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Plant Focus

A guest post by Matt Candeias, host of the In Defense of Plants podcast and blog

A Hybrid with a Twist

Quercus ×kewensis acorns harvested in Grigadale Arboretum, April 2018. Note the twist in the tip, particulalry in the one on the left.

This year I harvested acorns in Grigadale Arboretum rather late in the season (April), at a stage when most species had dropped them. As trees are planted quite close together, in some cases I had to distinguish acorns based on their shape in order to collect from a particular tree rather than its neighbor. When it came to Quercus ×kewensis, there was a mass of acorns on the ground, many from several neighboring Q. cerris. I found that I could pick out the Q. ×kewensis because of their odd shape. The acorns of the hybrid are not quite straight: the tip lists to one side, so that the remnant of the style is not on the "axis" of the nut.

Quercus ×kewensis is an oddball: it was raised in Kew Gardens in 1914 from an acorn gathered from a specimen of Q. wislizeni, an evergreen section Lobatae oak from California. Based on leaf shape and growth rate, it was clear that it was not true to the mother’s species and according to the description in Bean it is “fairly certain” that the paternal genes came from a Turkey oak (Q. cerris) that stands 40 yards away. This makes it a rare hybrid for oaks, as the two species are in different sections of the genus, and according to the new infrageneric classification published last year by Denk et al.[1] (and summarized by Béatrice Chassé in the recent issue of International Oaks[2]), the species are actually in different subgenera: Q. wislizeni in subgenus Quercus and Q. cerris in subgenus Cerris. I had understood that this was the only example of an intersectional hybrid, i.e., one involving species in different sections[3], but according to the new classification, the long-established hybrid Q. ×turneri (Turner’s Oak, raised before 1780 according to Bean) is now also an intersectional hybrid, and like Q. ×kewensis also involves parent species in different subgenera: Q. robur (subgenus Quercus, section Quercus) and Q. ilex (subgenus Cerris, section Ilex). In fact, the only two well-known examples of hybrids of species from different sections[4], involve a cross between species from different subgenera, implying that the barriers against fertilization between sections are stronger than those between subgenera. I wonder whether this phenomenon has been explained or researched by the scientists working on the new classification.

Immature Quercus ×kewensis acorn, Grigadale Arboretum, March 2014The tip is slightly off-axis. (Click on image to enlarge.)
Quercus wislizeni leaves and acorn © Benny White CC BY-SA 3.0 from Creative Commons

Intrigued by the shape of Q. ×kewensis acorns, I looked up images of Q. wislizeni acorns and found some examples that show long acorns that veer to one side towards the tip. So this characteristic is typical of the species (also found exceptionally in Q. agrifolia, apparently), and confirms one side of the cross. I was surprised Bean’s description of the acorn of Q. ×kewensis does not mention this characteristic, stating only:

Acorns 3⁄4 to 1 in. long, 1⁄2 in. wide, taking two years to reach maturity like those of Q. wislizenii [sic]

Based on what I can find on the web, Q. ×kewensis acorns are different to Q. wislizeni acorns in that they are slightly wider, closer to Q. cerris in that respect.

It would be interesting to hear from anyone else who has noticed this odd shape in Q. ×kewensis acorns, or in those of Californian species.

With thanks to Allen Coombes for providing the reference to intersectional hybrid oaks in Estremadura.

Photos © Roderick Cameron unless specified

[1] T. Denk, G.W. Grimm, P.S. Manos, M. Deng, and A.L. Hipp. An Updated Infrageneric Classification of the Oaks: Review of Previous Taxonomic Schemes and Synthesis of Evolutionary Patterns. Oaks Physiological Ecology. Exploring the Functional Diversity of Genus Quercus L., 2017, pp. 13–38. Cham, Switzerland : Springer International Publishing AG. doi:10.1007/978-3-319-69099-5_2.

[2] B. Chassé. Updated Classification of Oaks: a Summary. International Oaks, 2018No. 29, pp. 11-18. 

[3] See also Jeroen Braakman’s article on hybrids in Oak News & Notes Vol. 21 No. 1

[4] Hybrids between Q. suber (section Cerris) and species in section Ilex have been reported in Estremadura, Spain. See here (article in Spanish).