Reticulate evolution and fruit crops citrus

The genus Citrus contains numerous cultivars that are grown and utilized extensively throughout the world. Indeed, as the United Nations Conference on Trade and Development reported, "Citrus fruits are the first fruit crop in international trade in terms of value" (UNCTAD 2005). Some of the Citrus species of most value, both as a source of food and economic strength, are the grapefruit, lime, lemon, pummelo, citron, mandarin, and sour and sweet oranges (Citrus paradisi, Citrus aurantifolia, Citrus limon, Citrus grandis, Citrus medica, Citrus reticulata, Citrus aurantium, and Citrus sinensis, respectively; Moore 2001). Although 140 countries contribute to the production of these cultivars, over two-thirds of all citrus fruits originate in Brazil, various Mediterranean countries, the United States, and China (UNCTAD 2005). Of the Citrus producing countries, those in the Mediterranean provide over half of the fresh fruit exports, while Brazil and the United States are centers for processed fruits (mostly represented by orange juice; UNCTAD 2005). Notwithstanding the use made of citrus fruits by the various cultivators, the nutritional and economic gains from this farming industry are undeniable. Once again we use the United States as an example of economic benefit: 2006 receipts for grapefruits, lemons, and oranges alone totaled > US$2,500 million (Pollack and Perez 2007).

The origin of the genus, Citrus, is hypothesized to have occurred in the subtropical and tropical zones of Southeast Asia, followed by dispersal to other continents (see Nicolosi et al. 2000 for references). However, the evolutionary relationships among the cultivated species have been difficult to discern, likely due to extensive hybrid lineage formation and introgression (Moore 2001). In this regard, Barrett and Rhodes (1976) argued for the recognition of only three "true biological species," those being C. grandis (pummelo), C. medica (citron), and C. reticulata (mandarin). The remaining Citrus domesticated species were recognized as having a hybrid derivation (Barrett and Rhodes 1976; Nicolosi et al. 2000; Moore 2001; de Moraes et al. 2007).

Figure 7.8 illustrates the complicated, reticulate evolutionary history for grapefruit, lime, lemon, sour orange, and sweet orange. The complexity of these derivations is obvious, but one example is indicative of the vagaries of the cultivars' origins. This involves the formation of the (very) different hybrid species recognized as C. aurantium and C. sinensis—that is, the sour and sweet orange, respectively. Although both of these species formed through hybridization between the pummelo and mandarin lineages (with most of their genomes

Sour orange Sweet orange

Grapefruit

Mandarin * Pummelo

Figure 7.8 The reticulate evolutionary history of grapefruit, lime, lemon, pummelo, citron, mandarin, and sour and sweet oranges (C. paradisi, C. aurantifolia, C. limon, C. grandis, C. medica, C. reticulata, C. aurantium, and C. sinensis, respectively). ??? indicates the genetic contribution by unidentified species to the formation of both limes and lemons. Arrows indicate the genetic contributions in the origin/evolution of the various cultivars (adapted from information in Barrett and Rhodes 1976, Nicolosi et al. 2000 and Moore 2001).

Figure 7.8 The reticulate evolutionary history of grapefruit, lime, lemon, pummelo, citron, mandarin, and sour and sweet oranges (C. paradisi, C. aurantifolia, C. limon, C. grandis, C. medica, C. reticulata, C. aurantium, and C. sinensis, respectively). ??? indicates the genetic contribution by unidentified species to the formation of both limes and lemons. Arrows indicate the genetic contributions in the origin/evolution of the various cultivars (adapted from information in Barrett and Rhodes 1976, Nicolosi et al. 2000 and Moore 2001).

derived from C. reticulata; Barrett and Rhodes 1976; Nicolosi et al. 2000), the resulting cultivars produce fruit with significantly different flavor characteristics (Moore 2001). The hypothesis constructed to explain such divergent evolutionary endpoints involved their derivation from introgression of pummelo into different mandarin subspecies (Barrett and Rhodes 1976). Finally, and somewhat ironically, genetic analyses have shown that mandarin as well (i.e., one of the three "true" species) contains numerous recombinant genotypes (Filho et al. 1998). Similarly, Barrett and Rhodes (1976), while naming the mandarin as one of the three biologically relevant lineages, also discussed the formation of the heterogeneous C. reticulata variants from introgression both among mandarin subspecies as well as from C. grandis.

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