About the genus
Cotoneaster is a member of the rose family (Rosaceae), in particular the pear-clade (Pyreae) that contains the following genera: pear (Pyrus), apple (Malus), quince (Cydonia), rowan (Sorbus), hawthorn (Crataegus), firethorn (Pyracantha), Photinia, Amelanchier, and Raphiolepis. DNA sequencing results place it closest to Eriobotrya and Rhaphiolepis (Potter et al. 2007; Lo and Donoghue, 2012).
Cotoneaster is distinctive among these other genera for the following set of features: the plants are shrubs or small trees, they never have spines on the branches, the leaves are simple and entire (never toothed or lobed), there are between 1 and 5 carpels with one style per carpel, and the carpels become hard and bony at maturity.
The near-relative of Cotoneaster most likely to be misidentified as a large-leaved Cotoneaster in New Zealand is Stranvaesia davidiana. This resembles C. xwatereri in leaf size and shape and also has pink tinting on the lower leaf surface midvein. It has 5 carpels which fuse on maturity (a single spherical pyrene), and 5 styles which are fused at their base, whereas they are always separate in Cotoneaster.
Most species of Cotoneaster occur naturally in the Himalayan mountain chain, particularly the Eastern Himalaya in China’s Yunnan and Sechuan provinces, but extending westwards through Bhutan, northern India and Nepal to Afghanistan. However, the genus is more widely distributed, extending into Western Europe and Taiwan, Korea and Japan, northwards into Tibet and the former Soviet Union, and southwards into Egypt, southern India, and Burma.
There is no agreement on the number of species in the genus. The only complete treatment of the genus (Freyer and Hylmo 2009) recognises 361 species. The Flora of China suggests the genus has about “90 species in the broad sense” of which 57 occur in China and Tibet (Lu and Brach 2003).
Classification within Cotoneaster
Cotoneaster has been divided into two subgenera. Subgenus Cotoneaster contains the species that have flowers with erect petals that are often tinted red or pink, and have white anthers. Subgenus Chaenopetalum contains the species that have flowers with spreading petals that are usually white and anthers that are pink to purple. However, the DNA results of Lo and Donoghue (2012) suggest that there are two clades which each contain species from the two subgenera. Within the two subgenera 11 sections and 37 series are recognised (Fryer and Hylmo 2009). New Zealand species are scattered over 13 of the 37 series.
About 10% of the species are diploid and are outcrossing. The other 90% are tetraploid (c. 75%) or have other ploidy levels (Fryer and Hylmo 2009). Probably all the non-diploid species are apomictic. That is, they produce seed by self-fertilisation. Plants of an apomictic lineage are genetically identical from one generation to the next, i.e., they are clonal. These lineages have some of the characteristics of species, since they maintain differences from other apomictic lineages, but these differences tend to be smaller than commonly seen in outcrossing species. Thus it is possible to recognise very consistent lineages in cultivation in New Zealand cotoneasters using quite subtle differences, but whether to recognise these as species is problematic
Apomixis explains much of the difficulty of deciding how many species there are in Cotoneaster. If every apomictic lineage is recognised, the number of species recognised will be much higher than if these lineages are somehow grouped into aggregate species.
Distinct lineages are recognisable in C. franchetii (var. franchetii and var. stearnianus), C. glaucophyllus, C. integrifolius, and C. simonsii (in the key as C. simonsii and C. marquandii). Cotoneaster microphyllus var. microphyllus is tetraploid while var. thymifolius is diploid. Cotoneaster dammeri is diploid but at least two very distinct forms occur in New Zealand. The less common form has larger leaves, more impressed sideveins, and a longer pedicel than the common smaller-leaved form.
Current thinking is that there are only two hybrids known in Cotoneaster, and both had their origin in cultivation The two recognised in Fryer and Hylmo (2009) are C. x watereri (C. frigidus x salicifolius) and C. xsuecicus (C. conspicuus x dammeri). The first is in New Zealand, the second has not been recognised as being in New Zealand.
Lo EY and Donoghue MJ 2012. Expanded phylogenetic and dating analyses of the apples and their relatives (Pyreae, Rosaceae). Molecular Phylogenetics and Evolution 63: 230–243.
Lu L and Brach AR 2003. Cotoneaster. – Pp. 85–107 in: Wu ZY, Raven PH & Hong DY (ed.), Flora of China 9. – Beijing & St. Louis. – Online at http://flora.huh.harvard.edu/china/PDF/PDF09/Cotoneaster.PDF
Potter D, Eriksson T, Evans RC, Oh SH, Smedmark JEE, Morgan DR, Robertson KR, Arsenault MP, Dickinson TA, and Campbell CS 2007. Phylogeny and classiﬁcation of Rosaceae. Plant Systematics and Evolution 266: 5–43.