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1. Introduction
Maize (Zea mays L.) is cultivated in almost every continent, showing specific adaptations to different climates and soils1)(2. In the Southern Cone (Argentina, Brazil, Bolivia, Chile, Paraguay and Uruguay) there is a wide variety of maize landraces3)(4 that have been kept by small-scale farmers in traditional ways over the centuries5. The region has been a meeting place of different ethnicities, through the colonization and immigration of different cultures6. This fact has led to an important development of maize landraces, evolving into a secondary center of genetic diversity of the crop7)(8)(9.
A comprehensive study of the races of maize that were grown in the South America lowlands4 identified 19 races and 15 sub-races. Since this classification, which was later expanded by Sánchez and others10, broad studies on the racial diversity of Zea mays L. in this wide geographic area have not been executed11. Recent studies with high sampling intensity in micro-regions in southern Brazil revealed a currently greater number of races of maize5)(12. Furthermore, the presence of wild relatives of maize in sympatric coexistence with landraces, along with the identification of new races12, has led to the indication of micro-centers of diversity2.
The lowland middle South America region holds an important locally adapted maize gene pool13, with many traits of agronomic interest that can be incorporated in breeding programs. This is the case of the Catetos, a group of races that shows high protein and carotenoid contents, aluminum tolerance14 among other qualities. Another race that has shown outstanding performance in grain production and forage yield is Blanco Dentado15)(16)(17.
The countries of the Southern Cone keep important ex situ maize collections in Germplasm Banks. Most of these collections were carried out in farmers’ fields in the 1970s, before the generalization of hybrids and transgenic cultivars. Ecogeographic data associated to the origin of the accessions has proved useful for germoplasm classifications in several crops18)(19)(20)(21)(22)(23. In Brazil, ecogeographic origin and kernel type, a highly discriminant variable24, were used to classify maize landraces25)(26 and select the Brazilian core collection27. This paper describes geographical patterns of diversity in maize germplasm collections from the Southern Cone, to contribute to its conservation and use. The aim of this study was to map the geographic distribution of these collections and to analyze its genetic diversity based on kernel type.
2. Materials and Methods
2.1 Collection databases
The databases used are the Maize Genetic Resources Catalogues from Uruguay, Chile and Bolivia15)(28)(29. Databases from Argentina and Paraguay were provided by partners from the Instituto Nacional de Tecnología Agropecuaria (inta) and the Centro Regional de Investigación Agrícola (cria), respectively (2003 conversation with A. Hourquescos; unreferenced; 2003 conversation with O. Noldin; unreferenced). The database from Brazil proceeds from the Banco Ativo de Germoplasma (bag), and was supplied by M. L. Burle (2004 conversation with M. L. Burle; unreferenced). The total number of accessions assessed in this research was 7,680. The Uruguayan collection consists of 852 accessions, the Chilean collection consists of 898; Bolivia has 601 accessions; Argentina has 2,111; Paraguay has 467, and the Brazilian collection consists of 2,751 accessions.
2.2 Geographic coordinate assignment
The different catalogues presented differences at the level of accuracy of their geographic coordinates: Paraguay and Brazil displayed accuracy at the level of seconds; Bolivia and Argentina at the level of minutes, and Chile and Uruguay at the level of degrees. In the case of the Uruguayan database, additional information regarding descriptions of the collection sites was available30. Thus, an ex post assignment of the approximate geographic coordinates was made, taking into account three sources of information: 1) gazetteers available on the internet31, 2) maps in paper format, where routes, roads and locations were positioned, and 3) a digital map of the world, Map Source32, where distances were measured and geographically located. As for Chile, in order to assign the geographic coordinates, gazetteers and country maps were used to identify the different locations (towns or villages).
2.3 Geographic analysis
The software diva-gis33 was used to map the geographic distribution of the accessions. Richness and Shannon Diversity Indices according to grain type (kernel type) were performed. Richness is defined as the Number of different classes: different classes of a variable (e.g. the different grain types in a dataset) present in each grid cell33.
The Shannon Index (H) is defined as:
H = - ∑ p i lnp i
Where p i is the proportion of observations of kernel type i34.
The study area was divided into equally sized rectangles (grids). A grid divides a geographic region into equal-sized cells. The advantage of using grids is that cells of the same size and shape allow more objective comparisons33.
3. Results
3.1 Geographic distribution of maize collections
The location of the collection site of one or several accessions is represented by a point on the map, and represents the maize collection of the countries of the Southern Cone (Figure 1). In the Uruguayan collection, accessions concentrated mainly in three regions: departments of Canelones and San José (South of the country); Tacuarembó and Rivera (Northeast), and Soriano and Colonia (West). Chile showed a concentration of accessions in the Central zone (los Lagos, Araucanía, Bio Bio, Maule, Libertador O' Higgins, Metropolitana de Santiago, Valparaíso, Coquimbo and Atacama). In the Bolivian collection, accessions concentrated in the Central and Southern regions (Cochabamba, Chuquisaca and Tarija). As for Paraguay, the East part of the country is subdivided in four regions: North (Concepción and San Pedro), Center-South (Caaguazu, Guaira, Paraguari, Caazapa and Misiones); Northeast (Amambay and Canindeyu) and Southeast (Alto Paraná and Itapua), where accessions showed a relatively uniform distribution. Accessions from the Argentinian collection mainly concentrated in the provinces of Jujuy, Salta, Catamarca, La Rioja, Misiones and Buenos Aires. In Brazil, accessions concentrated in the Southeast, South and Northeast of the country (states of Sao Paulo, Minas Gerais; Río Grande do Sul and Northeast of Bahía, Alagoas, Pernambuco, Paraiba and Ceará).
The distribution of kernel types is represented on the map in Figure 2. Floury kernel types were mainly distributed in Central areas of the Southern Cone. Important concentrations of floury types were found in Paraguay, Bolivia, and North and Central Chile. In Argentina, this type of grain was restricted to areas bordering Uruguay (Buenos Aires province) and areas bordering Paraguay and Brazil (Chaco and Misiones). On the other hand, flint kernels were distributed throughout the entire region. In Brazil, an important group was observed in the Southeast and some accessions were scattered along the Atlantic coast. In Uruguay, flint kernels represented more than half of the collection, and were distributed throughout the country. In Chile, flint kernel type represented approximately forty percent of the national collection, with a dispersed distribution. In Argentina, these types were restricted to Buenos Aires province and to the North of the country. Dent kernel types concentrated along the Atlantic coast from Brazil to Uruguay. A small number of representatives of this grain type were registered in Bolivia and Chile. In Argentina and Uruguay dent kernel types were found in an isolated manner. Pop kernel types concentrated in areas that were formerly occupied by Guaraní indians. This type of grain was also found in the Central areas of Chile and Argentina, and in a scattered way in Brazil and Uruguay.
3.2 Richness and Diversity analysis
Red cells on the map show the areas where the highest Richness and Shannon Diversity Index values for kernel type were registered; whereas green cells reflect the lowest values (Figures 3 and 4). The maximum Richness values (7 different kernel types) were observed in the central region of Bolivia (Cochabamba, Chuquisaca, Potosí) and in the O’Higgins region in Chile. In Uruguay, up to six different kernel types were found in the South (departments of Canelones and San José), Northeast (Tacuarembó and Rivera) and West of the country (Soriano and Colonia) (Figure 3). The Shannon Diversity Index shows a similar pattern, with the highest values found in Chile (1.676), Bolivia (1.535 and 1.523), Paraguay (1.486), North of Argentina (1.386) and Uruguay (1.255) (Figure 4).
4. Discussion
Maize diversity is associated with farmers’ preferences and environmental conditions1)(35)(36)(37. The distribution of kernel types (pop, flint, floury, dent and sweet) is related to different stages in the process of domestication of the crop; it is assumed that primitive maizes were pop types, then appearing flints, floury types and dents3)(38. According to Paterniani and Goodman,4 Guaraní Indians grew a floury maize belonging to the Avatí Morotí race, a white flint from the Avatí Tupí race and two kinds of pop: one with rounded grains (Avatí Pichingá Ihú) and the other with pointed grains (Avatí Pichangá). As a general rule, floury types prevail among populations of indigenous descent, whereas most of the materials used in breeding programs are dents and flints that evolved in North and Central America.
The distribution of maize diversity is not homogenous throughout the Southern Cone; micro-centres of diversity are verified in the Central regions of Chile and Bolivia, Southern Paraguay, North of Argentina and Uruguay. The diversity values registered in this study are comparable to those obtained by Li and others39, who recorded average Shannon Index values of 1.408 for American countries, 1.077 for accessions from Europe, and 1.370 for Asia, Africa and Oceania. The results obtained in this work are consistent with the notion that the crop experienced a post-domestication diversity increase in this region40. Ecogeographic conditions define that certain genotypes are best adapted than others, in some cases due to long periods of interaction with the environment41. Socio-economic factors (including field size, grain destination and selection practiced by farmers, among others) have also been identified as of great significance in defining genetic diversity in crops41)(42.
Bolivia presents drastic variations in climates and elevations, with altitudes ranging from 100 to 6000 meters above sea level. An important cultural and linguistic diversity is found in this country, with the most important agricultural regions situated in the Altiplano and in the Temperate and Tropical Highland Valleys43. The country has the largest number of maize races recorded in America (77 races), surpassing Mexico, that has 65 races44. In Chile, mountains make up about 70 percent of the country, with elevations ranging from sea level to more than 6000 meters. Chile has a rich cultural pattern and agricultural system. Most of its agriculture is confined to the Central Valley, where Indians traditionally cultivated maize as one of the most important staple crops45. Paraguay is naturally divided by the Paraguay River into two distinct regions: The Eastern and the Western region (the Chaco region). There are substantial geographic, topographic and climatic differences between these two regions46. Conditions in the eastern region are generally favorable for maize production47. Maize has historically been one of the most important agricultural crops in Paraguay. Production practices and farming systems vary considerably in this country. In this sense, small-scale farmers, which cultivate landraces of maize as a subsistence crop coexist with medium to large-size famers, which grow this crop mainly for animal feed46. According to Morris and Alvarez47, by the year 1991 around 48 percent of the maize area was planted with local varieties, known by their Guaraní language names, whereas improved varieties and hybrids occupied 25 and 27 percent, respectively.
In the pre-Columbian era, a trail referred to as Peabiru path was constructed, connecting the Atlantic and the Pacific Oceans. A study of alcohol dehydrogenase 2 (ADH2) allele sequences48)(49 in landraces from Peabiru and archaeological specimens from the Andes suggests an early exploitation of maize around this historical path, involving Incas and Guaraní indigenous peoples, in the transition area around the Brazil-Paraguay border50. This proposed path displays a similar pattern as the observed concentrations of maize diversity based on kernel type.
Patterns of diversity can also be explained by the effect of sampling density. Uruguay occupies approximately 1,3 percent of the territory and contains 11 percent of the total accessions. This represents an important sampling density and possibly explains the high diversity registered. Brazil, on the other hand, occupies 65 percent of the area and holds 35 percent of the accessions. Therefore, a low sampling density could explain the lower observed diversity in this country, in comparison to the rest of the region. Apart from this, popcorn races were not included in the phenotypic evaluations performed by Paterniani and Goodman4, as it was considered at that time that pop grain types did not have immediate use for breeding programs in Brazil.
A broader understanding of maize diversity in the Southern Cone can increase the accessibility of ex situ collections41)(51 by encouraging germplasm use in breeding programs. This can lead to the development of new cultivars, with specific adaptations to environmental conditions, disease and pest resistance or improved quality or yield. Landraces are significantly more diverse than cultivars52, and can show adaptations to marginal environments, constituting an important resource for agriculture under adverse conditions53, including climate change scenarios54. Crops and agricultural systems are continuously evolving, and this has profound implications for the conservation of biological diversity55. At present, in many regions of South America, maize is grown in large extensions for animal feed, replacing traditional farmer’s landraces with hybrid cultivars and transgenics, leading to a decrease in the overall genetic diversity. A practical application for this research is to propose prospection plans and in situ conservation strategies for the traditional varieties that are still being grown. Also, local characterizations of maize diversity through census of diversity could be performed, which is perceived as a crucial step for strengthening on farm conservation strategies2. This paper contributes with a broader understanding of maize diversity in the Southern Cone and seeks to revalue micro-centres where traditional agriculture is still practiced. Integrating different types of information allows a more rational use, and contributes to the establishment of adequate conservation goals.
5. Conclusion
Maize germplasm collections from the countries of the Southern Cone show an important dispersion, covering different geographic areas. The kernel type distribution (floury, flint, dent, pop and sweet) shows patterns of concentration. A general observation is that floury and pop accessions are principally found in areas that were inhabited by different Indian groups (Guaraní, Tupí Guaraní, Caingang and Chavante). Flint maize is dispersed throughout the region, and predominates in Southeastern Brazil and along the Atlantic coast. Dent maize distributes along the Atlantic coast, and was presumably introduced from the United States. Based on kernel type, maize diversity is not evenly distributed throughout the Southern Cone, and micro-centres of diversity can be identified in the Central regions of Chile and Bolivia, Southern Paraguay, North of Argentina and Uruguay. It is assumed that geographic and socio-economic factors play a significant role in determining the distribution of genetic diversity. This research contributes to a broader understanding of maize diversity in the Southern Cone and seeks to highlight the strategic value of maize genetic resources, enhancing regions where traditional agriculture is still practiced.
