Origin, TAXONOMY, cytogenetics and evolution of arachis

Origin and evolution of oil yielding crop- groundnut

1. Ground Nut – ORIGIN, EVOLUTION, TAXONOMY and CYTOGENETICS

2. INTRODUCTION Scientific name – Arachis hypogaea L Common names _Ground nut,peanut, monkey nuts,goober nuts,earth nuts.. Ground nut is a major world crop,ranking thirteenth among the food crops. It’s high oil and protein content – needs for food, energy and in industry. It is native to south America ( Brazil) but it now cultivated in over 80 countries both tropical and warm temperature regions. Peanut is one of the world’s major source of vegetable oil.

3. Groundnuts are rich in nutrients, providing 30 essential nutrients and phytochemicals. They are a good source of niacin which plays a role in brain health and blood flow. Groundnuts contain 25 % protein, a higher proportion than any nut. Recent research on groundnut, in general, has found antioxidants and other chemicals. Scientific name – Arachis hypogea ( Arachis and hypogea are Greek words, Arachis means- legumes and hypogea means below the ground ).

4. Origin and Domestication Peanut is a large – growing annual or perennial plant that is distinguished from most other species by producing aerial flowers but fruiting below the soil. Arachis hypogaea L is the only domesticated species in the genus. The primary centre of diversity for the species is the chaco region between southern Bolivia and north west Argentina. For edible seed- ( A.villosulicarpa and A. stenosperma ) and For Forage- ( A.glabrata , A . Pintoi ).

5. The domesticated peanut is attacked by many disease and insect pests, with early leaf spot ( cercospora arachidicola ),late leaf spot ( cercosporidium personatum ) ,rust ( puccinia Arachis) and virus being widespread wherever the crop is grown. Many other problems are regional in nature,with sclerotina blight ( sclerotinia minor) ,white mold or stem rot ( sclerotium rolfsii ) ,nematodes ( Melodogyne spp ) being most important diseases in the US. Aflatoxin is a major problem that solved by testing seed samples from farmers field and remove the contaminated ones. Most cultivated one- Arachis hypogaea L.

6. IN INDIA.. Andhrapradesh, Maharashtra, Gujarat and Madras states have the largest acreages. Although important acreages are grown also in Madhya Pradesh, Utter Pradesh and Mysore. Plant was introduced by the Portuguese Jesuit father’s in 16 th century,who followed Vasco de gama shortly after his landing in Malabar coast. While used primarily as a oilseed crop in India,large quantities of ground nut are consumed directly as food .

7. Centres of Diversity 6 organised gene centers for cultivated ground nut in south America. 1 .The Guareni region. 2. Goias and Minas Gerais ( Brazil) 3. Rondonia and northwest Mato Grosso ( Brazil). 4. The eastern foothills of the Andes in Bolivia. 5. Peru 6. Northeastern Brazil, secondary centre of diversity ( Africa)

8. EARLY HISTORY The most conclusive evidence for origin of cultivated ground nut from peru. First, established in south America and as for North as Mexico before the arrival of Europeans. It was regularly cultivated for over 3500 years ,during which time numerous morphological forms have evolved. It is believed that ground nut were first domesticated in North – western Argentina and southern Bolivia,the probable area of origin. Subsequently spread of the crop to other agroclimatic regions.

9. In 1838, Bentham published his ‘ flora Braziliensis ’ describing five species of Arachis,all from Brazil. Arachis hypogea is thought to have arisen as 4000 years ago from a single hybridization event between two diploid Arachis species. A genome from A.duranensis ,B genome from A. ipaensis followed by spontaneous chromosome doubling of the sterile hybrid to form a fertile allotetraploid . A.monticola is the only tetraploid known to be cross compatible with A.hypogaea . Africa represents another centre of diversity for the cultivated peanut.

10. Most authorities believe that the Portuguese carried two seeded ground nut varieties from the east coast of South America to Africa,to the Malabar coast of South – Eastern India Middle of 16 th century, ground nut had made their way to North America from Africa as well as from the Caribbean islands, central America and Mexico and were then distributed world wide. By 19 th century,the ground nut had become an important food crop in west Africa, India and the USA .

11. RECENT HISTORY The genetic resources in ground nut include landraces and wild Arachis species from South America,land races from the centre of diversity in Africa and breeding material more recently developed in several countries. ICRISAT consist 13,000 accessions from 89 countries and function as the major world ground nut germplasm repository. Initial genetic improvement in the crop was made by individual plant and pure line selection.

12. Improvement through hybridization was initiated in the USA by Hull and Carver and spread else where after the 2 nd World war. Traditionally, Hybridization has been followed by pedigree selection. However,with changing requirements many new methods have been adopted eg , convergent crossing, composite crossing,interspecific hybridization,single seed descent ,early generation selection and development of synthetic varieties, multiple lines. These efforts lead to the development of many varieties from different parts of the world.

13. Genetic Evolution of Arachis The cultivated peanut Arachis hypogaea is considerd a segmental allotetraploid composed of the A and B genomes, with probable origin by amphidiploidization of an AB hybrid. Cultivated peanuts ( A.hypogaea ) arose from a hybrid between two wild species of peanuts, probably A. duranensis and A. ipaensis . The initial hybrid would have been sterile,but spontaneous chromosome doubling restored its fertility . Genetic analysis suggests the hybridization event probably occurred only one and gave r is e to A . moticola ,a wild form of arachis ,that occurs in a few restricted location in northern Argentina by artificial selection to A.hypogaea . The process of domestication through artificial selection made A.hypogaea dramatically different from its wild relatives.

14. Most of the species are diploids with 2n = 2x = 20, but there are at least two allotetraploids with 2n = 2x = 40, including cultivated Arachis hypogea L . and wild Arachis monticola . The cultivated groundnut was probably derived from the wild tetraploid, A. monticola , which is native to north-western Argentina, and freely hybridizes with it. For a long time, the most likely diploid progenitors were thought to be Arachis cardenasii and Arachis batizocoi , but more recent molecular and cytological data indicate that the progenitors are more closely related to Arachis duranensis and Arachis ipaensis . A. duranensis carries the A genome, while A. ipaensis possesses the B genome.

15. TAXONOMY

16. The cultivated groundnut an annual herb belonging to the family Fabaceae (Leguminosae ), is classified into two subspecies,subsp. Fastigiate( bunch type ) and subsp.hypogaea ( runner type ). The subsp.fastigiate contains 4 botanical varieties , var.vulgaris ,var.fastigiate ,vr. Peruviana and var. aequatoriana (Krapovikas and Gregory in 1994). The subspecies hypogaea contain 2 varieties ,var.hypogaea and var. hirsute. Each of these botanical types has different plant, pod and seed characteristics .

17. Ground nut belongs to leguminosae family.Height – 6-12 inches. Produce angular,hairy stems,with spreading and erect branches. It has a relatively deep tap root system with well developed lateral root system. Leaves – pinnate, opposite, pairs, alternately occur,one at each node. Flowers borne at the axils of the leaves either above or below the ground. Solitary or clusters of three or more .

18. Flowers are self pollinated,open between 6-8 Am.have yellow petals,Two wing and two keel petals. Androecium and gynoecium enclosed in keel petals. Stames- 10 in number,monadelphous bundle,8 stamens are functional. Long calyx tube. Unilocular ovary – at the base a meristemic region grows and becomes a stalk like structure called gynophore,that bends downwards and forces the ovary in to the soil. Gynophore referred to as ‘ peg ’. The fruit is an indehiscent pod containing one to five seeds. The nut is composed ot two cotyledons which contains oil and other food materials.

19. Ground nut Gene pool Based on the cross compatibility relationships ,the Arachis gene pool has been classified into primary, secondary and tertiary gene pools. Arachis hypogaea and A. monticola are two tetraploid species of section Arachis grouped under primary , while all other diploid species of section Arachis fall in secondary gene pool. Species that belong to section other than Arachis are grouped under tertiary gene pool.

20. Cytogenetics The chromosomes of groundnut are small, ranging from 1.3 to 6.0 µm in length, mostly metacentric and are difficult to karyotype. The genus Arachis is classified into seven sections.three series of annuals, perennials and amphidiploids. There are 4 annual diploids ( 2n= 20) ,several perennial diploids and two annual tetraploids (2n = 40). Chromosome number in A . hypogaea is 2n= 4x= 40. Allopolyploid. These chromosomes pair mostly as bivalents. Somatic chromosome are small and most have median centromere. Aneuploids are reported as a result of interspecific hybridization .

21. the cultivated peanut ( Arachis hypogaea  L.) is of hybrid origin and has a polyploid genome that contains essentially complete sets of chromosomes from two ancestral species. The genome has evolved through mobile-element activity, deletions and by the flow of genetic information between corresponding ancestral chromosomes (that is, homeologous recombination). Genetic exchange between ancestral genomes could be inferred towards the ends of colinear pairs of homeologous chromosomes. In these regions, the genome structure was not the expected AABB, but may be better described as AAAA or BBBB, that is, ‘ tetrasomic ’ conformations. 

22. Groundnut is an segmental allotetraploid ( 2n=2x=40 )with AA and BB genomes. ex-A.hypogaea (oil seed ), A.glabrata (fodder and cover crop ). All species except the cultivated species in section Arachis are diploid (2n=2x=20) ex- A . repens (fodder and cover crop ). The diploid progenitors , A.duranensis and A.ipaensis , contributed “AA” and “BB” genome respectively

23. Raman and kesavan (1962) reported the first hybrid among wild species in the genus Arachis between A . duranensis and A . villosa var. correntia .hybrids were fertile. The most extensive single hybridization program conducted by Gregory and Gregory (1979) who reported the cross compatibility relationships among 91 accessions of Arachis species. A . monticola is a tetraploid, is the only wild Arachis taxon which can be readily crossed with A . hpogaea to produce fertile progeny. First chromosome count reported for a wild species was 2n = 40 for A . glabrata .

24. Two distinct set of chromosomes are found in ground nut ,one marked By a pair of chromosomes significantly smaller( A chromosomes) and a set that carries a pair with a secondary constriction ( B chromosomes).

25. Lavia (1996, 1998) reported a chromosome number of x=9 for A. palustris and A . praecox (section Arachis), which was also found for A. décora ( section Arachis) by Penaloza et al. It reveals that there are two series of chromosome numbers that appear to occur in the genus (2n=2x=20 and 2n=4x=40). Lavia (1998) was of the opinion that since the diploid forms are more predominant, the basic chromosome number is believed to be x=10 and proposed that basic chromosome number x=9 in species A. palustris and A. praecox might have originated by loss of a chromosome from the other species having n=10. The presence of two basic chromosome number (x=9 and x=10) and less existence of polyploid species indicate that aneuploidy has played a key role in the evolution and speciation of Arachis species rather polyploidization.

26. Therefore, the species diversity of Arachis may be mainly due to structural chromosomal rearrangements. Aneuploid complements have been reported in A. hypogaea sporadically. Husted (1936) first reported a plant showing 2n=41 plus a chromosome fragment. The most extensive reports of aneuploidy in the genus has arisen as a result of interspecific hybridization. Eight different trisomics or doubled trisomics (2n+1+1) were reported by Stalker (1985). Cytologically, the extra chromosome behaved as a trisomic

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