In 2013 I was the first author on a paper on the phylogeny of Boechera, which included splitting the genus into three: Boechera s.s., Borodinia, and Yosemitea. Since then, Mike Windham, a coauthor on that paper, has used microsatellite data to learn much more about intergeneric hybrids involving Boechera s.s. At the time the 2013 paper was written, we thought there was one (Boechera tiehmii), but didn’t have much clarity on what was going on in that case. Mike sorted out the origin of Boechera tiehmii (which I suppose must receive either a hybrid formula or nothogenus at this point) and uncovered several more hybrids and their origins. Noteworthy for the split of Boechera s.l., some of these hybrids are between members of Boechera and Borodinia. Mike gave a presentation at Botany 2014 on these hybrids, and he has interpreted these hybrids as making the separation of Boechera and Borodinia untenable. This counterargument is not explicit in the publicly-accessible abstract of the presentation, which I provide below in case the link disappears, but has been floating around in “pers. comm.” land as a nomenclatural asterisk, a suggestion that this separation has been rejected by further research without a clear explanation of the reasoning involved. Since I disagree with Mike on the nomenclatural question here, I thought it might be worthwhile to write and publish here my best attempt to explain the situation. I should first make it clear that my disagreement with Mike is quite narrow. I think Mike is almost always right on taxonomic questions, and even in this case I have no doubt regarding the hybrid lineages he has uncovered and disagree only on their nomenclatural implications.

The Botany 2014 abstract:

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.

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. or Borodinia 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 in Borodinia. I don’t know the details of the backcrosses to Boechera s.s., but in the research I’m aware of there is no known mechanism for gene flow to get from an allopolyploid back into sexual diploid Boechera. This leaves 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 nomenclatural points necessary for his argument, 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 is 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 is only an example; I do not know if such a lineage 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.

I still think the split of Boechera s.l. into Boechera s.s., Borodinia, and Yosemitea is the best solution to this nomenclatural puzzle. I don’t think it is unequivocally correct or that there isn’t room for reasonable disagreement, but in my mind the uncertainty revolves around the poor resolution at basal nodes in Boechereae. I stand by the reasoning I was following in the 2013 paper–if we want to have monophyletic genera and our best guess (without good resolution to nail things down!) is that Boechera s.l. is not monophyletic, recognizing these three segregates gives us a generic nomenclature that is unlikely to become untenable if we get better resolution and sort out those uncertain nodes. The split might well become unnecessary once those nodes are resolved, but it would still be a viable option, and at present is the option that is most likely to be stable across different possible resolutions of these nodes. The hybridization Mike uncovered between Boechera and Borodinia certainly adds more nuance and complexity to an already complicated situation and furthers our understanding of Boechereae, but I don’t think it forces our hand in nomenclature and it doesn’t change my understanding of the phylogenetic situation.