Groundnut Taxonomy
The genus Arachis is a member of family Fabaceae (synonym: Leguminosae) subfamily Papilionoidae, tribe Aeschynomeneae and subtribe Stylosanthinae (Polhill and Raven, 1981). At the time Linnaeus first named the cultivated groundnut Arachis hypogaea L., it was the only member of the genus. One hundred years later Bentham (1841) produced the first taxonomic treatment of the genus. Subsequent taxonomic treatments include Chevalier (1933), (1934), (1936) Hoehne (1940) and Hermann (1954). These treatments are now largely outdated because of the number of new taxa that have been collected and described in the past forty to fifty years (Gregory et al., 1973). Unfortunately during this surge in germplasm collection, many new taxa were invalidly described, other unofficial names came into common use and different names were sometimes assigned to the same species and vice versa (Resslar, 1980). The basis for the confusion was a lack of recognised differentiating morphological descriptors, as well as fragmentary early collecting and the representation of species by seedling specimens.
The genus was in a state of chaos until 1994 when Krapovichas and Gregory published a taxonomic revision of the genus. This work took them nearly thirty-five years, and involved re-visiting and collecting specimens at the type locations of each known species. This taxonomic revision recognises 69 species in 9 sections. Distinctions are made on the basis of morphological characters and life cycle attributes, although eco-geographic distribution, crossability evidence, cytological information, plant form, as well as chromatographic and antigenic reactions were all considered in the groupings. The currently recognised classification of the genus Arachis can be found here, and a dichotomous key here. All species, except the cultivated species and A. monticola, in Section Arachis, and certain species in Section Rhizomatosae, are diploid (2n=2x=20).
Section Arachis contains the cultivated groundnut A. hypogaea, which is itself divided into two subspecies, subsp. fastigiata Waldron and subsp. hypogaea Krap. et Rig. Previously each subspecies was divided into two varieties (Gregory et al., 1973); subspecies hypogaea contained var. hypogaea and var. hirsuta, and subsp. fastigiata contained var. vulgaris and var. fastigiata. However in 1994, Krapovickas and Gregory proposed two new varieties of subsp. fastigiata in addition to the existing ones, namely var. peruviana and var. aequatoriana. A key to the currently recognised botanical varieties of A. hypogaea can be found here.
Wild Arachis species are endemic to South America, occurring in Argentina, Bolivia, Brazil, Paraguay and Uruguay. Both Krapovickas (1969), (1973) and Gregory et al., (1980) postulated a planalto profile from Corumba to Joazeiro, Brazil, as the centre from which distribution of Arachis occurred. Distribution maps of species can be generated through the wild Arachis database, here. The geocarpic habit of peanuts (developing seeds underground) is an unusual feature of their life-cycle that has interesting consequences for dispersal and population structure. Seeds appear to be largely dispersed by water and species distribution co-incides to a large extent with the main river systems. Different sections are associated with different river basins (Gregory et al., 1973).
The cultivated groundnut is thought to have originated from the domestication of A. monticola, the only other tetraploid species in Section Arachis (Smartt et al., 1978; Singh, 1986,1988). Both A. monticola and the cultivated groundnut are allotetraploids, which means they have four sets of chromosomes (four times the haploid chromosome compliment), with two of these chromosome sets being identical (named A), but differing slightly from the other two sets (named B), which are also identical. The ‘A’ set has a pair of chromosomes that are conspicuously smaller than the other chromosomes. The allotetraploid form is likely to have arisen from the combination of unreduced gametes of two diploid species (two times the haploid chromosome compliment) through a process known as amphidiploidisation. The search for these two donor species, the wild ancestors of the cultivated groundnut, has been long and hard. For many years the donor of the B genome was thought to be A. batizocoi (Singh, 1988). It was only in 1996, when Kochert et al. provided RFLP and cytological evidence that it became accepted that A. duranensis and A. ipaensis contributed the A and B genomes respectively, to the cultivated groundnut, with A. duranensis being the female parent.
Not much is known about the origin and subsequent domestication of peanut. The centre of origin is thought to be in the region of Southern Bolivia and northern Argentina Krapovickas, 1969; Gregory et al., 1980; Kochert et al., 1996). This is an important centre of diversity for cultivated groundnut and A. duranensis occurs throughout the region. A. ipaensis has only been found in southern Bolivia, and A. monticola has been found in northern Argentina (see the wild Arachis database here).
Domestication probably first took place in the valleys of the Parana and the Paraguay river systems in the Gran Chaco area of South America. Early European explorers found native Indians cultivating this crop in many islands in the Antilles, on the northeastern and eastern coasts of Brazil in all warm regions of the Rio de la Plata basin, extensively in Peru and sparsely in Mexico (Hammons, 1994).
The centre of diversity of the genus is in an area encompassing parts of Western Brazil, Bolivia, Paraguay and Northern Argentina (Gregory et al., 1980). Singh and Simpson (1994), classified the genetic diversity of the genus in to four genepools: