Chloroplast diversity in the genus Malus: new insights into the relationship between the European wild apple (Malus sylvestris (L.) Mill.) and the domesticated apple (Malus domestica Borkh.)

Abstract

To unravel the relationship between the European wild apple, Malus sylvestris (L.) Mill., and its domesticated relative M. domestica Borkh., we studied chloroplast DNA variation in 634 wild and 422 domesticated accessions originating from different regions. Hybridization between M. sylvestris and M. domestica was checked using 10 nuclear microsatellites and a Bayesian assignment approach. This allowed us to identify hybrids and feral plants escaped from cultivation. Sixty-eight genotypes belonging to 12 other wild Malus species, including 20 M. sieversii (Ledeb.) Roem. accessions were also included in the analysis of chloroplast diversity. Marker techniques were developed to type a formerly described duplication and a newly detected transversion in the matK gene. Chloroplast DNA variation was further investigated using PCR-RFLP (Polymerase Chain Reaction-Random Fragment Length Polymorphism), and haplotypes were constructed based on all mutational combinations. A closer relationship than presently accepted between M. sylvestris and M. domestica was established at the cytoplasmic level, with the detection of eight chloroplast haplotypes shared by both species. Hybridization between M. sylvestris and M. domestica was also apparent at the local level with sharing of rare haplotypes among local cultivars and sympatric wild trees. Indications of the use of wild Malus genotypes in the (local) cultivation process of M. domestica and cytoplasmic introgression of chloroplast haplotypes into M. sylvestris from the domesticated apple were found. Only one of the M. sieversii trees studied displayed one of the three main chloroplast haplotypes shared by M. sylvestris and M. domestica. This is surprising as M. sieversii has formerly been described as the main maternal progenitor of the domesticated apple. This study hereby reopens the exciting discussion on the origin of M. domestica.

Malus ×oxysepala (M. domestica Borkh. × M. sylvestris Mill.) – a new spontaneous apple hybrid

“…This is not surprising in view of that indicate a considerable participation of M. sylvestris in the formation of new cultivars [17]. Generally, M. sieversi and M. orientalis are considered as the main ancestor for cultivated M. domestica [2,23,24]. On the other hand, studies based on the analysis of chloroplast DNA indicate much closer relations than previously believed between M. sylvestris and M. domestica in terms of the origin [25], which clearly indicates the possible spontaneous hybridization between them.…”

The genome of the domesticated apple (Malus × domestica Borkh.)

“…This could explain apple's lack of domestication syndrome, which is the loss of sexual reproduction, seed dispersion and seed dormancy. Despite evidence of intrageneric hybridizations 14,45 , the possibility of substantial genetic contributions to the domestic gene pool of other wild Malus species, such as M. sylvestris 14 , was rejected in our analysis.…”

“…Although M. sieversii has been considered to be the ancestor of the domesticated apple 13 , this has been challenged by the identification of molecular similarities between domestic apple and M. sylvestris 14 . To test these two hypotheses, we surveyed molecular differences at 23 genes across the genus Malus (Supplementary Table 22 A r t i c l e s modern apple breeding 15 (Supplementary Note).…”

Assessment of Genetic Diversity in Crop Plants - An Overview

“…Further, these markers may be based on variation in genomic DNA/ RNA, ribosomal RNA or organelle genome sequences. Chloroplast microsatellites have been developed [5] and used in assessment of genetic diversity at intra-specific level in wheat [23], barley [24], apple [25], rice [26], pearlmillet [27] etc. Mitochondrial DNA in plants, in contrast, has been demonstrated to be an unsuitable tool for studying genetic diversity, being quantitatively scarce.…”