Stern's Medlar, Revisited

Way back in the Dark Ages of the Fruit Blog, back when I actually wrote long articles, I wrote an post on medlars, which mentioned Stern's medlar, aka Mespilus canescens. I've always been fond of Stern's medlar, mostly because I'm fond of outliers in general, and this thing is definitely an outlier.

The genus Mespilus has long been one of those single species genera, consisting only of M. germanica, the European medlar. So the discovery in 1970 of a medlar species, a hemisphere away from all other naturally occuring medlars, in Arkansas of all places, is certainly surprising. The fact that it exists only on one site, with only 25 individuals, that it only rarely flowers, and that it turns out to be triploid, are that much more unusual.

In fact, probably too unusual to be the real deal. A little while ago I had a great e-mail from Thomas Frothingham, who has worked with M. canescens, with an update on the latest thinking on the species. I'll repost it here verbatim, rather than just paraphrasing his whole letter for you:

I happened across your website today, and I thought I would pass along some information about the Stern's medlar. In a former job, I worked extensively with this species in situ, at the Konecny Grove Natural Area. There has always been controversy about the taxonomy of this very rare plant, but the latest thinking, based on the genetic analysis of the entire Crataegus tribe by Dr. J.B. Phipps [EFL: The paper is actually by Lo et al., not Phipps, though Phipps has been a prolific researcher in the field of Crataegus and Mespilus taxonomy], is this: the Stern's medlar is an accidental hybrid between The blueberry hawthorn (C. brachyacantha) and the European medlar, Mespilus germanica. I can send you the original article if you're interested. It seems likely to me that the germanica plants were brought to the area early in the 20th century, when there was an influx of immigrants from the area of the Czech/Austrian border region. The influence of these settlers is very evident today: the nearest community is the town of Slovak, there is an Eastern Orthodox church nearby, the nearest cemetery is named the Czech National Cemetery, etc. Also the area is still populated with people of Czech ancestry (the Konecnys, Orliceks, etc).

At one time the population of Stern's medlar was managed as one of the rarest tree species in the world. If the hybrid theory is correct, it may change that perception. On the other hand, the blueberry hawthorn is tracked as a species of concern, and has been extirpated from the Grand Prairie region of east Arkansas (the original prairie has been almost entirely converted to rice production). So the Stern's medlar is at least preserving some of the genetics of a rare species.

To answer some of the questions posted on your blog:

To my knowledge, the Stern's medlar is not available commercially. Any attempt to market it would have to take into account the ownership rights of the landowners (the Konecny Grove Natural Area is privately owned, managed through a conservation easement by a state agency. It is also not open to the public). The plant also has some limitations as an ornamental: the period of bloom is very short, and the flowers smell bad. It has so far proven impossible to propagate by cuttings or tissue culture, although I assume it could be grafted. The only reliable method so far is to carefully dig up the suckers from around the base of the parent plant. There are some specimens at the Center for Plant Conservation in Missouri, and also I believe at the National Center for Germplasm Research in Corvallis, Oregon.

The reason it hasn't bloomed in recent years is probably due to the succession process, as the canopy closes and the plants are increasingly shaded. The use of prescribed fire and removal of surrounding trees has been implemented to reverse this process. The other problem with managing this plant in the wild has been the presence of invasive Japanese honeysuckle and privet. These plants have been removed manually, which is very labor intensive. The last time I saw it, the grove was in good shape. This natural area is also the habitat for a number of other rare plants, and so it will continue to be managed carefully, regardless of the conservation status of the Stern's medlar.

It would be extremely interesting to replicate the original cross between the two presumed parent species-in other words, cross pollinate between a European medlar and the blueberry hawthorn. The seedlings from such a cross would presumably be Stern's medlars.

He also passed along a copy of the Lo et al. paper* (which I would be happy to share with any one interested). It's not the easiest reading, at least from my point of view (as much as I like taxonomy I find taxonomy papers mind-numbing), but buried in there are the evidences for a hybrid origin of M. canescens:

1. Nuclear sequence data showed M. canescens shared a recent common ancestor with the M. germanica samples, but chloroplast sequence was actually much more closely related to C. brachyacantha.


2. Stern's medlar shares many characteristics with C. brachyacantha that it does not with M. germanica: petals that turn orange upon drying, multiflorous inflorescences.


3. C. brachycantha occurs naturally in Louisiana, eastern Texas, and adjacent portions of Oklahoma and Arkansas. M. germanica, while native to Europe, is recorded to have been grown in Louisiana as long ago as 1893, placing the two species in close proximity of the only known site of Stern's medlar.


4. Both sexual (×Crataemespilus) and graft (+Crataegomespilus) hybrids between Crataegus and Mespilus. (I've discussed the general promiscuity of the whole Mespilus / Crataegus / Sorbus / Pyrus / Amelanchier club before).


5. There are a lot of reasons to reconsider Mespilus' standing as a distinct genus and include it in Crataegus.

They basically propose the following scenario for the origin of M. canescens:

• Sometime, probably 150-200 years ago, pollen from cultivated medlars was transferred to a flower of blueberry hawthorn, resulting in hybrid seed.


• The resulting seed produced hybrid individuals. However, because of differences between the parent species, meiosis was irregular and the hybrids were largely infertile.


• The only viable gametes produced by this primary hybrid would have been unreduced, and thus diploid.


• With these unreduced ova, occasional seed set might have occurred, and in those cases the pollen parent could have been either a medlar or another native diploid hawthorn (possibly red-fruited, like M. canescens).


• Stern's medlar is the result of these secondary hybridizations.

I have to admit they lay out a pretty compelling case, both for revoking species status for M. canescens (they suggest designating it Crataegus ×canescens) and for absorbing Mespilus into Crataegus. But my personal taxonomic system is based more on sentiment than on anything else, and I'm definitely a splitter, not a lumper. So I'm going to keep Mespilus, and I'm going to keep calling Stern's medlar a medlar, because it's cooler that way.

*Lo, E.Y.Y., Stefanovic, S., and Dickinson, T.A. (2007) Molecular reappraisal of relationships between Crataegus and Mespilus (Rosaceae, Pyreae)—Two genera or one? Systematic Botany 32(3) 596-616.