Rosaceae and the systematics of fruit crops

Subfamily Amygdaloideae (within Spiraeoideae)

This is the subfamily with the largest number of economically important genera. It includes the former Prunoideae and Maloideae, as well as a number of genera traditionally assigned to Spiraeoideae (Potter et al., 2007).

Genus Prunus L. — the largest genus of the subfamily, including about 200 species (Kalkman, 2004). Many of them are the most important fruit crops of the temperate zone. The subgenus Prunus (plum) includes P. domestica (European plum) and P. salicina (Japanese plum). Plum fruits rank third among stone fruit crops by world production volume after peach and nectarine (Gull et al., 2023). Plum is used fresh, for drying (prunes), canning, and the production of juices and alcoholic beverages (Gull et al., 2023). The subgenus Cerasus (cherry) includes P. avium (sweet cherry) and P. cerasus (sour cherry). Sweet cherry is valued for its large sweet fruits consumed fresh; sour cherry is more acidic and widely used for processing (Rieger, 2010). The subgenus Amygdalus (peach and almond) includes P. persica (peach) and P. dulcis (almond). Peach is one of the most popular stone fruit crops, cultivated in warm temperate and subtropical regions (Rieger, 2010). Almond is cultivated for its edible seeds (kernels), which are used in the confectionery industry (Rieger, 2010). The subgenus Armeniaca (apricot) is represented by P. armeniaca, whose fruits are consumed fresh and dried (dried apricots, apricot leather), and are also used for the production of juices and jams (Rieger, 2010). The subgenera Padus (bird cherry) and Laurocerasus (cherry laurel) are mainly of ornamental and medicinal importance (Kalkman, 2004).

The genus Prunus is monophyletic, but its subgenera require revision; in particular, the subgenus Amygdalus may be paraphyletic (Potter et al., 2007; Bortiri et al., 2001). This is important for breeding, since interspecific hybrids within the genus are widely used to create new cultivars (Gull et al., 2023).

Genus Malus Mill. — apple. This is the most important pome crop of the temperate zone. World apple production exceeds 70 million tonnes per year, and the planted area is about 5 million ha (Colavita et al., 2021). The cultivated apple Malus domestica Borkh. (often designated as Malus × domestica) originates from the wild species M. sieversii (Ledeb.) M. Roem., growing in the mountains of Central Asia (Luby et al., 2001; Ferree and Warrington, 2003). Apples are consumed fresh, dried, canned, and used for the production of juices, cider, puree, and vinegar (Colavita et al., 2021). Apple cultivars are extremely numerous; only a few dozen are used in commercial horticulture, while more than 7500 cultivars have been described in total (Colavita et al., 2021). Apple is also an important object of breeding for resistance to diseases, especially scab (Venturia inaequalis) (Brown and Maloney, 2003; Ferree and Warrington, 2003).

Genus Pyrus L. — pear. The second most important pome crop after apple. The European pear Pyrus communis L. is cultivated in the temperate latitudes of Europe, North America, and other regions (Jackson, 2003). Asian pears (nashi, P. pyrifolia, P. ussuriensis, and others) are cultivated in East Asia (Jackson, 2003). Pear fruits are consumed fresh and used for canning, drying, and the production of juices and cider (Rieger, 2010). Pear is less widespread than apple because of its greater heat requirements and shorter storage life of the fruits (Jackson, 2003). Rootstocks are also important, in particular quince (Cydonia oblonga), used for grafting pear (Jackson, 2003).

Genus Cydonia Mill. — quince. The single species Cydonia oblonga Mill. is an ancient fruit crop originating from Northern Iran, Turkmenistan, and Transcaucasia (Abdollahi, 2021). Quince fruits are hard and astringent, used mainly after culinary processing — for jam, jelly, marmalade, and also as an additive to apple and pear products (Abdollahi, 2021). World quince production is relatively small (about 650 thousand tonnes in 2014), but it is of great local importance in the countries of the Middle East, the Caucasus, and Central Asia (Abdollahi, 2021). Quince is also widely used as a rootstock for pear, especially in Europe (Jackson, 2003; Abdollahi, 2021).

Genus Mespilus L. — medlar. Mespilus germanica L. is a small tree or shrub whose fruits are eaten after prolonged storage (so-called “bletting”), when they become soft and sweet (Kalkman, 2004). It is currently cultivated to a limited extent, mainly in Southern Europe and Asia Minor (Kalkman, 2004). The closely related genus Eriobotrya (loquat) includes E. japonica, whose fruits are popular in the subtropics (Kalkman, 2004).

Genus Crataegus L. — hawthorn. Numerous species (about 200) are distributed in the temperate zone of the Northern Hemisphere (Kalkman, 2004). The fruits of some species are edible and are used to make jams, jellies, and liqueurs. Hawthorn is widely known as a medicinal plant used for cardiovascular diseases (Rieger, 2010). Many species and cultivars are cultivated as ornamental plants because of their beautiful flowers and fruits, and also as hedges (Kalkman, 2004).

Genus Sorbus L. — rowan. It includes trees and shrubs distributed in the temperate zone. Rowan fruits contain a significant amount of vitamins and are used as food after processing (juices, jam, liqueurs), and also as a medicinal remedy (Kalkman, 2004). Some species, for example S. aucuparia, are cultivated as ornamentals (Rieger, 2010). The genus is taxonomically complex because of hybridization and polyploidy (Kalkman, 2004; Potter et al., 2007).

Subfamily Rosoideae

It includes several economically important genera, including berry and ornamental crops.

Genus Fragaria L. — strawberry. The cultivated strawberry Fragaria × ananassa (Duchesne) Guedes is a hybrid that arose in Europe in the 18th century from a cross between the North American F. virginiana and the Chilean F. chiloensis (Rieger, 2010; Kalkman, 2004). The strawberry fruit is false: the juicy enlarged receptacle (hypanthium), on the surface of which the true fruits — achenes — are located (Kalkman, 2004). Strawberry is one of the most popular berry crops, cultivated both in open ground and under protected conditions (Rieger, 2010). World strawberry production exceeds 4 million tonnes (Rieger, 2010). The berries are consumed fresh and used for freezing, canning, and the production of jams, juices, and desserts (Rieger, 2010).

Genus Rubus L. — raspberry, blackberry, stone bramble, etc. It includes several hundred species distributed almost cosmopolitanly (Kalkman, 2004). Cultivated raspberry (R. idaeus L.) and blackberry (Rubus subg. Rubus) are important berry crops. The fruit is an aggregate of drupes consisting of many small drupelets (Kalkman, 2004). Raspberry and blackberry are used fresh, frozen, and for the production of jam, juices, syrups, and liqueurs (Rieger, 2010). Some species, for example R. chamaemorus (cloudberry), are collected in the wild (Kalkman, 2004). The genus has a complex taxonomy because of apomixis and hybridization (Kalkman, 2004; Potter et al., 2007).

Genus Rosa L. — rose, dog rose. It includes about 100–150 species distributed in the temperate zone of the Northern Hemisphere (Kalkman, 2004). Rose is the most important ornamental crop; tens of thousands of cultivars have been bred (Kalkman, 2004). Dog rose fruits are rich in vitamin C and are used in medicine and the food industry (Rieger, 2010). Essential oil (rose oil), used in perfumery, is obtained from the petals of some species (Kalkman, 2004). Rose also serves as a rootstock for cultivated cultivars (Kalkman, 2004).

Subfamily Dryadoideae

Genus Dryas L. — mountain avens. Small creeping dwarf shrubs distributed in the Arctic and alpine regions of the Northern Hemisphere (Potter et al., 2007; Kalkman, 2004). They have no economic importance, but play an important role in tundra and high-mountain ecosystems. They are of interest for studying symbiotic nitrogen fixation (Potter et al., 2007).

Subfamily Spiraeoideae (in the narrow sense, excluding Amygdaloideae and Maloideae)

Genus Spiraea L. — spiraea. Numerous species (about 80) are distributed in the temperate zone of the Northern Hemisphere (Kalkman, 2004). They are widely cultivated as ornamental shrubs because of their abundant and prolonged flowering (Kalkman, 2004). Some species are used in folk medicine (Rieger, 2010). They have no economic importance as fruit crops.

Thus, the economically most important genera of Rosaceae are concentrated in the subfamilies Amygdaloideae (Prunus, Malus, Pyrus, Cydonia, etc.) and Rosoideae (Fragaria, Rubus, Rosa). The subfamilies Dryadoideae and Spiraeoideae (in the narrow sense) are represented mainly by wild and ornamental plants. Understanding the phylogenetic relationships between these genera is important for breeding, since it allows the targeted use of wild relatives to improve cultivated cultivars.

Life forms

Within Rosaceae, almost all main life forms of flowering plants occur. Among them there are large trees: apple (Malus), pear (Pyrus), plum, cherry, sweet cherry, peach, apricot (Prunus), rowan (Sorbus), hawthorn (Crataegus) (Kalkman, 2004; Potter et al., 2007). Shrubs are widely represented: raspberry and blackberry (Rubus), rose and dog rose (Rosa), spiraea (Spiraea), cotoneaster (Cotoneaster), quince (Cydonia) (Kalkman, 2004). There are also herbaceous plants: strawberry (Fragaria), lady's mantle (Alchemilla), cinquefoil (Potentilla), avens (Geum) (Kalkman, 2004; Rieger, 2010). Finally, there are creeping dwarf shrubs, for example mountain avens (Dryas) from the subfamily Dryadoideae (Potter et al., 2007). It is believed that herbaceousness within Rosaceae is an apomorphic, secondarily derived state, while woody forms are plesiomorphic (Kalkman, 2004).

The hypanthium — a key structure

The unifying principle for all these forms is the structure of the flower, above all the presence of a hypanthium. The hypanthium is a cup-shaped, tubular, or bell-shaped structure formed from the fused bases of sepals, petals, and stamens (Kalkman, 2004; Potter et al., 2007). The perianth and androecium are attached to its rim, and the inner surface is often lined with nectariferous tissue (Kalkman, 2004). The hypanthium may be free from the ovary or fuse with it to varying degrees. It is precisely the degree of fusion of the hypanthium with the ovary and the number of carpels that determine the fruit type (Kalkman, 2004; Potter et al., 2007).

In Prunus, the hypanthium is free, the ovary is superior, and the fruit is a drupe. In Malus and Pyrus, the hypanthium fuses with the ovary almost halfway or more, the ovary is inferior, and the fruit is a pome. In Fragaria, after fertilization the hypanthium grows into a juicy, fleshy receptacle bearing numerous small achenes on its surface. In Rubus, the hypanthium is convex, and the fruit is an aggregate of drupes consisting of many small drupelets (Kalkman, 2004; Rieger, 2010). Thus, the same organ — the hypanthium — gives rise to different fruit types in different phylogenetic lineages.

Main fruit types in Rosaceae

The family contains practically all main fruit types known in flowering plants (Potter et al., 2007; Rieger, 2010).

Follicle — a dry, dehiscent fruit formed by one carpel. It is characteristic of Spiraea, Gillenia, Exochorda, Vauquelinia, Kageneckia (Potter et al., 2007; Kalkman, 2004). Several follicles developing from one flower form a follicular aggregate fruit. The follicle is considered the most plesiomorphic fruit type in the family (Potter et al., 2007; Kalkman, 2004).

Achene — a dry, indehiscent, one-seeded fruit. It occurs in Fragaria, Potentilla, Alchemilla, Acaena, etc. (Kalkman, 2004). In strawberry (Fragaria), the achenes are embedded in the enlarged fleshy receptacle, which is often mistakenly taken for the fruit. In fact, the strawberry fruit is an aggregate of achenes, and the juicy flesh is the enlarged hypanthium, that is, a false fruit (Kalkman, 2004; Rieger, 2010).

Drupe — a juicy fruit with a fleshy mesocarp and a lignified endocarp (stone) containing one seed. It is characteristic of Prunus — plum, cherry, sweet cherry, peach, apricot, almond (Kalkman, 2004; Potter et al., 2007). Almond is an example of a drupe in which the fleshy mesocarp (hull) dries and splits at maturity, and the edible part is the seed enclosed in the woody endocarp (Rieger, 2010).

Aggregate of drupes — a collective fruit consisting of many small drupelets located on a common receptacle. It is typical of Rubus — raspberry, blackberry, stone bramble (Kalkman, 2004; Rieger, 2010).

Pome — a juicy fruit formed by the enlarged hypanthium fused with the ovary. It is characteristic of Malus, Pyrus, Cydonia, Mespilus, Crataegus (Kalkman, 2004; Potter et al., 2007). In apple and pear, the edible flesh is largely formed by the hypanthium, while the carpels themselves form the leathery or cartilaginous core (Kalkman, 2004). In hawthorn, the fruit is a multi-drupe with woody pyrenes surrounded by a fleshy hypanthium (Kalkman, 2004).

Drupe-like fruit with a leathery endocarp — in Cotoneaster, Pyracantha, Heteromeles, etc. (Potter et al., 2007). In Heteromeles, the fruit is a fleshy hypanthium containing two almost free, leathery fruitlets (Potter et al., 2007).

Evolution of fruits

Traditional ideas about the evolution of fruits in Rosaceae were simple: from the follicle as the initial type to achenes, drupes, and pomes, each of which arose once (Schulze-Menz, 1964; Kalkman, 2004). However, molecular phylogenetic studies have shown that the evolution of fruits in the family was much more complex and was accompanied by multiple independent origins of similar types (Potter et al., 2007; Morgan et al., 1994).

According to the reconstruction performed by Potter et al. (2007) based on analysis of nuclear and chloroplast sequences, the ancestral fruit type for Rosaceae was probably a follicle or a follicular aggregate fruit. Achenes, drupes, and pomes arose independently in different clades. For example, a drupe-like pericarp (lignified endocarp) occurs in four clades: in Pyrinae (hawthorn, etc.), Kerrieae, Osmaronieae, and Amygdaleae (Prunus). However, in each of these groups the drupe-like fruit is accompanied by different additional characters: in Pyrinae by a fleshy hypanthium enclosing the fruit, in Kerrieae by the absence of a fleshy mesocarp, in Amygdaleae by a free hypanthium and superior ovary (Potter et al., 2007). This means that similarity of fruits does not always reflect close relationship, but may be the result of convergent evolution.

Another example is juicy fruits. The pome (Malus, Pyrus) and the drupe (Prunus) are outwardly and functionally similar: both attract animal dispersers with juicy flesh. However, their morphogenesis is fundamentally different. In apple, the juicy flesh is formed mainly from the hypanthium, while in Prunus it is formed from the mesocarp of the carpel. This once again confirms that fruit type cannot serve as the only criterion for distinguishing large taxa (Potter et al., 2007; Kalkman, 2004).

Why fruit morphology is important for systematics and breeding

For systematics, fruit morphology has a dual significance. On the one hand, fruit type is a convenient and easily observed character that has traditionally been used to delimit subfamilies and tribes (Kalkman, 2004). On the other hand, molecular data have shown that similar fruit types arose repeatedly, and therefore they cannot be used to distinguish monophyletic groups without additional characters (Potter et al., 2007). The modern classification of Rosaceae is based primarily on molecular phylogenetic data, while fruit morphology is used as an additional, but not determining, criterion (Potter et al., 2007; APG II, 2003).

For the breeding of fruit crops, knowledge of fruit morphology and genetics is of paramount importance. Fruit type determines its economic use: drupes (Prunus) are suitable for fresh consumption, drying, and canning; pomes (Malus) for long storage and the production of juices, cider, and puree; aggregate drupes (Rubus) for freezing and processing; strawberry achenes (Fragaria) for desserts (Rieger, 2010). Breeders take into account fruit size, juiciness, sugar and acid content, storability, and disease resistance (Brown and Maloney, 2003; Colavita et al., 2021). In addition, understanding the evolutionary relationships between fruit types allows the targeted use of wild relatives for introgression of genes controlling desirable fruit traits (Potter et al., 2007; Luby et al., 2001).

Thus, Rosaceae demonstrates an amazing combination of a uniform floral ground plan with enormous diversity of life forms and fruits. The hypanthium acts as a key structure whose variations determine the morphological diversity of fruits. Modern phylogeny shows that this diversity arose as a result of multiple independent evolutionary events, rather than a single transformation of an ancestral type. Understanding these patterns is necessary both for constructing a natural system of the family and for practical breeding of fruit crops.

Morphological similarity does not always mean relationship

The main reason for the discrepancy between traditional morphological classification and modern phylogeny is convergent evolution. Similar fruit types arose independently in different phylogenetic lineages as a response to similar selective pressures (Potter et al., 2007). For example, juicy, fleshy fruits attracting animal dispersers formed repeatedly: in Malus and Pyrus (pome), in Prunus (drupe), in Rubus (aggregate drupe), in Fragaria (false fruit). In each case, morphogenesis is different, but the functional result is similar (Kalkman, 2004; Potter et al., 2007).

Traditional systematics, based on fruit type, did not distinguish homologous and analogous characters. Molecular data made it possible to identify which similarities are inherited from a common ancestor (homologies) and which arose independently (homoplasies). It turned out that many morphological characters used to distinguish subfamilies are homoplasies (Potter et al., 2007; Morgan et al., 1994).

Example 1: Polyphyly of Spiraeoideae

Traditional Spiraeoideae were distinguished on the basis of the presence of follicles — dry, dehiscent fruits. It was believed that this was the most primitive group, from which the other subfamilies originated (Kalkman, 2004). However, molecular data have shown that Spiraeoideae in the traditional circumscription are polyphyletic: genera with follicles are distributed among several independent clades (Morgan et al., 1994; Potter et al., 2002; Potter et al., 2007). For example, Spiraea and Holodiscus turned out to be closer to Dryas and Cercocarpus (Dryadoideae) than to Gillenia or Kageneckia (Potter et al., 2007). Thus, the presence of follicles is a plesiomorphic character retained in different lineages, rather than evidence of close relationship.

Example 2: Maloideae and Prunoideae nested within Spiraeoideae

Traditionally, Maloideae (apple, pear, hawthorn) and Prunoideae (plum, cherry, peach) were considered separate subfamilies opposed to Spiraeoideae. Molecular data have shown that all of them represent clades within one large group, to which the name Spiraeoideae must be assigned according to the rules of priority (Potter et al., 2007). In other words, the “spiraeoid” genera with follicles do not form a single group; some of them are closer to apples and plums than to other spiraeas (Potter et al., 2007; Morgan et al., 1994).

This discovery has important nomenclatural consequences. In the modern system proposed by Potter et al. (2007), the subfamily Spiraeoideae includes seven tribes, including Amygdaleae (former Prunoideae) and Pyreae (former Maloideae). The names Maloideae and Prunoideae are not used as subfamilies, since they do not correspond to monophyletic groups at the rank of subfamily (Potter et al., 2007).

Example 3: The genus Prunus is monophyletic, but the subgenera require revision

The genus Prunus in the traditional understanding includes five subgenera: Prunus (plum), Amygdalus (peach, almond), Cerasus (cherry, sweet cherry), Padus (bird cherry), and Laurocerasus (cherry laurel) (Kalkman, 2004). Molecular data confirm the monophyly of the genus Prunus in the broad sense, including Maddenia and Pygeum (Potter et al., 2007; Bortiri et al., 2001). However, the boundaries of the subgenera turned out to be less unambiguous. In particular, the subgenus Amygdalus may be paraphyletic with respect to Prunus and Cerasus (Bortiri et al., 2001; Potter et al., 2007). This means that the traditional division of the genus into subgenera, based on fruit morphology (drupe with juicy or dry mesocarp, presence or absence of grooves on the stone), does not reflect real phylogenetic relationships.

Example 4: The problem of the origin of Maloideae

One of the most striking examples of the contradiction between morphology and molecular data is the question of the origin of Maloideae (apple tribe). Traditionally, it was believed that Maloideae arose as a result of distant hybridization between an ancestral “prunoid” plant with x = 8 and a “spiraeoid” one with x = 9, which led to polyploidy and a base chromosome number of x = 17 (Sax, 1933; Phipps et al., 1991; Kalkman, 2004). This hypothesis of wide hybridization was long generally accepted.

However, analysis of sequences of the GBSSI gene (granule-bound starch synthase) showed that Gillenia (x = 9) is the sister group to the clade including taxa with x = 15 and x = 17, and not to the “prunoid” ancestors (Evans and Campbell, 2002; Potter et al., 2007). This means that the origin of Maloideae is connected not with wide hybridization between distant lineages, but with hybridization and polyploidization among closely related spiraeoid ancestors, one of which was close to Gillenia (Evans and Campbell, 2002; Potter et al., 2007). Moreover, the data indicate a North American origin of this group, since Gillenia, Lindleya, and Vauquelinia are distributed in North America, and Kageneckia in South America (Potter et al., 2007).

This example shows how molecular data can not only refine but also fundamentally change our understanding of the evolutionary history of groups that seemed well studied.

Example 5: Chromosome number as an alternative criterion

Even in early molecular works it was noted that chromosome number in Rosaceae may be a better indicator of relationship than fruit type (Morgan et al., 1994). Traditionally, genera with x = 9 but with achenes (for example, Adenostoma, Coleogyne) were assigned to Rosoideae, and genera with x = 15 and 17 but with follicles (for example, Vauquelinia, Kageneckia) to Spiraeoideae. Molecular data have shown that taxa with x = 9 producing achenes are actually closer to spiraeoid genera with follicles, and taxa with x = 15 and 17 producing follicles are closer to Maloideae with pomes (Morgan et al., 1994; Potter et al., 2007). Thus, chromosome number turned out to be a more conservative and phylogenetically informative character than fruit type.

Why this is important for systematics and breeding

The contradiction between morphology and molecular data does not mean that morphology is “wrong.” It means that morphological characters may be subject to convergent evolution and do not always reflect phylogeny. To construct a natural system, it is necessary to use a complex of characters, giving priority to those less subject to homoplasy (Potter et al., 2007; Judd et al., 2018).

For the breeding of fruit crops, this has practical significance. Traditionally, breeders crossed species within one subfamily, believing that closely related species would produce more viable offspring. However, molecular data show that some intergeneric crosses that were considered impossible or unlikely (for example, between Maloideae and Spiraeoideae) may be quite feasible, since these groups are phylogenetically closer than previously thought (Potter et al., 2007; Evans and Campbell, 2002). This opens new possibilities for introgression of valuable genes, for example, disease resistance, from wild spiraeoid relatives into cultivated apples and pears (Luby et al., 2001; Brown and Maloney, 2003).

In addition, understanding phylogeny makes it possible to predict which wild species may be donors of economically valuable traits. For example, Malus sieversii — the wild ancestor of the apple from Central Asia — turned out to be a valuable source of disease resistance and is currently actively used in breeding programs (Luby et al., 2001; Ferree and Warrington, 2003). Similarly, wild species of Pyrus and Prunus are used to improve cultivated cultivars (Jackson, 2003; Gull et al., 2023).

Thus, the modern phylogeny of Rosaceae does not reject morphology, but supplements and refines it. It shows that traditional morphological classifications based on individual characters (fruit type, chromosome number) may be artificial, and offers a more reliable basis for constructing a natural system. For systematics, this means a transition from “morphological” thinking to “phylogenetic” thinking, and for breeding, an expansion of possibilities for using the genetic resources of wild relatives.

Use of wild relatives for disease resistance

One of the main directions of modern breeding of fruit crops is increasing resistance to diseases and pests. Cultivated cultivars, having undergone long selection for productivity and fruit quality, often lose the resistance possessed by their wild relatives. Therefore, breeders actively involve wild species for introgression of resistance genes.

Apple (Malus)

The cultivated apple Malus domestica Borkh. originates from the wild species M. sieversii (Ledeb.) M. Roem., growing in the mountains of Central Asia (Luby et al., 2001; Ferree and Warrington, 2003). The genetic diversity of M. sieversii is enormous, and many trees of this species in nature have fruits resembling cultivated cultivars (Luby et al., 2001). This makes M. sieversii an extremely valuable source of genes for breeding, including disease resistance genes. For example, in US breeding programs (Cornell University, USDA-ARS), M. sieversii is used to create cultivars resistant to scab (Venturia inaequalis), powdery mildew (Podosphaera leucotricha), and fire blight (Erwinia amylovora) (Brown and Maloney, 2003; Ferree and Warrington, 2003).

Other wild Malus species also serve as donors of resistance. Malus floribunda Siebold ex Van Houtte is a source of the Vf gene, which provides resistance to scab; this gene has been transferred to many commercial cultivars, such as Florina, Liberty, Freedom, Goldrush (Brown and Maloney, 2003; Ferree and Warrington, 2003). Malus baccata (L.) Borkh. is used to increase frost resistance and scab resistance; Malus micromalus Makino is a source of the Vm gene, which also provides scab resistance (Brown and Maloney, 2003). Malus robusta (Carrière) Rehder is a donor of fire blight resistance used both in cultivar breeding and rootstock breeding (Brown and Maloney, 2003).

Pear (Pyrus)

The European pear Pyrus communis L. is cultivated in the temperate latitudes of Europe and North America (Jackson, 2003). Its wild relatives, primarily P. pyrifolia (Burm.) Nakai (Asian pear, nashi), P. ussuriensis Maxim., P. betulifolia Bunge, and P. calleryana Decne., possess valuable traits: disease resistance, winter hardiness, and adaptation to various soils (Jackson, 2003; Bell et al., 1996). For example, P. ussuriensis is distinguished by high frost resistance and is used in breeding to create winter-hardy cultivars; P. calleryana is resistant to fire blight and is used as a rootstock (Jackson, 2003; Rieger, 2010). P. pyrifolia served as the basis for creating Asian pear cultivars, which in recent decades have become increasingly popular in the world (Jackson, 2003).

Stone fruits (Prunus)

The genus Prunus L. includes the most important stone fruit crops: plum, cherry, sweet cherry, peach, apricot, almond (Gull et al., 2023; Kalkman, 2004). Wild species of this genus are widely used in breeding. For example, Prunus cerasifera Ehrh. (cherry plum) is a wild diploid species that, along with P. spinosa L. (blackthorn), is considered one of the ancestors of the cultivated plum P. domestica L. (Gull et al., 2023). Prunus cerasifera is also used as a source of disease resistance and as a rootstock (Gull et al., 2023). Prunus spinosa is a tetraploid species with high winter hardiness and resistance to some diseases; it also participated in the origin of P. domestica (Gull et al., 2023). Prunus salicina Lindl. (Japanese plum) originates from China and is an important source of genes for plum breeding in regions with a warm climate (Gull et al., 2023). For breeding peach (P. persica) and apricot (P. armeniaca), wild species from Central Asia and China are used, possessing disease resistance and low requirements for growing conditions (Rieger, 2010).

Hybridization and polyploidy in breeding

Hybridization is the main method of breeding fruit crops. It may be intraspecific (between cultivars of one species) and interspecific (between different species). Interspecific hybridization makes it possible to combine valuable traits of different species in one genotype, for example, high fruit quality of a cultivated cultivar and disease resistance of a wild species (Brown and Maloney, 2003; Rieger, 2010).

However, interspecific hybridization is associated with difficulties: incompatibility in crossing, sterility of hybrids, and disturbed segregation of traits in the progeny. To overcome these problems, special methods are used: “bridge” crosses, colchicine treatment to double the chromosome number, and embryo culture (Brown and Maloney, 2003; Rieger, 2010).

Polyploidy plays an important role in the evolution and breeding of Rosaceae. Many cultivated cultivars are polyploids. For example, among apples there are diploids (2n = 34), triploids (2n = 51), and tetraploids (2n = 68) (Colavita et al., 2021; Brown and Maloney, 2003). Triploid cultivars, such as Jonagold, Mutsu, Bramley's Seedling, are distinguished by large fruits and high yield, but cannot be used as pollinators because they form unbalanced gametes (Brown and Maloney, 2003; Colavita et al., 2021). In apple breeding, triploids are obtained by crossing diploids with tetraploids or through culture of unreduced gametes (Brown and Maloney, 2003). Triploid cultivars are also known in pear, for example, Beurré Bosc (Jackson, 2003). Among stone fruit crops, polyploidy is common in plum: P. domestica is hexaploid (2n = 48), P. cerasifera is diploid (2n = 16), P. spinosa is tetraploid (2n = 32) (Gull et al., 2023). Polyploidy can lead to an increase in fruit size, changes in ripening time, and increased resistance to unfavorable conditions (Gull et al., 2023; Rieger, 2010).

Cultivar assortment and its taxonomic basis

The cultivar assortment of fruit crops is the set of cultivars approved for use in a particular region. It is formed on the basis of breeding achievements and includes both traditional local cultivars and new ones created as a result of targeted breeding (Colavita et al., 2021; Gull et al., 2023).

The taxonomic basis of the assortment is important for cultivar identification, their legal protection, and breeding work. Each cultivar belongs to a certain species or hybrid. For example, all apple cultivars belong to the species Malus domestica (or Malus × domestica), pear cultivars to Pyrus communis or P. pyrifolia, plum cultivars to Prunus domestica or P. salicina (Kalkman, 2004; Rieger, 2010). Within a species, cultivars differ in morphological, physiological, and economic characters. For cultivar identification, morphological descriptors (fruit shape, skin color, ripening time) and molecular markers (SSR, AFLP, RAPD) are used (Potter et al., 2007; Gull et al., 2023).

The modern assortment of fruit crops is characterized by narrowing genetic diversity. For example, in world apple production a few cultivars dominate: Delicious, Golden Delicious, Fuji, Gala, Granny Smith, McIntosh, Jonagold (Colavita et al., 2021). These cultivars originate from a limited number of ancestors, which creates genetic vulnerability to new diseases and pests. Expanding the assortment through new resistant cultivars created with the involvement of wild relatives is an important task of modern breeding (Brown and Maloney, 2003; Luby et al., 2001).

In recent decades, so-called “club” cultivars have become widespread; they are protected by trademarks and licensed to a limited circle of producers. Examples include Pink Lady (Cripps Pink), Envy, Kanzi, Jazz, Ambrosia, SweeTango, Honeycrisp (Colavita et al., 2021). Club cultivars make it possible to control product quality and obtain higher prices, but at the same time limit producers' access to genetic resources (Colavita et al., 2021; Brown and Maloney, 2003).

For breeding, it is important to take into account the taxonomic affiliation of cultivars, since the possibilities of crossing and inheritance of traits depend on it. For example, triploid apple cultivars cannot be used as pollinators; cultivars obtained from interspecific hybrids may have complex segregation of traits in the progeny (Brown and Maloney, 2003; Rieger, 2010). Understanding phylogenetic relationships between species makes it possible to predict compatibility in crossing and the probability of successful gene introgression (Potter et al., 2007).

Conclusion

Systematics and breeding of fruit crops are interconnected and mutually enrich each other. Systematics provides the breeder with knowledge about relationships, evolutionary history, and genetic diversity of plants, which makes it possible to purposefully use wild relatives to improve cultivated cultivars. Breeding, in turn, expands our understanding of the limits of species variability, the possibilities of hybridization, and the role of polyploidy in evolution. Modern molecular phylogenetic studies refine the taxonomic basis of the assortment, which is of paramount importance for the identification, protection, and breeding of fruit crops. Thus, knowledge of the systematics of Rosaceae is a necessary condition for successful breeding work and sustainable production of fruit products.