Abstract
The genusPhaseolus, native to the Americas, is composed of more than eighty wild species, five of which were domesticated in pre-Columbian times. Since the beginning of domestication events in this genus, ample opportunities for gene flow with wild relatives have existed. The present work reviews the extent of gene flow in the genusPhaseolusin primary and secondary areas of domestication with the aim of illustrating how this evolutionary force may have conditioned ecological fitness and the widespread adoption of cultigens. We focus on the biological bases of gene flow in the genusPhaseolusfrom a spatial and time perspective, the dynamics of wild-weedy-crop complexes in the common bean and the Lima bean, the two most important domesticated species of the genus, and the usefulness of genomic tools to detect inter and intraspecific introgression events. In this review we discuss the reproductive strategies of severalPhaseolusspecies, the factors that may favor outcrossing rates and evidence suggesting that interspecific gene flow may increase ecological fitness of wild populations. We also show that wild-weedy-crop complexes generate genetic diversity over which farmers are able to select and expand their cultigens outside primary areas of domestication. Ultimately, we argue that more studies are needed on the reproductive biology of the genusPhaseolussince for most species breeding systems are largely unknown. We also argue that there is an urgent need to preserve wild-weedy-crop complexes and characterize the genetic diversity generated by them, in particular the genome-wide effects of introgressions and their value for breeding programs. Recent technological advances in genomics, coupled with agronomic characterizations, may make a large contribution.
Highlights
The genus Phaseolus is native to the Americas with wild species growing from Connecticut in the United States [for P. polystachyus (L.) B.S. & P.: Dohle et al (2019)] down to the province of Córdoba in Argentina [for wild P. vulgaris L.: Drewes (2008)]
Two independent domestications occurred for both the Lima bean (Motta-Aldana et al, 2010) and the common bean (Chacón et al, 2005; Bitocchi et al, 2013) in Mesoamerica and the Andes of South America, while a single domestication occurred for the tepary (Muñoz et al, 2006), the scarlet runner (GuerraGarcía et al, 2017) and the year-bean (Schmit and Debouck, 1991), all of them in Mesoamerica
Landraces were diffused outside their areas of origin to different places in the Americas, and after 1493, P. vulgaris, P. lunatus and to a lesser degree P. coccineus were introduced in different parts of the Old World, as cultivated varieties for their edible seeds (Kaplan and Kaplan, 1992)
Summary
The genus Phaseolus is native to the Americas with wild species growing from Connecticut in the United States [for P. polystachyus (L.) B.S. & P.: Dohle et al (2019)] down to the province of Córdoba in Argentina [for wild P. vulgaris L.: Drewes (2008)]. Due to the widespread distribution of wild Phaseolus species in the Americas, the multiple cases of domestication and the worldwide diffusion of cultivated forms, the genus Phaseolus offers a great opportunity to examine the role and extent of gene flow in primary and secondary centers of diversity and its evolutionary consequences. The fourth and fifth sections review in more detail the dynamics of wild-weedy-crop gene flow in the two most studied Phaseolus species, the common bean and the Lima bean. For the Lima bean, the Yucatán Peninsula represents a natural laboratory to study wild-weedy-crop gene flow in this species. This is the region that houses the largest number of native varieties of this crop in all of Mexico, which grow sympatrically with wild populations. The identification of these events will open hypotheses of spontaneous gene flow or early breeding efforts and can facilitate the identification of genetic drivers for different traits
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