Hybridization and genetic variation in Danish populations of European crab apple (Malus sylvestris)

Abstract

The aim of the present study was to investigate the genetic variation in Danish populations of the endangered European crab apple (Malus sylvestris). Special emphasis was given to hybridization between the wild species and its cultivated relative Malus ×domestica. A total of 178 wild individuals from four Danish populations were studied along with a reference sample of 29 old cultivars. The genetic variation within and among samples was studied at ten microsatellite marker loci. Additionally, a morphological analysis was carried out to identify hybrids and test for correspondence between phenotypic and genotypic indices of hybridization.From application of ordination and a model based cluster analysis to the molecular data, two clusters were identified consisting of wild and cultivated individuals respectively. This indicates that pronounced admixture between the two species is not present. At the population level, a high correspondence was found between geographic isolation from M. ×domestica and genotypic and morphological indices of hybridization. As expected, isolated populations appeared less affected by hybridization than poorly isolated populations. Isolated 'pure' M. sylvestris populations could thus be identified. However, morphological and molecular evidence of hybridization was found to be divergent at the individual level. This is suggestive of some historical introgression into the M. sylvestris gene pool, and indicates that relying exclusively on either morphological or molecular characters as diagnostic markers in studies of hybridization between M. ×domestica and M. sylvestris might lead to fallible results. Combined application of genetic and morphological markers is therefore recommended.3

Genetic diversity and apple leaf spot disease resistance characterization assessed by SSR markers

“…Guilford et High level of genetic diversity was expressed also in terms of total expected heterozygosity (He=0.84). The high level of heterozygosity of the cultivars is in accordance with former studies on M. sylvestris and M. domestica (Coart et al 2003, Larsen et al 2006, Koopman et al 2007). Allelic frequencies showed wide variations, ranging from 0.017 to 0.828.…”

Introgression and Long-Term Naturalization of Archaeophytes into Native Plants - Underestimated Risk of Hybrids

“…Introgression between cultivated apple (Malus pumila, syn. M. ×domestica) and a wild relative (Malus sylvestris) in Europe is already known (Coart et al, 2003Larsen et al, 2006). These cases are comparable to our case of Pyrus ussuriensis in terms of introgression between archaeophytic cultivated fruit trees and wild relatives.…”

Crossability Between Wild (Malus sylvestris) and Cultivated (M. x domestica) Apples

“…Where such information is not available, it is recommended to perform an investigation by morphological and /or molecular means in order to establish the hybridization status of the candidate populations (cf. COART et al, 2003;LARSEN et al, 2006). However, the observed high degree of crossability, combined with apparently relatively few hybrids occurring in at least some of the Danish populations, indicates that other types of reproductive barriers must provide some protec- Brought to you by | MIT Libraries Authenticated Download Date | 5/9/18 6:27 PM tion against hybridization.…”

“…This is because of the predominance of short distance pollination events in natural populations (LARSEN and KJAER, unpublished). In addition, differences in flowering phenology between the two species ('isolation in time') are also thought to play a role (LARSEN et al, 2006).…”

“…Ontogenetic and phenotypic variation however, is likely to hamper such studies. Therefore, these studies have recently been supplemented by molecular studies which demonstrate that it is still possible to find genetically authentic populations of M. sylvestris with only small traces of hybridization or introgression (COART et al, 2003;WAGNER et al, 2004;LARSEN et al, 2006). At least this is true for the more or less dense populations that have been included in the reported studies.…”