Reticulate evolution and fruit crops coconut

The coconut palm, Cocos nucifera, exemplifies again observations made repeatedly in this and the two preceding chapters. First, this species has benefited numerous tropical, H. sapiens populations as a source of food and economic stimulus in countries such as India, Sri Lanka, Malaysia, Indonesia, Nigeria, Jamaica, and Brazil (Teulat et al. 2000; Manimekalai and Nagarajan 2006b). In 2003, 10.6 million hectares were utilized for C. nucifera plantations, yielding ~53 million nuts (Manimekalai and Nagarajan 2006a). The second observation that can be drawn from an analysis of coconut is that, though I have placed this species into the category of food source, coconut palm is a species with a multitude of uses. For example, parts of the coconut palm are utilized to produce drink containers, alcohol, furniture, activated charcoal, ropes, doormats, woven articles, thatch, and biodiesel (Bruman 1945; Corpuz 2004; Baudouin et al. 2006).

The domesticated and wild coconut palms are both placed within C. nucifera, but nonetheless reflect well-differentiated genetic lineages (Zizumbo-Villarreal and Piñero 1998; Zizumbo-Villarreal et al. 2002, 2005, 2006; Perera et al. 2003; Manimekalai and Nagarajan 2006a). Furthermore, their genetic differentiation is reflected in divergent morphologies, with the cultivars possessing the Niu vai-type and the wild populations the Niu kafa-type morphology (Zizumbo-Villarreal et al. 2006). A portion of the morphological differences between wild plants and cultivars likely reflects the enhanced ability of the Niu kafa-types to disperse by floating (Zizumbo-Villarreal et al. 2006) thus allowing oceanic colonization of islands and continents (e.g., the Cocos Islands in the Indian Ocean; Leach et al. 2003).

The ability of the wild coconut to disperse by floating could bring these lineages into contact with domesticated forms that have been introduced by humans thus facilitating introgressive hybridization. Furthermore, introgression between different cultivars brought together by the establishment of multiple plantations could also contribute to C. nucifera (wild and domesticate) genetic diversity. Indeed, it is well recognized that both wild-domesticate and domesticate-domesticate introgression have contributed to the genetic variation in contemporary wild and cultivar populations (Lebrun et al. 1998; Teulat et al. 2000; Perera et al. 2003; Manimekalai and Nagarajan 2006b). However, the avenue by which the cultivated and wild lineages have been brought together has more frequently involved the human-mediated transport of domesticated coconuts into areas previously occupied by wild C. nucifera (Zizumbo-Villarreal et al. 2006). Humans have catalyzed additional genetic exchange between "wild" and domesticated genomes by the importation and cultivation of both Niu vai-type and Niu kafa-type plants. This has been the case for Mexican populations of coconuts, which were introduced to both the Gulf of Mexico and Pacific coasts (Niu kafa and Niu vai, respectively; Zizumbo-Villarreal and Piñero 1998; Zizumbo-Villarreal et al. 2005). Subsequent to their importation, introgression occurred not only between different cultivars, but also between plants belonging to the Niu vai-type and Niu kafa-type lineages (Zizumbo-Villarreal et al. 2006). Numerous modes for genetic exchange have thus been identified among the worldwide C. nucifera populations.

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