The Biology of Malus domestica Borkh. (Apple)
5. Related Species of Malus domestica
As mentioned earlier, the number of species in the genus Malus varies widely, with different taxonomic treatments recognizing anywhere from 8 to 78 primary species (see Section 2.1). Many of the crabapple species can be difficult to differentiate due to the lack of distinguishing characters (Dickson et al. 1991).
Within Canada, there are two native species: the sweet crabapple, M. coronaria, native to Ontario, and the Oregon or Pacific crabapple, M. fusca, native to BritishColumbia. In addition, there are three introduced species that are considered naturalized in Canadian floras: the Siberian crabapple, M. baccata, the plum- or pear-leaved crabapple, M. prunifolia, and the cultivated apple, M. domestica (often listed under synonyms M. pumila auct.; M. sylvestris auct., Pyrus malus L.) (Brouillet et al. 2010+; CFIA and NRCan/CFS 2011+; Duncan and Duncan 1988; Kartesz 1999; Scoggan 1979; Sudworth 1967; USDA-NRCS 2012) . The distribution of these species in Canada is shown in Table 2.
|Species||Nativity||Distribution by province|
|Malus baccata (L.) Borkh.||Introduced||ON, QC, NB, NS, ?NL|
|Malus coronaria (L.) Mill.||Native||ON|
|Malus domestica Borkh.Table Note 1||Introduced||BC, MB, ON, QC, NB, NS, PE, ?NL|
|Malus fusca (Raf.) C. K. Schneid.||Native||BC|
|Malus prunifolia (Willd.) Borkh.||Introduced||NB,NS|
- Table Note 1
Distribution collated from reports of synonyms Malus pumila Mill. and Malus sylvestris (L.) Mill. in the literature.
In the United States, there are two additional native species, the southern crabapple, M. angustifolia (Aiton) Michx. and the prairie crabapple, M. ioensis (Alph. Wood) Britton, as well as 14 additional introduced species and hybrids that are reported in floras: M. × arnoldiana (Rehder) Sarg. ex Rehder, M. × dawsoniana Rehder, M. floribunda, M. halliana Koehne, M. hupehensis (Pamp.) Rehder, M. × magdeburgensis Hartwig, M. mandshurica, M. × platycarpa Rehder, M. sargentii, M. × soulardii (L. H. Bailey) Britton, M. spectabilis (Aiton) Borkh., M. sylvestris, M. toringo (Siebold) de Vriese, and M. zumi (Kartesz 1999; Kartesz 2011; USDA-ARS 2012; USDA-NRCS 2012).
In addition, there are a large number of species that are grown and traded for ornamental purposes in North America (Bailey and Bailey 1976), as well as several crabapple species that are grown as pollen donors in commercial apple orchards (Kron and Husband 2007). An online search of nursery databases indicates that, in addition to M. domestica, many other species are commonly available at wholesale and retail nurseries in the U.S. and Canada, including: M. angustifolia, M. baccata, M. coronaria, M. floribunda, M. fusca M. hupehensis, M. ioensis, M. kansuensis (Batalin) C.K. Schneid., M. × micromalus, M. mandshurica, M. × purpurea (A. Barbier) Rehder, M. × robusta (Carriere) Rehder, M. sargentii, M. sikkimensis (Wenz.) Koehne ex C. K. Schneid., M. sylvestris, M. tschonoskii (Maxim.) C. K. Schneid., and M. zumi (Isaacson and Allen 2007).
5.1 Inter-species/genus hybridization
Most species in the genus Malus can be readily hybridized (Hancock et al. 2008; Luby 2003). The capacity for inter-species hybridization within the genus Malus is evident by the numerous named hybrids among Malus spp. (e.g., Korban 1986; Schuster and Büttner 1995) (see also Appendix 1). The majority of Malus spp. are diploid and inter-fertile, as there are no apparent physiological or genetic barriers (Korban 1986). Reports of natural hybrids are common and artificial interspecific hybrids are easily produced (Luby 2003). M. domestica, which is thought to be of hybrid origin (Korban 1986), is able to interbreed with its congeners in the genus Malus (Korban 1986; Kron and Husband 2009). Interest in controlled hybridization for the improvement of cultivated apples dates back to the 1700s, and reports of successful experimental interspecific hybridizations began in the late 1800s (Korban 1986). Since then, interspecific hybridization has played a major role in genetic improvement and a large number of crosses have been made among Malus spp. in research and breeding programs throughout the world, primarily to improve the cultivated apple or to develop new hybrid species with distinctive characteristics (Korban 1986). A list of experimental interspecific hybrids that have been documented in the genus Malus is provided by Korban (1986), and includes about 60 different species combinations. Those involving species known in Canada are provided in Table 3 (for the full list see Korban 1986).
|Malus baccata||Malus domestica||2864 pollinations; 840 fruits matured||(Crandall 1926; Korban 1986)|
|Malus baccata||Malus prunifolia||No data||(Korban 1986)|
|Malus coronaria||Malus domestica||54 pollinations; 5 fruits matured||Crandall 1926; Korban 1986)|
|19 pollinations; 10 fruits matured||(Kron and Husband 2009)|
|Malus domestica||Malus baccata||734 pollinations; 113 fruits matured||Crandall 1926; Korban 1986)|
|Malus domestica||Malus coronaria||22 pollinations; 0 fruits matured||Crandall 1926; Korban 1986)|
|Malus domestica||Malus fusca||No data||(Korban 1986)|
|Malus domestica||Malus prunifolia||1234 pollinations; 313 fruits matured||Crandall 1926; Korban 1986)|
|Malus fusca||Malus domestica||117 pollinations; 11 fruits matured||Crandall 1926; Korban 1986)|
|Malus prunifolia||Malus baccata||No data||(Korban 1986)|
|Malus prunifolia||Malus domestica||181 pollinations; 54 fruits matured||Crandall 1926; Korban 1986)|
Outside of the genus Malus, the potential for natural hybridization with other genera appears to be limited. While there has been extensive intergeneric hybridization reported among closely related taxa (e.g., in the former subfamily Maloideae), a summary presented by Robertson et al. (1991) indicates no intergeneric crosses involving Malus spp. outside of breeding programs. A reported Malus × Chaenomeles hybrid was subsequently discounted by Rudenko (1976, cited in Robertson et al. 1991), and a proposal that the species M. florentina (Zuccagni) C. K. Schneid. was the product of hybridization between Malus and Sorbus sect. Torminaria (called ×Malosorbus) was also subsequently challenged by several authors who considered it a relictual species of Malus (e.g., Huckins 1972, cited in Robertson et al. 1991). This has been further supported by recent taxonomic work (Qian et al. 2008).
Breeding programs have produced intergeneric hybrids between apple and pear (Malus × Pyrus) and apple and hawthorn (Malus × Crataegus) as reported in Robertson et al. (1991), however these have been met with many difficulties and successful intercrosses relied on techniques such as embryo rescue (Banno et al. 2003). Forced intergeneric hybridization of Cydonia (quince)with Malus results in fertile genotypes which have been identified as an artificial hybrid genus ×Cydomalus, though the seedlings produced are generally weak and of low viability and germinability (Bell and Leitão 2011).
5.2 Potential for introgression of genetic information from Malus domestica into relatives
There does appear to be potential for gene introgression from M. domestica into its congeners in Canada, although the extent to which this might happen is still unclear. A study conducted by Kron and Husband (2009) in southern Ontario examined populations of the introduced diploid M. domestica and the native tetraploid crabapple M. coronaria, and found that their geographic ranges and flowering times overlapped sufficiently for cross-pollination to occur. In addition, 27.7% of seed from open-pollinated fruit was found to be of hybrid origin. This suggests that inter-crossing does happen in natural populations, and that there is potential for gene flow from domestic apples into native crabapple populations. However, the ability of the resulting hybrid plants to survive and backcross with M. coronaria is unknown at this time, and the adult trees within the populations were all identified to be distinct species (either M. domestica or M. coronaria).
This correlates with a number of European studies, which have suggested that the gene pools between wild and cultivated Malus spp. remain fairly distinct when feral M. domestica trees are present in native Malus populations (Coart et al. 2006; Coart et al. 2003; Larsen et al. 2006; Larsen et al. 2008) . These studies used molecular and other analyses on adult trees from natural populations and showed that hybridization between M. domestica and native Malus species is possible but occurs at low frequencies. Coart et al. (2006) evaluated hybridization between M. domestica and the European wild crabapple M. sylvestris by looking at nuclear microsatellites from a large sample of trees, the majority from Belgian forests, and found that 11% of the sampled M. sylvestris trees were of hybrid origin (Coart et al. 2006). In the case of Larsen et al. (2008), the study also investigated reproductive success of hybrid seed originating from handmade interspecific crosses between M. domestica and M. sylvestris, measuring fruit set, seed production, germination percentages of resulting seeds and subsequent seedling development. Results showed that interspecific hybridization yielded viable seeds which exhibited normal growth and development up to young seedlings and the authors note that it could be expected that substantial hybridization will occur given the overlap in geographical distribution and flowering time of the two species. However, the authors also note that the lack of observed hybrid individuals in natural populations suggests the possibility of some other type of reproductive barrier operating to maintain genetically distinct populations (Larsen et al. 2008). The nature of this potential barrier is unknown at this time.
5.3 Summary of the ecology of relatives of Malus domestica
There are four Malus species present in Canada in addition to M. domestica (see Section 5). Two of these are native to North America (M. coronaria and M. fusca), while the other two are introduced (M. baccata and M. prunifolia) (Brouillet et al. 2010+; CFIA and NRCan/CFS 2011+; Kartesz 1999; Scoggan 1979; USDA-NRCS 2012).
M. coronaria (sweet crabapple) occurs in eastern North America, with a range that corresponds to the Carolinian forest zone (Kron and Husband 2009; Little 1979). It is considered "uncommon to common" within this zone (Little 1979). It is reported from 23 U.S. states bordered by Kansas and Wyoming in the west and Alabama and Georgia in the south, continuing up the eastern seaboard to New York, Michigan and Wisconsin in the north (USDA-NRCS 2012). In Canada, it is at the northern edge of its range, and is present only in southern Ontario (CFIA and NRCan/CFS 2011+; Kron and Husband 2009). Its habitat is described as "low ground, thickets, and clearings" (Scoggan 1979) and "woods and thickets" (Gleason and Cronquist 1991). In Ontario, it occurs in areas that have been subject to European farming practices for at least 200 years, and where feral populations of M. domestica are common (Kron and Husband 2009). Today, about half of its range falls within major commercial fruit-growing regions, and in about two thirds of this area M. domestica is the primary fruit crop (Kron and Husband 2009).
M. fusca, the Oregon or Pacific crabapple, occurs in western North America with a range that includes California, Oregon and Washington, as well as coastal British Columbia and southern Alaska (Brouillet et al. 2010+; CFIA and NRCan/CFS 2011+; Kartesz 1999; Scoggan 1979; USDA-NRCS 2012) . It is adapted to wet, often disturbed habitats, described as "moist woods, streambanks, swamps and bogs" (Scoggan 1979) and "moist to wet, open forests, streambanks, upper beaches, shoreline thickets, estuary fringes, swamps and bogs in the lowland zone" (E-Flora BC 2012). In British Columbia, it is common on coastal islands and the adjacent mainland (E-Flora BC 2012).
M. baccata, the Siberian crabapple, is introduced in North America and has spread from cultivation to become established in the northeastern U.S. and Canada (Scoggan 1979; USDA-NRCS 2012). It is reported from 13 northeastern U.S. states, from Minnesota and Missouri in the west to Kentucky in the south, Massachusetts on the east coast, and Maine, New York and Michigan in the north (Kartesz 1999; USDA-NRCS 2012). In Canada, it is reported from Ontario, Quebec, New Brunswick, Nova Scotia, and possibly Newfoundland and Labrador (Brouillet et al. 2010+; CFIA and NRCan/CFS 2011+; Kartesz 1999; Scoggan 1979; USDA-NRCS 2012) . It occurs in thickets and clearings, and along the banks of rivers and old abandoned railways (Scoggan 1979).
M. prunifolia, the plum- or pear-leaved crabapple, is also introduced in North America and like M. baccata, has spread from cultivation to become established in the northeastern U.S. and Canada (Scoggan 1979; USDA-NRCS 2012). It has a similar range in the U.S., reported from 13 states from Minnesota to South Carolina, up to Massachusetts on the east coast, and Maine, New York and Michigan in the north (Kartesz 1999; USDA-NRCS 2012). In Canada, it is reported from New Brunswick and Nova Scotia (Brouillet et al. 2010+; CFIA and NRCan/CFS 2011+; Kartesz 1999; Scoggan 1979; USDA-NRCS 2012) . It occurs along roadsides, and in thickets and riverbanks (Scoggan 1979).
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