Ploidy, Genome Size, and Cytogenetics of Apple

Abstract

AbstractApple (Malus) is one of the most important fruit crops. The cultivated apple, M. × domestica, can be either diploid or triploid, whereby the latter accounts for 10% of all currently cultivated apples. In this chapter, we will explore the hypothesis on the origin of the apple genus and its haploid chromosome number of x = 17. Furthermore, special attention will be paid to genome size which is commonly evaluated using flow cytometry. Genome size is a multipurpose parameter used in many fields of plant science regarding species evolution and taxonomy, cell biology, cell cycle, endoreplication, and organ development as well as in breeding efforts. Based on data collected for 120 Malus taxa in The Plant DNA C-values Database for species and hybrid species with the same number of chromosomes, it is found that the rate of genome size variation is relatively low. The reported nuclear genome sizes for Malus accessions, including species, hybrids, and cultivars of M. × domestica, range from 1.50 to 1.78 pg for diploids, 2.27–2.58 pg for triploids, and 3.13–3.37 pg for tetraploids. Differences between the smallest and the largest nuclear DNA contents are as follows: 0.45 pg (36%), 0.45 pg (21%), and 0.34 pg (12%) for diploids, triploids, and tetraploids, respectively. Furthermore, the roles of mitotic and meiotic polyploidization of apple involved in new assortment development as well as the influence of genome size/ploidy level on phenotypic traits (nucleotypic effects) will be discussed. It is observed that ploidy level significantly influences resistance to biotic and abiotic stresses, as well as secondary metabolite production, among other traits. Much attention will be devoted to genome downsizing of polyploids, as well as to methods of ploidy level evaluation, including chromosome count, flow cytometry, and molecular markers. In addition, this chapter presents the findings of cytogenetic studies of the Malus genus. As apple chromosomes are small, numerous, and difficult to analyze with classical methods, the use of new molecular cytogenetic techniques, such as fluorescence in situ hybridization (FISH), has enhanced investigations of genome composition in apple. Moreover, further progress has been achieved by exploiting bacterial artificial chromosomes (BACs) along with fluorescence in situ hybridization, BAC-FISH, thereby enabling the determination of physical locations of useful gene loci in the Malus genome.