
Mike's interpretation, based on a couple of emails he sent on the topic, is that the hybridization scenario here makes it impossible to recognize Borodinia as a segregate genus based on a particular interpretation of the nomenclatural possibilities. He believes that a Borodinia × Boechera hybrid could be assigned to a nothogenus but a (Boechera × Borodinia) × Boechera or (Boechera × Borodinia) × Borodinia hybrid could not. If we can't assign such hybrids to a nothogenus and we must assign all plants to a genus, it follows that we cannot maintain Boechera and Borodinia as separate genera.
If Mike is correct, we would have to combine these two genera but it is not clear what circumscription of Boechera would result. Simply combining Boechera and Borodinia is viable if we’re willing to accept a genus that may or may not be monophyletic, but not viable if we restrict ourselves to genera that existing phylogenetic analyses support as monophyletic. If we restrict ourselves to monophyly, Boechereae as a whole becomes a single genus (for which the name Phoenicaulis would have priority). A third possibility here is that we prioritize stability over monophyly and retain the nomenclature in place prior to Alexander et al. (2013) until we can be certain that two conditions hold: Boechera s.l. (i.e., as treated in the Flora of North America and in the series of papers by Al-Shehbaz or Windham & Al-Shehbaz leading up to the Flora of North America treatment) is not monophyletic, and a monophyletic Boechera smaller than Boechereae can be identified.
I do not agree with Mike on either of the two necessary nomenclatural points here, that the backcrosses cannot belong to a nothogenus used for hybrids between Borodinia and Boechera, or that all plants must be assigned to a genus. The first point is covered by ICNafp article H.4: “a nothotaxon is circumscribed so as to include all individuals recognizably derived from the crossing of representatives of the stated parent taxa (i.e. not only the F1 but subsequent filial generations and also back-crosses and combinations of these)”. A possible objection in the case Boechera and Borodinia might be that there are different parental taxa at the species rank. For instance, a hybrid between Boechera stricta and Borodinia laevigata would not have the same parents as a hybrid between Boechera stricta × Borodinia laevigata and Boechera collinsii (this latter formula is only an example; I do not know if a lineage corresponding with it is known). Article H.6, covering the names of hybrid genera, removes this possible objection. Parental genera, not parental species, play a role in the names of hybrid species; e.g., ×Agropogon is Agrostis × Polypogon. At the generic rank, (Boechera stricta × Borodinia laevigata) × Boechera collinsii is simply Boechera × Borodinia. The second point is covered by article H.2, “A hybrid between named taxa may be indicated by placing the multiplication sign × between the names of the taxa; the whole expression is then called a hybrid formula,” and article H.3.1, “Hybrids between representatives of two or more taxa may receive a name.” There is no obligation to create a name for any particular hybrid. We can name a nothogenus for Boechera × Borodinia if we like, or refrain from doing so and use the hybrid formula. If we did name such a nothogenus, it would include all hybrids between Boechera and Borodinia.
Thanks for this additional and detailed commentary! I agree that the hybrid nomenclature issue is addressed in the Code, and besides should not drive the decision on generic taxonomy.
I do wonder about 'canadensis' and entity 'macrophylla', which jump around rather badly in the 4 trees presented in Alexander et al. (2013). I wonder what some new analyses (with different statistical foundations, like Bayesian) on existing data would show with those.
I wasn't thrilled with the genus-level resolution in that paper, but I think using multiple phylogenetic methods is kind of like asking for a second opinion on a medical diagnosis. If the second opinion agrees with the first, confidence in the diagnosis goes up. If they disagree, you shouldn't be confident in either answer but, people being people, you might choose to believe the more optimistic one, or the one coming from a more charismatic doctor.
People seem to still use Boechera laevigata (15 results in last 5 years, Google Scholar), only using Borodinia laevigata very recently (probably in response to POWO: 5 results in last 5 years, mostly in last 2 years). But I can't find a source explicitly disagreeing with the 2013 transfers of some Boechera species to Borodinia, so I feel like we might as well follow POWO's lead here.
The other Boechera --> Borodinia transfers from the same 2013 paper (also accepted by POWO):
B. burkii
B. canadensis
B. dentata
B. missouriensis
B. perstellata
B. serotina
Also:
Boechera repanda --> Yosemitea repanda
Alexander, P. J., Windham, M. D., Beck, J. B., Al-Shehbaz, I. A., Allphin, L., & Bailey, C. D. (2013). Molecular Phylogenetics and Taxonomy of the Genus Boechera and Related Genera (Brassicaceae: Boechereae). Systematic Botany, 38(1), 192–209. doi:10.1600/036364413x661917