The North American Plums ( Prunus Spp.): A Review of the Taxonomic and Phylogenetic Relationships

North America is a center of diversity for Prunus species. Tree architecture, chilling requirement, heat requirement, fruit development period, fruit size, fruit texture, disease resistance, and adaptive changes to multiple environmental condi-tions are a few examples of the traits of which tremendous genetic variability is available in the native plum species. Wild native Prunus species constitute an important potential source of genetic diversity for stone fruit breeding and selec-tion. A review of the North American plum taxonomic treatment and phylogenetic studies is described. Various studies have been done with three major groups being identified: Americana series, Chickasaw series, and Beach series.


Introduction
The genus Prunus L. belongs to the subfamily Amygdaloideae (=Prunoideae) of the Rosaceae. It has a worldwide distribution with approximately 200 species. Edible species are mostly distributed in the northern hemisphere [1][2][3][4][5]. The genus contains species that are important in the production of fruit, nuts, and lumber. Plums, cherries, almonds, apricots, and peaches are the most commonly known fruit and nuts in this genus. The world's net production of almonds, apricots, cherries, peaches, nectarines, plums, and sloes in 2010 was approximately 40.8 million tons. Peach and nectarine production was the largest in the world with 20.5 million tons. US peach and nectarine production was approximately 1.3 million tons, with a farm gate value of $683 million dollars [6].
North America is an important center of diversity for plum species adapted (native) to widely divergent climates and soils representing an important potential source of genes for plant breeding. In [7], Layne and Bassi reported high levels of variation in the Prunus germplasm for tree size, growth habit, flower size and color, chill hour requirement, fruit size, flesh texture, flesh color, resistance to diseases, and adaptability to diverse climatic and geographic regions. Plums are the stone fruit with the greatest diversity of flavor, aroma, texture, color, form, and size [2,8].
Stone fruit breeders have used this tremendous genetic variability through interspecific hybridizations (in particular with species in the subgenus Prunus or  [8,22] (Table 1).

Prunus phylogenetic studies
Phylogeny and systematics in the genus Prunus was reported by [23]. They employed isozymes to study the phylogenetic relationships in Prunus. Section Prunocerasus was found to be polyphyletic, with a clade formed by P. americana, P. munsoniana, P. hortulana, P. subcordata, and P. angustifolia, and a clade formed by P. maritima Marshall and P. umbellata.
Chloroplast DNA is an alternative source of genetic variation and is maternally inherited in Prunus. Chloroplast DNA is highly conserved and in relative abundance in the cell as compared with the nuclear DNA. Kaneko et al. [24] and Uematsu et al. [25] used cpDNA to classify cherries, apricots, and wild and cultivated peaches in Japan. In [26], Badenes and Parfitt reported a phylogeny similar to Mowrey and Werner [23]. All the Prunus species were grouped as in conventional subgenus classifications [3]. Prunus persica L.-P. dulcis (Mill.) D.A. Webb, P. domestica L.-P. salicina Lindl., and P. cerasus L.-P. fruticosa Pall were monophyletic. Lee and Wen's [27] phylogenetic analysis of the genus Prunus using ITS sequences recognized two major groups: the Amygdalus-Prunus group, and the Cerasus-Laurocerasus-Padus group. The results were not congruent with Rehder's [3] taxonomic treatment.
Bortiri et al. [28] used the nuclear gene s6pdh, which encodes NADP + -dependent sorbitol-6-phosphate dehydrogenase, to assess the lack of support for deep nodes in the clade subgenera Prunus-Amygdalus-Emplectocladus (as reported in previous data). The phylogenies based on ITS, cpDNA trnL-trnF, and s6pdh sequences were compared and combined. Phylogenetic analysis of the combined data supported two major clades: subgenera Cerasus-Laurocerasus-Padus and subgenera Amygdalus-Emplectocladus-Prunus. Section Microcerasus (subgenera Cerasus) was reported nested within subgenus Prunus.
Similarly, a survey of cpDNA haplotypes available within section Prunocerasus was reported by Shaw and Small [30]. The cpDNA rpL16 region was sequenced for 207 accession representatives of 17 North American plums, including P. texana D. Dietr. (as described before). More than one of the three primary cpDNA haplotypes was found in many of the taxa.
Bortiri et al. [31] studied the evolution of vegetative and morphological characters of 37 species of Prunus and other genera of Rosaceae. Morphological characters were combined with ITS, trnL-trnF, and trnS-trnG data from previous studies [1,28]. The addition of the morphological data with trnS-trnG supported some nodes that were found in ITS and trnL-trnF studies. Three clades were reported: "Clade A" with subgenera Padus and Laurocerasus; "Clade B" with subgenera Amygdalus, Emplectocladus, and Prunus; and "Clade C" with subgenera Cerasus. "Clade B" was characterized by the production of three axillary buds. Padus and Laurocerasus were not supported as monophyletic (high homoplasy).
Genetic diversity within Prunus cerasifera (cherry plum) was studied using morphological characters, cytometry, cpDNA, and SSR markers [32]. Morphological characters showed differences between clones. Analysis of cpDNA reported 15 haplotypes clustered in 3 groups. Considerable diversity among accessions was reported based on these studies.

Phylogeny in classification
Genus

Notes
The congeneric relationship of plums to peach and cherry allowed the successful use of these primers in section Prunocerasus.
Microsatellites are evolving too rapidly to be truly useful at resolving species phylogeny The common practice of choosing one specimen to represent a taxon can be misleading in closely related groups. Choosing different genotypes could have resulted in a different result than previous studies The 9.1 kb region between psbA and atpA genes would be useful tool to study the cpDNA evolution in Prunus

Morphology and molecular Molecular
Analytical methods
Chavez et al. [39] identified genomic regions that provided the greatest number of characters and variability and improved the phylogenetic signal at the low level in Prunus section Prunocerasus relationships. The American and the Chickasaw clades were identified. An outgroup clade was comprised by P. persica and P. fasciculata. The results reported were similar to those reported by Mowrey and Werner [23].
Previous studies demonstrated the value of morphology, cytometry, nuclear DNA, and cpDNA as data for phylogenetic studies in Prunus. Most of the previous phylogenetic research used Mason's [21] and Rehder's [3] taxonomic classification. A complete summary of Prunus phylogenetic research is summarized in Table 2.

Final remark
The subgenus Prunus section Prunocerasus (the North American plums) constitutes important genetic resources (gene pool) of unique traits such as tree architecture, chilling requirement, heat requirement, fruit development period, fruit size, fruit texture, disease and insect resistance, and adaptive changes to multiple environmental conditions, among others. These species could be used in the breeding of improved stone fruit cultivars in the future. The summary of the taxonomic and phylogenetic relationships presented in this chapter provides a base to understand the species relationships. In addition, it will help for the conservation and maintenance of a broader germplasm base within Prunus.

Author details
Dario J. Chavez 1 * and José X. Chaparro 2 1 University of Georgia, USA 2 University of Florida, USA *Address all correspondence to: dchavez@uga.edu © 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.