Biology, speciation and utilization of peanut species

Abstract

Peanut, also known as groundnut (Arachis hypogaea L.), is a native new world crop. Arachis species originated in South America and are found in tropical and subtropical areas. Eighty-one species have been named (Krapovickas and Gregory, 1994; Valls and Simpson, 2005; Valls et al., 2013), including the domesticated peanut, A. hypogaea L. Species have evolved in highly diverse habitats and both annual and perennial types exist. New species are being discovered in areas that previously were very difficult to reach because of poor roads and transportation. It is likely that the genus originated in the highlands in the southwestern Mato Grosso do Sul region of Brazil close to Gran Pantanal where the most ancient species of the genus (Arachis guaranitica Chodat. and Hassl. and Arachis tuberosa Bong. Ex Benth.) are found (Gregory et al., 1980; Simpson and Faries, 2001). Subsequently, as the planalto continued to be uplifted coupled with water flow, the genus spread into the drier lowlands of South America (Gregory and Gregory, 1979; Stalker and Simpson, 1995; Simpson et al., 2001). The genus likely originated in tropical wetland areas and subsequently adapted for survival in dry environments. Species in the genus Arachis are widely distributed in South America from Northeast Brazil to southern Uruguay and from the Andean lowlands in the west to the eastern Atlantic coast, and the distribution is continuous across this region (Valls et al., 1985). Species grow in deep friable sand to thick, gummy clay and on schist rocks with virtually no soil, suggesting that species have adapted to highly diverse and harsh environments (Simpson et al., 2001). Fruiting below ground likely protected the seeds from predators and the many root adaptations (e.g., rhizomes, tuberous roots) likely helped species to adapt to new habitats. Conversely, the geocarpic fruit impeded rapid spread into new environments. Krapovickas and Gregory (1994) indicated that the most defining morphological features of the genus are underground plant parts, including the fruits, rhizomatous structures, root systems, and hypocotyls. The center of origin for the cultivated species A. hypogaea is believed to be southern Bolivia to northwestern Argentina based on the occurrence of the two progenitor species Arachis duranensis and Arachis ipaënsis, and archaeological evidence gathered in this region (Hammons, 1982; Stalker and Simpson, 1995). Simpson et al. (2001) also suggested that the eastern slopes of Cordillera may be a possible area for origin of A. hypogaea because of the favorable environment for peanut growth. Advances in the peanut genome sequence and the availability of new genomic tools will help clarify the origin and evolution of the cultivated and wild species of the genus Arachis. Wild peanut species were important as sources of food in pre-Columbian times and several taxa are still widely used as forages or for their aesthetic value as a ground cover. Arachis glabrata and Arachis pintoi are utilized for grazing and Arachis repens is used as a ground cover in residential areas and roadsides in tropical regions (Mathews et al., 2000; Hernandez-Garay et al., 2004). Two wild species (Arachis villosulicarpa Hoehne and Arachis stenosperma Krapov. and W.C. Gregory) were cultivated by indigenous people in Brazil for food and medicinal use, albeit on a limited scale (Gregory et al., 1973; Simpson et al., 2001), but only A. hypogaea is economically important today as a human food source. Importantly, many Arachis species have extremely high levels of disease and insect resistances that are not present in cultivated peanut.