Abstracts

ARTIGO CONVIDADO INVITED ARTICLE

Marcos VBM Siqueira

USP-ESALQ, Lab. Ecologia Evolutiva e Genética Aplicada, C. Postal 83, 13400-900 Piracicaba-SP; marcos.morruga@gmail.com

ABSTRACT

In Brazil current studies and investments on yams are incipient. Similarly, the literature in recent decades lacks adequate information on this group of plants. The existing literature, on its turn, requires more than ever to be revised and organized. Yams have joined the so-called "neglected" group of crops for several reasons, but particularly because they are associated with poor and traditional communities. Many vegetables introduced in Brazil during the colonization period have adapted to different cropping systems, yams being an excellent example. This diversity resulted very widespread, yet poorly recognized in the country. In turn, the gardens using traditional farming systems continue to maintain and enhance yam local varieties. Studies from other countries, with an emphasis on characterization and genetic breeding, brought to light an urgent need for Brazil to invest in yams as a food rich in carbohydrates, even to the point of alterations in food public policy. Reversal of the yam's current stigma is both a challenge to the scientific community and to the population as a whole. This paper aims to raise pertinent questions about Dioscorea species, an important key group for many communities in tropical countries, yet still unrecognized as so in Brazil.

Keywords:Dioscorea spp., tubers, traditional agriculture, agrobiodiversity, molecular markers.

RESUMO

No Brasil, estudos e investimentos ao inhame são incipientes. Similarmente, a literatura nas últimas décadas apresenta informações insuficientes para este grupo de plantas. A literatura existente, por sua vez, exige mais que nunca ser revisada e organizada. O inhame tem-se unido ao grupo de culturas ditas "negligenciadas" por diversas razões, mas particularmente devido ao fato de estar associado às comunidades pobres e tradicionais. Muitos vegetais introduzidos no Brasil durante o período da colonização têm-se adaptado a diferentes sistemas de cultivo, sendo o inhame um excelente exemplo. Esta diversidade é resultado de uma ampla dispersão, ainda pouco conhecida no país. Por sua vez, as roças usam o sistema de agricultura tradicional de forma a manter e aumentar as variedades locais de inhame. Estudos de outros países, com ênfase na caracterização e melhoramento genético, trouxeram à luz uma necessidade urgente de o Brasil investir em inhame como uma rica fonte de carboidratos, mesmo apesar das mudanças na política alimentar pública. Reverter o atual estigma do inhame é um duplo desafio para a comunidade científica e população como um todo. Este artigo objetiva-se a trazer questões pertinentes sobre as espécies do gênero Dioscorea, um importante grupo para muitas comunidades em países tropicais, contudo ainda pouco conhecido no Brasil.

Palavras-chave:Dioscorea spp., tubérculos, agricultura tradicional, agrobiodiversidade, marcadores moleculares.

Certain crops, such as yams, are associated with low-income communities, being a subsistence food source for many Brazilians. Despite the many contributions to human health from the active compounds found in several species, yams continue to be marginalized, hence have joined one of the many "orphan" crops. The yam is an underutilized crop, somewhat forgotten by society and not considered priority in political agendas (Siqueira & Veasey, 2009). Yams are not even included in agricultural, government, or economic policies, which act as financial protectors and allow for trade of resources, especially exportable monocultures. Additionally, the species Colocasia esculenta is also called "yam" in some regions of Brazil, making it difficult, for example, to distinguish the statistical production figures of these two different crops.

This neglected species is increasingly "forgotten" on the table of the rural population and still utterly unrecognized in large cities, hence the need for resurgence of its popularity. The spread of monocultures, the disappearance of traditional communities, changes in nutritional habits among younger generations, and lack of incentives for public and private institutions to focus on yam cultivation research, have made the following questions difficult to answer: (i) how this crop has diversified over the years in tropical countries, (ii) if there is wide biodiversity in these regions, as the question of genetic erosion risk rises, and (iii) how genetics can manifest itself in favor of yams.

Through advances in molecular biology, scientific data has shown the importance of cultivars beyond the perimeters of food. Other research, with reinforcement from new fields, such as ethnobotany, involves local communities that have genetic resources of inestimable value as local varieties (Peroni & Hanazaki, 2002; Bressan et al., 2005; Zannou et al., 2006). However, how long will this gene pool exist in backyards and gardens throughout the tropics? The stimulus of applied studies (such as in vitro cultivation and genetic breeding), the strengthening of the entire supply chain (surely that would help marketing in an increasingly competitive market), and an effective conservation policy (a set of measures that focus on the small and medium farmer's role to maintain local varieties), are the main premises that can bring value back to yams. This resolution and other parallel issues would certainly put yams back in the spotlight, both domestically and internationally.

Thus, the goal of the present study was to review concepts regarding Dioscorea spp., focusing on how scientific research has been debated, with particular emphasis on issues related to in situ and ex situ conservation.

The yam today - Yam is currently the fourth most important tuber-root crop in the world, after potato (Solanum tuberosum L.), cassava (Manihot esculenta Crantz), and sweet potato (Ipomoea batatas L.). In 2008, the estimated world production of yams was 51.7 million tons, with Africa leading the production. The Brazilian production, in 2008, was 250,000 tons within a cultivated area of 27,000 ha (FAOSTAT, 2010). Thriving well in tropical and subtropical environmental conditions, varied yam species are satisfactorily developed in the Brazilian ecosystems, especially in the Northeast region of the country, where agriculture presents great economic potential (Santos, 1996). The highest production of yams in Brazil occurs mainly in the States of Paraíba, Pernambuco, Alagoas, Bahia, and Piauí, followed by others at a lesser scale (Mesquita, 2002).

The Dioscorea species has long been cultivated for their medicinal properties (sapogenin steroids, used in production of cortisone and synthetic hormones) in the following order of importance: D. bulbifera, D. cayenensis, D. dumentorum, D. alata, D. trifida, D. laxiflora, and D. microbotrya (Karnick, 1969; Pedralli, 2002). Wu et al. (2005) analyzed the consumption of D. alata by post-menopausal women and concluded that although the mechanisms are not yet fully understood, its consumption reduces the risk of breast cancer and cardiovascular disease. Among many of the yam medicinal properties, the prevention of diseases like malaria, yellow fever, and dengue is mentioned. Bhandari & Kawabata (2004) have presented interesting information about the nutritional composition of several wild species and their possible application in modern medicine. Traditional medicine uses yams to cleanse skin impurities, for example, rashes. Nutritionists stress the importance of yams in treating anemic patients given their wealth of nutrients, vitamins, and minerals (Okwu & Ndu, 2006). In Africa, it was found that the tuber is responsible for increased fertility in women who habitually consume it (Balbach & Boarim, 1993).

Yam species, considered somewhat irrelevant on a commercial scale, began to receive recognition for their importance, as exemplified in D. bulbifera, from which diosgenin can be extracted (Narula et al., 2007) or even in its use as an anti-tumor agent (Gao et al., 2007). Among many other species of the same genus, is D. balkan, endemic to the Balkans and currently protected, however, sustainably extracted to obtain high concentrations of diosgenin (Fodulovicé et al., 1998). This example serves to illustrate the rich and untapped genetic resources found within the genus Dioscorea.

Even with all the economic and cultural significance that this crop entails for tropical countries, few studies using biochemical approaches or molecular markers have been conducted to understand the relationships and extent of genetic similarity between cultivated and wild yams (Tamiru et al., 2008). Consequently, farmers in some countries have reported the disappearance of many cultivars due specially to pests and diseases, which has led to significant genetic erosion (Mignouna & Dansi, 2003).

Despite the minor effects of pests on yam cultivars in comparison to other crops, phytosanitary problems represent the main difficulties for producers. The virus [Yam mosaic virus (YMV)], anthracnose (caused by the fungus Glomerella cingulata), nematodes (Meloidogyne spp., Pratylenchus spp., and Scutellonema bradys), and tuber rot (Penicillium sclerotigenum and Rhizopus oryzae) are the main diseases and account for the highest field and storage losses in susceptible cultivars (Abang et al., 2003; Amusa et al., 2003). Anthracnose stands out as the principal phytosanitary problem in several areas of yam cultivation and production. Nematodes, which interact with fungi and bacteria, attack the tubers in the field and continue to cause damage in post harvest. The dry rot disease is also a limiting factor in yam cultivation and negatively influences the market value of the product, especially when found in conjunction with the nematode Scutellonema bradys, which acts as its etiologic agent. This disease is a serious problem because it still lacks effective countermeasures, hence remains lethal. Yam meloidogynoses are diseases caused by nematodes of the genus Meloidogyne, which show high incidence and severity in production, causing heavy losses in yam production and marketing (Abang et al., 2002; Mignouna et al., 2003b).

Micropropagation and organogenesis are two of many biotechnological techniques that could be adopted to obtain healthy seedlings, increasing the potential of cultivation (Alizadeh et al., 1998; Chen et al., 2003; Royero et al., 2007), especially in commercially valuable species, such as D. alata (Balogun et al., 2006).

Although of great importance, especially to subsistence communities, for its high nutritional quality and medicinal properties, there are unfortunately few institutions involved in research related to yam in Brazil. For this reason, as previously mentioned, further study is necessary to improve the knowledge on this species.

Origin, dispersal of species, and genetic contribution - With approximately 600 yam species, only few are known for their use in human consumption (D. alata, D.cayenensis, D. mummularia, D. opposita, D. rotundata, D. transverse, D. esculenta, D. bulbifera, D.trifida, and D. pentaphylla). The genus is quite dispersed and can be found throughout tropical, sub-tropical, and temperate regions (Lebot, 2009). Different yam cultivation practices, similar to historical and socio-cultural profiles, define an important force in the creation and maintenance of genetic diversity between each group in several areas of Africa (Baco et al., 2007). It is believed that a similar chain of events has also occurred in Brazil.

According to Lebot (2009), the genus dispersed worldwide at the end of the Cretaceous period, evolved in different directions throughout the New and Old World, and resulted in distinct species. The main dispersal regions for many of these species included the Americas, Africa, Madagascar, South and Southeast Asia, Melanesia, and Australia. According to Coursey (1967), separation of the Asian and African species occurred later, during the Miocene period. Furthermore, according to Coursey (1967), the species, D. alata and D. esculenta, originated from Burma and Assam, localities of Southeast Asia. Degras (1993) noted that D. cayenensis is of African origin, given that wild species can be found on the continent. The origins of other yam cultivated species are African and Asian, except for D. trifida, whose origin is South American (Lebot, 2009). In these regions, the yam has been cultivated by humans since the beginning of civilization, where its importance to the African peoples' diet has always been valued. Although cultivated since antiquity by native Indian communities, the yam cultivars only entered Western civilization when the black slave trade intensified. It is most likely that yams were introduced to Europe by merchants, especially by black slave traders. In Brazil, a similar history ensued, however native Indians blended with introduced slaves in various states of the country during colonization (Madeira et al., 2008). According to Silva (1971), in the early twentieth century, the Rondon Commission found isolated tribes in the northwest of Mato Grosso, Brazil, cultivating the species D. trifida where they named it "cará mimoso".

Dioscorea spp. is commonly known as "cará" or yam in Brazil. Due to the ethnic richness in tropical countries, there is a wide diversity of vernacular names assigned to species (Pedralli et al., 2002; Bressan et al., 2005; Tamiru et al., 2008). The name "igname" or "yam" seems to have an African origin. Thus, it appears that the word "inhame" is a translation of the terms, "yam" or "igname", used originally in English and French colonies in Africa. As for the word "cará", based on historical Brazilian documents, it seems to be of native Indian origin (Cascudo, 1983). In Portuguese, especially in the Northeast region of Brazil, there is a tendency to apply the name "yam" to large tubers of D. cayenensis and the name "cará" to the smaller tubers of D. alata (Peixoto Neto et al., 2000).

One aspect that makes yams well known in Brazil is the strong African influence on the Northeastern cultures. Despite being increasingly abandoned, communities still maintain their dietary value and use yam in cultural dishes. Recently, it is notable that there is a presence of yams in some supermarket chains, small retail markets, and farmers markets, which has encouraged some agriculturists to increase cultivation of this species. At first one might think that the explanation for this may be related to the diet of the Northeastern population. However, in recent years, the media has explored several issues related to health and nutrition, which may be stimulating yam consumption in the pursuit of alternative starch sources (Hsu et al., 2004; Ukpabi, 2010).

In Brazil, no data exists yet about the diversity of the yam based on molecular markers. The most relevant studies came from Africa, the main origin and dispersal centers of some yam species. Studies of genetic diversity in 269 cultivars of D. alata from the South Pacific, Asia, Africa, the Caribbean, and South America (Lebot et al., 1998), concluded that many yams exhibited diverse variations, most likely due to human selection. In Brazil, isozymes were used to study the genetic diversity among local varieties of D. alata, D. bulbifera, D. cayenensis, and D. trifida in the Ribeira Valley, on the coast of São Paulo State. The results of this study showed a high genetic diversity maintained by farmers in this region, and that this variability was not structured in space (Bressan, 2005). RAPD markers were used to evaluate intraspecific variation in accessions of D. alata from Jamaica (Asemota et al., 1996), to compare Dioscorea species from Africa, Asia and Polynesia (Ramser et al., 1996), and to characterize accessions in the D. cayenensis-rotundata complex (Hamon & Toure, 1990; Dansi et al., 2000b). AFLP markers were used to assess the genetic diversity of D. alata in relation with nine other species of edible yams (Malapa et al., 2005) and to study domestication of genus Dioscorea (Scarcelli et al., 2006). Egesi et al. (2006), using these markers, were able to show, from 53 accessions of D. alata in west and central Africa, that each group formed was a mixture of different geographical origins, indicating that geography has played a central role in species differentiation.

Isozymes, RAPD and microsatellites have contributed to the determination of the D. cayenensis-rotundata complex, which some have considered not to be two separate species, but as a species complex (Dansi et al. 2000a,b; ChaÊr et al., 2005; Obidiegwu et al., 2009a). Fundamentally, microsatellite markers have been used to study the segregation patterns and characterization of several Dioscorea species (Mignouna et al., 2003a; Mignouna et al., 2003b; Scarcelli et al., 2005; Hochu et al., 2006; Obidiegwu et al., 2009b).

Part of the complexity in genetic studies of the yam is due to its ploidy. The basic chromosome number of Dioscorea species is considered to be x = 10 and x = 9, with a high frequency of polyploid species (Abraham, 1998). Tetraploid species are most frequent, followed by types 2x, 6x, and 8x in similar proportions. The basic number of chromosomes (x = 10) is found in 52% of African species and 13% of American species. The remaining African and American species have the basic number x = 9. However, recent studies show two new basic chromosome numbers, x = 6 (Segarra-Moragues et al., 2004) and x = 20 for D. rotundata (Scarcelli et al., 2005) and D. trifida (Bousalem et al., 2006). If these basic numbers are confirmed in a large proportion of species, the basic chromosome number of the genera may be reconsidered, leading to a decrease in the ploidy level in some species (Bousalem et al., 2006; Arnau et al., 2009).

Accurate information on genetic diversity is critical to the success of breeding programs, since genetic divergence produces high heterotic effects and, therefore, desirable segregants for the purposes of each program. In this sense, molecular markers have contributed to the advancement of measures not only for protection, but improvement, which many producers have already benefited from. The literature shows actions and consequences of the replacement of local varieties by other more productive by gene banks and research centers, bringing losses to yam ethnovarieties (Tamiru et al., 2008). As a consequence of stress agents, the issue of genetic erosion of local varieties generates a heated debate, demanding effective measures by government and more interaction among the few gene banks of the genera.

Final Considerations - Based on what has been reported, it becomes clear that studies involving yam cultivation, whether for conservation or breeding, especially in Brazil, are still incomplete. Urgent government intervention with a number of measures to aid researchers, extension workers, and farmers is necessary for several reasons, and the risk of genetic erosion is one of the main issues. In this context, it is hoped that the Yam and Taro Symposium will return to Brazil, where the second, and last, took place in 2002. This would be an excellent opportunity to share problematic yam insights; helping traditional, local, and large-scale farmers.

The unknown diversity of Dioscorea in Brazil, with a wide range of local varieties and agroecosystems, is an information blank for breeding and conservation programs. Thus, coupled with an urgency to reinforce to the new generations how important is the yam as a healthy food source, multidisciplinary analysis is crucial for future progresses.

ACKNOWLEDGEMENTS

The author would like to thank Elizabeth Ann Veasey, Paulo César Tavares de Melo and Josh Halsey for contributions to the manuscript. The author is also grateful to The São Paulo Research Foundation (process 2007/07222-8).

(Recebido para publicação em 1 de setembro de 2010 ; aceito em 2 de fevereiro de 2011)

(Received on September1, 2010; accepted on February 2, 2011)

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