Cotoneaster is a genus of woody plants composed of ≈400 species that range in habit from tight, impenetrable groundcovers to airy shrubs and medium-sized trees. The center of species diversity is the Himalayas and mountains of Yunnan and Sichuan provinces of China. The distribution encompasses the temperate zones of Eurasia and Northern Africa. The northern end of the range stretches from Spain to Siberia, and the southern limit extends from Morocco to the southern tip of India and South Korea (Fryer and Hylmö, 2009).
Although there are hundreds of species of Cotoneaster, a relatively small percentage are commonly grown in ornamental landscapes, as illustrated by Dirr (2009) listing only 14. These species were selected for their multiseason interest from flowers, fruit, and plant habit. In the 2014 Census of Horticultural Specialties (U.S. Department of Agriculture, 2014), Cotoneaster sales were estimated to exceed $7 million in the United States, although the value is likely greater because this figure accounted only for sales of Cotoneasters classified as “broadleaf evergreens” and many species are deciduous or semievergreen depending on climate and environmental factors.
Cotoneaster is a member of Rosaceae, subfamily Maloideae, and appears to be most closely related to Pyracantha (firethorn) and Heteromeles (christmas berry) (Robertson et al., 1991; Rohrer et al., 1992). Taxonomy at the family level is complicated, with interspecific and intergeneric hybridization being common. Interspecific hybrids of several species of Cotoneaster have been reported, and Cotoneaster melanocarpus has reportedly hybridized with Sorbus acuparia ssp. siberica to form the intergeneric hybrid ×Sorbocotoneaster (Fryer and Hylmö, 2009). Within Cotoneaster, there are two subgenera, Chaenopetalum and Cotoneaster, which are primarily defined by floral morphology. These subgenera have been further divided into 11 sections based on botanical characteristics, and further dissected into 37 series based on botanical characteristics and geographic origins of the species (Flinck and Hylmö, 1966). However, keys associated with this treatment are ambiguous and often of limited use for species identification. We are collaborating with Hoyt Arboretum (Portland, OR) to identify and evaluate our germplasm collection, with little success in identifying unknown samples.
The base chromosome number of Maloideae is 17 and is thought to be of allopolyploid origin—perhaps derived from a hybridization event between other subfamilies in Rosaceae [Rosoideae (x = 7, 8, 9), Spiraeoideae (x = 9), Amygaloideae (x = 8)] followed by a whole genome doubling event (Dickson et al., 1992; Sax, 1954). Cotoneaster species show a ploidy series, with estimates of 70% tetraploid (2n = 4x = 68), 15% triploid (2n = 3x = 51), and 10% diploid (2n = 2x = 34), and the remaining species of greater ploidy level (Fryer and Hylmö, 2009). Apomixis is common in Cotoneaster and appears to be associated with polyploidy, as the tetraploids and triploids are frequently obligate or rarely facultative apomicts, while diploid progeny are sexually derived (Bartish et al., 2001; Czapik, 1996; Hjelmqvist, 1962; Nybom and Bartish, 2007).
Because apomixis is so common in polyploid Cotoneaster, knowledge of ploidy level is essential for breeders to design crosses with hopes of hybrid seed, as the female must be a sexually fertile diploid. In addition, information on ploidy level, genome size, and bp composition may give taxonomists and phylogeneticists insight to the evolution and organization of the genus and related taxa. Previous reports of genome sizes in Cotoneaster are limited; therefore, our goals were to determine relative genome sizes and produce ploidy estimates across a wide selection of Cotoneaster including its breadth of taxonomic groups.
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