Dickinson Lab

Hybridization in Crataegus monogyna

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History Leaf shape Phenolics Isozymes Credits

Update (2016)

Since this page was drafted molecular data have become available that have enable us to to conclusively demonstrate the hybridity of the entities described below. This work is described in the following paper:
Christensen, K. I., M. Zarrei, M. Kuzmina, N. Talent, C. Lin and T. A. Dickinson 2014. Crataegus ×ninae-celottiae and C. ×cogswellii (Rosaceae, Maleae), two spontaneously formed intersectional nothospecies. Phytokeys 36: 1-26. doi:10.3897/phytokeys.36.6784

Please visit this site to see a list of more recent papers from this lab, notably those by Nadia Talent (and others) that describe the use of flow cytometry as a tool with which to rapidly obtain information on ploidy level and breeding system in hawthorns.


Hybridization has been studied in detail only in situations involving the Eurasian diploid species C. monogyna Jacq. not only in Europe (Raunkaier 1925; Bradshaw 1971; Byatt 1975; Christensen 1982, 1996) but also in North America where it has been introduced and hybridizes with native diploid species (Love & Feigen 1978; Wells & Phipps 1989). Up until the last of these studies the data used came from floral morphology and leaf shape, as indexed by the measurements shown in red at right (these measurements exemplify the data used in traditional morphometrics, discussed elsewhere). We are using these and other sources of data, as well as additional analytical methods, in order to document hybridity and relate it to features of reproductive architecture and biology.

(Left) Short shoot leaf spectrum of Oregon Crataegus monogyna. At the left, the most basal leaf. Fruits in this individual would be red and have 1 pyrene.
(Left) Short shoot leaf spectrum of Oregon C. monogyna x C. suksdorfii hybrid. At the left, the most basal leaf. Fruits in this individual were red-purple and had 2-4 pyrenes. (Above) Leaf measurements used by Love & Feigen (1978) to quantify leaf shape in two Crataegus species and their putative hybrid.
(Left) Short shoot leaf spectrum of Oregon Crataegus suksdorfii. At the left, the most basal leaf. Fruits in this individual would be purple-black and have 4-5 pyrenes.  
We are comparing C. monogyna with native diploids like C. punctata Jacq. in Ontario and C. suksdorfii (Sarg.) Kruschke in Oregon. In particular we seek to understand more about the dynamics of C. monogyna colonization and hybridization. Preliminary results (Celotti 1995) provide evidence of severe pollen tube competition in the style, and of the role of the obturator in directing pollen tubes toward the micropyle of the lower ovule. Nadia Talent has shown that the monogyna x punctata hybrids are diploid, like the parent species.  
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Leaf shape

Short shoot leaves (above) were collected from hawthorns at the Cogswell-Foster Reserve in the Willamette River Valley (Love & Feigen 1978), and at the Upper Elk Meadows Research Natural Area in the Cascade Mountains, both in Oregon, U.S.A. At the first site C. monogyna has become established and has hybridized with native C. suksdorfii (Love & Feigen 1978). Eighty-seven short shoot leaves were collected from a random sample of 20 trees that proved, on the basis of fruit color and composition, to comprise seven monogyna, one suksdorfii, and 12 hybrid individuals. At the second site only C. suksdorfii is present, and 71 short shoot leaves were collected from 13 individuals.

Since landmarks are present in the lobed leaves of C. monogyna but not in the unlobed ones of C. suksdorfii (leaf spectra, above), recourse was made to outline-based morphometric methods. For us, the simplest and best approach was to obtain elliptic Fourier coefficients and to calculate a single parameter describing the departure from circularity (Dickinson 1993).

Elliptic Fourier coefficients (for 20 harmonics) and their amplitudes (calculated as described by McLellan & Endler 1998) were obtained and summarized as PCA scores. The inverse of the dissection index that was described by Kincaid and Schneider (1978), i.e. inv(D.I.), where A and P are the leaf outline area and perimeter, respectively, obtained with MorphoSys is calculated as shown below:

(Above) Leaf outline summarized using 10 harmonics (elliptic Fourier analysis carried out using EFAWIN on outline data obtained with TPSDIG; both programs - and many others - can be downloaded from the SUNY Stony Brook Morphometrics Website).

As described by Kincaid & Schneider (1978) the original index contrasts a circle of unit area with any shape of equal area and larger perimeter over the interval {1, +infinity}. The advantage of calculating the inverse of this function (above) is that the values obtained lie in the interval {0,1}, corresponding to a shape with an infinitely large perimeter, and to a circle, respectively. The inverse dissection index thus does not require further scaling to be made either comparable between studies, or commensurate with other descriptors.

(Left) The inverse of the dissection index of Kincaid & Schneider (1978) plotted against the PC1 from a principal components analysis of Fourier amplitudes (McLellan & Endler 1998), for 87 short shoot leaves collected from a random sample of 20 trees that proved, on the basis of fruit color and composition, to comprise seven C. monogyna (red), one C. suksdorfii (green) and 12 hybrid individuals (open). Box plots are shown for values of the inverse of the dissection index for the leaves in each of the three groups (G, L, and H, respectively). The PCA was calculated from the correlation matrix for 39 Fourier amplitudes. PC1 accounts for 38% of the total variance. By the broken-stick criterion (Legendre & Legendre 1998) only PC1 and PC2 (8%) are significant.
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Leaf phenolics have often been used in studies of hybridity. Paper chromatography of phenolics was used previously in Crataegus in the examination of putative hybrids between Ontario C. monogyna and native C. punctata (Wells and Phipps 1989). As this example and others illustrate (e.g. Phipps 1984), hybridization can definitely occur in hawthorns. However, the extent to which hawthorns hybridize in the wild and the potential evolutionary importance of those hybrids is unknown. To address the question of the frequency of hybridity in Crataegus and its potential significance, four putative hybrid cases in Crataegus were examined by Eric Harris using two-dimensional thin-layer chromatographic patterns of leaf phenolics. Three known hybrids, C. monogyna x C. punctata, the C. monogyna x C. suksdorfii example illustrated here, and C. macracantha x C. section Crus-galli, were confirmed by their additive or intermediate spot patterns. One new putative case in section Coccineae, between C. chrysocarpa and C. series Tenuifoliae, was discovered. The latter case illustrates the utility of chemotaxonomic data for indicating potential cases of hybridity because in this case, the morphology of the putative hybrid was not intermediate with respect to its parents.

Image © 2002 Eric Harris.
(Left) Chromatographs (under UV light, 365 nm) of leaf phenolics from individuals of C. monogyna x C. suksdorfii, C. monogyna, and C. suksdorfii. Red arrows indicate spots shared by C. suksdorfii and the hybrid. Yellow arrows indicate spots shred by C. monogyna and the hybrid. Green arrows indicate two of the spots shared by the parent taxa.

Chemotaxonomic characters in hybrids, like morphological ones, are usually intermediate or additive with respect to their putative parents (Harborne and Turner 1984). Intermediacy in phenolic pattern can be observed in the chromatograms of C. monogyna x C. suksdorfii and its putative parents, and is also apparent in a principle coordinates analysis of all the spots observed in the chromatograms (below).

(Left) Principal coordinates analysis of the phenolic patterns of C. monogyna x C. suksdorfii and its putative parents.

Image © 2002 Eric Harris.

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Isozyme variation within and between C. monogyna, native North American diploid Crataegus species, and their hybrids is currently being investigated. Results will be presented here at a later time.

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Jenny Bull, Nina Celotti, Eric Harris, Rhoda Love, Nadia Talent.

back to "The Crataegus problem" - history


Bradshaw, A. D. (1971). The significance of hawthorns. In Hedges and local history. S. C. f. L. History. London, National Council of Social Service: 20-29.

Byatt, J. I. (1975). Hybridization between Crataegus monogyna Jacq. and C. laevigata (Poiret) DC. in southeastern England. Watsonia 10: 253-264.

Christensen, K. I. (1982). A biometric study of some hybridizing Crataegus populations in Denmark. Nordic Journal of Botany 2: 537-548.

Christensen, K. I. (1992). Revision of Crataegus Sect. Crataegus and Nothosect. Crataeguineae (Rosaceae-Maloideae) in the Old World. Systematic Botany Monographs 35: 1-199.

Christensen, K. I. (1996). A reanalysis of the status of Crataegus eremitagensis, C. raavadensis and C. schumacheri (Rosaceae). Acta Univ. Ups. 31: 211-220.

Dickinson, T. A. (1993). Utility of landmark and non-landmark methods in assessing leaf shape intermediacy in plant hybrids (abstract). AIBS Contributed Paper, Ames IA, 1-5 August 1993.

Harborne, J. B. & B. L. Turner (1984). Plant Chemosystematics. Toronto, Academic Press.

Love, R. & M. Feigen (1978). Interspecific hybridization between native and naturalized Crataegus (Rosaceae) in western Oregon." Madro–o 25: 211-217.

Kincaid, D. T. & R. B. Schneider (1978). Quantification of leaf shape with a microcomputer and Fourier transform. Canadian Journal of Botany 61: 2333-2342.

Legendre, L. & P. Legendre (1998). Numerical Ecology, 2nd ed. Amsterdam, Elsevier Scientific.

McLellan, T. & J. A. Endler (1998). The relative success of some methods for measuring and describing the shape of complex objects. Systematic Biology 47(2): 264-281.

Phipps, J. B. (1984). [Studies in Crataegus (Rosaceae: Maloideae) VII.] Problems of hybridity in the cladistics of Crataegus (Rosaceae). Plant Biosystematics. W. F. Grant. Toronto, Academic Press Canada.

Raunkaier, C. (1925). Eremitageslettens Tjorne. Isoreagensstudier 1. . Biol. Medd. 5: 1-76.

Wells, T. C. & J. B. Phipps (1989). Studies in Crataegus (Rosaceae: Maloideae). XX. Interserial hybridization between Crataegus monogyna (series Oxyacanthae) and Crataegus punctata (series Punctatae) in southern Ontario. Canadian Journal of Botany 67: 2465-2472.

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