Coastal dwarf mistletoe (Arceuthobium littorum) - taxonomic history
I'm not an expert on mistletoes. But I noticed an inconsistent classification of dwarf mistletoes (Arceuthobium) on the Monterey Peninsula pines in iNaturalist, so I researched the literature and have concluded that in iNaturalist, at least for the time being, we should classify all dwarf mistletoes infecting Pinus radiata and Pinus muricata on Monterey Peninsula as Arceuthobium littorum - this being one of the essential conclusions of the last five journal papers that address the taxon, as far as I have been able to find.
I'll attempt to update this post from time to time - e.g. upon receipt of informative comments. I started the posted on 5-Sep-2022 and tagged a few observations with it. If I tagged one of your IDs, and you have an opinion, please chime in!
A timeline of much of the main relevant literature appears near the bottom of this post. To summarize, a portion of the genus has oscillated between "lumped" and "split" perspectives. The epithet "littorum" did not appear until 1990, but numerous other taxa in the "campylopodum complex" were being lumped (e.g. by Kuijt) and split (e.g. by Hawkswoth et al.) for decades prior to 1992. The "split" perspective began as morphological one (Hawksworth & Wiens 1970, 1972) and then became supported by some genetic analyses, but not all of them. In the general trend toward increasing support for littorum as a distinct taxon, there are perhaps two notable contrapoints: one is where Nickrent et al. (2004) reached a lumped conclusion based on genetic analysis; the other is where Kuijt's second edition Jepson Manual treatment re-affirmed a lumped classification in the same year that Nickrent (2012) re-split the complex. The newest work supports the split perspective and pointedly challenges Kuijt's (2012) re-lumping in the latest Jepson Manual. This work is by Mathiasen, Kenaley, and Daugherty in several journal papers (2009, 2013, 2015, 2016) including one review paper (2021) and separates littorum based largely on morphology but noting the genetic evidence reported as early as 1990 by Nickrent & Butler. Of the major first authors on the taxon, recognition of littorum as a distinct taxon in some way (either as a species or subspecies) is supported in the most recent publications on the matter led by Mathiasen (2021), Nickrent (2012), and Hawksworth (1993). Job Kuijt (last relevant publication 2012?) is the outlier; he's 92, but was co-authoring with Dan Nickrent as recently as 2019. iNaturalist uses Plants of the World Online (POWO) for taxonomy, and includes littorum as a species (as I write in September 2022).
In the field in Monterey County, my understanding from reading the literature below is that a dwarf mistletoe specimen could be classified entirely based on the host species. If the host is Pinus radiata or Pinus muricata (as it would be anywhere among native pines of the Monterey Peninsula, Point Lobos, etc.), you have yourself an Arceuthobium littorum. Morphological differences are included in the Monterey County flora (Matthews & Mitchell), but these have barely changed since the 1997 first edition, and presumably could benefit from a revision based on the recent morphological studies by Mathiasen et al. which lead to a new taxonomic key to the genus based on host and morphology (Mathiasen & Kenaley 2015). Interestingly, while color appears early in the keys as an indicative character, it has not yet formed part of the quantitative basis for a morphological analysis. So, arguably characteristics other than color might be more definitive for the time being, if one were to attempt to key a specimen based on morphology and not host plant or location.
Timeline of of much of the main relevant literature:
- 1953 to 1970 - Kuijt single-authored 10 publications on Arceuthobium or relevant to Arceuthobium.
- 1970 - Hawksworth & Wiens - journal paper: "New Taxa and Nomenclatural Changes in Arceuthobium (Viscaceae)". 13 species in "Series Campylopoda" of Genus Arceuthobium. Does not mention epithet "littorum" anywhere.
- 1972 - Hawksworth & Wiens - lengthy Forest Service report: "Biology and Classification of DWARF MISTLETOES (Arceuthobium)". Many species covered, but epithet "littorum" not mentioned. Indicates A. occidentale, not A. campylopodum, as the taxon that infects Pinus radiata.
- 1984 - Hawksworth & Wiens - conference proceedings: "Biology and Classification of Arceuthobium: an update". Epithet "littorum" not mentioned.
- 1990 - Nickrent & Butler - journal paper. "Allozymic Relationships of Arceuthobium campylopodum and Allies in California". First (?) introduction of the eipthet "littorum". Suggested that it be recognized as a species based on morphology (including color) and genetic evidence.
- 1991 - Nickrent & Butler - journal paper. "Genetic Relationships in Arceuthobium monticola and A. siskiyouense (Viscaceae): New Dwarf Mistletoe Species from California and Oregon". Demostrated genetic differentiation between littorum and other taxa in campylopodum complex.
- 1992 - Hawksworth, Wiens, & Nickrent - journal paper: "New Western North American Taxa of Arceuthobium (Viscaceae)". Describes A. littorum as a new species (separating it from A. occidentale).
- 1993 - Hawksworth & Wiens, in Jepson Manual 1st ed.: 12 species and 2 subspecies. Recognizes littorum as a species. Uses common name "Coastal dwarf mistletoe" - which may be the first use of this name.
- 1994 - Nickrent et al. - journal paper: "A Molecular Phylogeny of Arceuthobium (Viscaceae) Based on Nuclear Ribosomal DNA Internal Transcribed Spacer Sequences". Excluded littorum, presumably because it had not yet been described at the time of initiation of their study. Did not cite the 1992 or 1993 publications by Hawksworth et al.
- 1997 - Matthews - book: "An illustrated field key to the flowering plants of Monterey County, and ferns, fern allies, and conifers". Recognizes littorum as a species i.e. following the statewide treatment in Jepson Manual 1st ed.
- 2004 - Nickrent, Garcia, Martin, Mathiasen - journal paper: "PHYLOGENY OF ALL SPECIES OF ARCEUTHOBIUM (VISCACEAE) USING NUCLEAR AND CHLOROPLAST DNA SEQUENCES". Lumped littorum to be part of species campylopodum, based on genetic analysis.
- 2012 - Nickrent - journal paper: "JUSTIFICATION FOR SUBSPECIES IN ARCEUTHOBIUM CAMPYLOPODUM (VISCACEAE)". Recognized subspecies within species campylopodum, including subspecies littorum as the only Arceuthobium to infect Pinus radiata and Pinus muricata.
- 2012 - Kuijt, in Jepson Manual 2nd ed.: Recognized only 3 species of Arceuthobium in California, lumped from previous 12 in Jepson 1st ed., based on "recent molecular studies". Does not recognize any subspecies. Does not recognize littorum as a taxon. This lumped treatment by Kuijt was considered "highly conservative" by Mathiasen & Kenaley (2015).
- 2009, 2013 - Mathiasen & Daugherty - journal papers that now serve as a preface to Mathiasen & Kenaley (2015).
- 2015 - Mathiasen & Kenaley - journal paper: "A MORPHOMETRIC ANALYSIS OF DWARF MISTLETOES IN THE ARCEUTHOBIUM CAMPYLOPODUM –OCCIDENTALE COMPLEX (VISCACEAE)". Elevates littorum back to species level, based on morphology. Provides a new taxonomic key to the genus, including littorum as one of 4 species.
- 2015 - Matthews & Mitchell - book - second edition of the Monterey County flora by Matthews (1997). Recognizes littorum as a species, unchanged from the 1st edition of the county flora, NOT following the most recent Jepson Manual (2nd edition) treatment, but following the emerging studies by Mathiasen et al.
- 2016 - Mathiasen, Kenaley, & Daugherty - journal paper. Similar to their previous publications but broader in scope.
- 2021 - Mathiasen and Kenaley - journal paper. Re-affirmed species status for littorum, in context of a comprehensive review.
Comments
Any thoughts @dan_nickrent ?
Irene Rosen and I had a version of this discussion last year when I hadn't really read anything except Jepson, which I consider the authority for California https://www.inaturalist.org/observations/49910233
This is a tough one Fred because mistletoes are plants that are parasitic on other plants. That leads to many potential problems in using morphological characters for resolving relationships in my opinion. Since selection is operating primarily on physiology and overcoming host defense, morphological reduction is a major trend as it is in most parasites. Furthermore they are, in this case, subject to the same suite of hormones and many similar regulators of gene expression as the host, so there are bound to be host effects on morphology and phenology. Without lengthy, difficult, and expensive cross-infection studies it would be hard to tease all that apart. And that's why I like the molecular genetic approach.
The Jepson Manual still is going with only three species as of September 5, 2022: https://ucjeps.berkeley.edu/eflora/eflora_keys.php?key=10279 Job Kuijt, whom you mention, is the author of that treatment.
Jepson (I assume Kujit) bases this lumping on a phylogeny constructed in the 2004 paper by Nickrent, García, Martín, and Mathiasen in the American Journal of Botany. That phylogeny was based on nuclear ribosomal ITS sequences and chloroplast tRNA sequences, which is a pretty solid approach although I would like to have seen a low copy number nuclear locus such as the pentatricopeptide repeat protein gene that has been so useful in resolving relationships in the Orobanchaceae.
I haven't looked at the papers after 2012, and I intend to, but if they are based on morphometrics I'd not be ready to abandon Jepson yet. I am also not an expert on mistletoes or even on botany... but I did get my PhD in the Lincoln Taiz lab so I have pretty strong biases :-)
Thanks Henrik! Please chime in again after you've read the papers from 2012 onwards. Let me know if you want me to send them to you. I have them all downloaded as PDFs.
It is perhaps worth noting that two of the authors on the 2004 paper you cite (Nickrent et al., including Mathiasen) have since published papers that separate littorum as either a subspecies (Nickent 2012) or a species (Mathiasen and various co-authors 2009, 2013, 2015, 2013, 2021).
I was able to retrieve them and they appear to be good studies. In the end, this is a question of who to consider the authority. Mathews and Mitchell's Monterey County Flora, The Jepson Manual online, Flora of North America online, or Plants of the World Online? iNat pretty much has to use Plants of the World Online given the world-wide scope and the fact that plants don't know about political borders. But I've become pretty addicted to using Jepson with the Keybase filters. Are we going to identify plants in California using Jepson? Plants of the World Online? Or the latest paper? They will all change and I am (perhaps mistakenly) putting my money on Jepson as generally being closest to what is the consensus state of taxonomy in California. And I wouldn't be surprised at all if next January Jepson splits littorum back out as a species. At which time I will begin naming these plants littorum again as I did in the past. The key will be simple if it's just based on who's the host.
There are many taxa in these sorts of limbo... Creosote is an interesting example: https://www.jstor.org/stable/41416947
Those in the the Mojave have 78 chromosomes,in the Sonoran Desert (southern AZ and CA) have 52 chromosomes, while those of west Texas (Chihuahuan Desert) have only 26. They still appear to us to be the same plants and so far have the same name, without even subspecies on Jepson or Plants of the World Online. However, there is evidence that galling insects can distinguish between plants with different chromosome numbers! https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6749999/ Will we soon be identifying the "species" of reproductively isolated Larrea based on their galls?
Please see my current summary notes/references for Arceuthobium littorum: https://www.inaturalist.org/observations/126496206, and newer research on A. littorum vrs. A. campylodum: https://www.inaturalist.org/journal/fredwatson/69693-coastal-dwarf-mistletoe-arceuthobium-littorum-taxonomic-history. An interesting discussion! Thanks @fredwatson for reviving it.
I agree @hkibak, the key (to differentiating the Dwarf Mistletoe species) would be so easy if humans would follow examples in nature--and base it on who is the Host plant. Nature shows us so many perfect examples: Monarch butterflies only laying eggs on Asclepias milkweed, . . .
I thought I'd revisit this post given that about 30 new campylopodus IDs have shown up in radiata / muricata habitat. To me, littorum still stands. I don't see any reason to defer to an outdated Jepson treatment.
Flora of North America recognizes the epithet littorum and was last edited in 2020, by a long-standing authority in the genus:
http://floranorthamerica.org/Arceuthobium_campylopodum
The Jepson treatment is now 11 years old, and was authored by someone who is now 93 years old (i.e. quite possibly no longer active and updating Jepson Online). It's my experience that in the modern era, Taxonomic revisions happen primarily in the journal literature, and these are then followed by the regional floras. Progress would be slowed if we had to wait for regional floras to be updated before adopting peer-reviewed treatments on each genus, particularly when there's been multiple such treatments. iNat doesn't even follow Jepson; it follows POWO, as far as recognition of taxa is concerned.
If I recall correctly, five papers have supported littorum since the last Jepson, and in combination with the FNA treatment, the authorship of these encompasses everyone recently active on this genus.
Just to be clear Fred, you are proposing that we identify all Arceuthobium growing on P. radiata and P. muricata as Arceuthobium littorum despite Flora of North America" calling them A. campylopodum ssp. littorum?
iNaturalist uses the Kew Royal Botanic Gardens "Plants of the World Online" as its authority, which in turn uses "Flora of North America North of Mexico" as the authority for Arceuthobium. On the forum there has been a lot of discussion on whether to use the latest journal papers for taxonomic updates and I think the iNat consensus has been to not go that route, but to wait for FONA and POWO to make the updates (example: https://forum.inaturalist.org/t/how-to-trace-authority-flow-for-taxon-changes/9492 ).
Nevertheless, I've just withdrawn the IDs on two of my Arceuthobium observations. I spent another couple of hours reading various treatments and couldn't really be convinced yet. I still feel pretty strongly about the weakness of using morphometrics to discriminate between plant parasites that are dependent on and subject to their hosts phytohormones. The molecular biology still isn't overwhelming either. But that's the great part about iNaturalist's "community ID" approach. And we are also able to tag experts if we would like their input on an ID.
You might also post a link to this journal entry on the forum. I think you might get additional input from knowledgeable people that way.
Yes, for two reasons.
Firstly, the iNat taxonomic authority for Tracheophyta is POWO, and POWO recognizes littorum as a species:
https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:300855-2
https://www.inaturalist.org/pages/curator+guide#authorities
I could be wrong, but I don't think it is generally true that POWO follows FNA. For one thing, FNA is not finished yet; many families are yet to be treated: http://floranorthamerica.org/Volumes_under_Production. Instead, each POWO taxon cites its own authorities. Sometimes this includes FNA, and sometimes not. For Dipterostemon capitatus, for example, POWO includes FNA in the authorities for the taxon: https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:82063-2. But for Arceuthobium littorum, POWO does not include FNA in the authorities. Instead, it cites the Mathiasen & Kenaley (2016) paper in my original reference list above: https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:300855-2.
Secondly, if we were to take FNA into account, then I think the most important part of that accounting is whether a distinct taxon is recognized at all; and secondary to that is the rank at which it is recognized. Epithet littorum is recognized by FNA, so if we were to follow FNA, we should ID observations with that epithet. The inconsistency that FNA recognizes the epithet at a lower rank than POWO (and therefore iNat) can be resolved at any time with a simple taxon swap. But it would be much trickier if the distinct littorum epithet had not been used in IDs in the first place, because potentially multiple identifiers would all have to agree to change their IDs based on their own interpretation of the authority at the the time for identification (i.e. using morphology, host species, or whatever the key said at the time).
In my reasoning here, I'm trying to steer clear of any biological arguments about whether or not littorum should be a distinct taxon. We should leave that to the journal peer-review process.
P.S: You mentioned POWO uses "Flora of North America North of Mexico" as an authority for Arceuthobium. I'm not seeing that in POWO. Can you post a link?
https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:17596-2
Also, according to the iNat curators guide,
A new plant name or classification is published in the peer-reviewed literature
The new name or classification is registered with the International Plant Name Index (IPNI)
After an intentional one-year lag to allow for synonimization the name is added to POWO
One can register the name with IPNI oneself if the author hasn't gotten around to it.
Thanks. So POWO authority for campylopodum includes FNA, WCSP, and M&K (2016).
But POWO authority for littorum just includes WCSP and M&K (2016), and doesn't include FNA.
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