Reticulate evolution and coffee

There are two Coffea lineages, C. arabica and Coffea canephora, used for the vast majority of coffee cultivation in both the Old and New Worlds. Raina et al. (1998) concluded that (1) coffee plants were cultivated in >50 countries and (2) this industry resulted in a trade of ~US$18 billion. More recently, Vega et al. (2003) quoted a value of US$70 billion for the retail value of coffee sales. Of this trade, C. arabica accounted for ~70% and C. canephora ~30% of the coffee products (Moncada and McCouch 2004). The greater proportion of the market captured by C. arabica reflected the higher quality beverage (i.e., low content of caffeine and pleasant aroma) produced by this cultivar (Raina et al. 1998). In the main, C. canephora's beans are utilized to manufacture instant coffee (Steiger et al. 2002). Of the more than 80 species of Coffea, C. arabica is unique in being the only tetraploid (44 chromosomes) and self-fertile form (Raina et al. 1998; Steiger et al. 2002). In contrast, the remainder of the species, including C. canephora, are self-sterile and diploid (22 chromosomes; Raina et al. 1998; Steiger et al. 2002). Though distributed in several areas of Ethiopia, it is likely that C. arabica originated in this country's southwestern highlands (Anthony et al. 2001). From this source, humans have moved C. arabica throughout the world. This movement began with its initial transport to Yemen (possibly as early as 575 AD) followed subsequently by importation into Java, Reunion Island, the Amsterdam Botanical gardens and, finally, the New World (Anthony et al. 2002). In the process of its cultivation and human-mediated migration, two varieties originated; these were termed "Typica" and "Bourbon" (Anthony et al. 2002).

Many studies have indicated the importance of natural hybridization in the evolution of genus Coffea and, in particular, the cultivars. Several data sets have detected signatures of introgressive hybridization involving various species, including genetic exchange between the cultivar and its diploid progenitors (Cros et al. 1998; Mahe et al. 2007). However, the most significant class of hybridization—in terms of the origin of the cultivar— was the allopolyploid formation of C. arabica. The origin of this taxon is now known to be from a natural allopolyploid speciation event (see discussion by Ruas et al. 2003). Though the allopolyploid derivation of this species is well established, identifying the diploid progenitors for this species has been problematic. Raina et al. (1998) applied in situ hybridization to test for the genomic components of C. arabica. Results from this analysis led to the inference of Coffea eugenioides and Coffea congensis as the progenitors of this tetraploid. In contrast, though Lashermes et al. (1999) also identified C. eugenioides as one parent of the cultivar, they identified the other major cultivated species, C. canephora, as the second progenitor lineage.

Recently, Ruas et al. (2003) applied inter-simple sequence repeat markers to test the earlier, conflicting hypotheses. Consistent with the findings of Lashermes et al. (1999), Ruas et al.'s (2003) data indicated the closest genetic associations between C. eugenioides, C. canephora, and C. arabica. Ruas et al. (2003) indicated additional support for C. eugenioides' contribution by reporting its status as the only wild, diploid species shown to produce a beverage with "fine aroma and flavor." They argued that one effect of the genome from this species in the allotetraploid C. arabica might be reflected in the presence of these aroma and flavor characteristics. However, the reticulation that led to the derivation of C. arabica also resulted in ecological novelty relative to its parents. Specifically, C. arabica is adapted to environmental conditions different from either C. eugenioides or C. canephora. This conclusion is supported by C. arabica's native range encompassing an area outside of the distribution of the diploid species (Lashermes et al. 1999). It thus appears likely that the allopolyploid speciation event resulting in C. arabica produced a lineage adapted to both a unique ecological setting and its use as a caffeine delivery system for humans.

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