Ensuring the genetic diversity of tomato Andreas W. Ebert and Lawrence Kenyon, AVRDC – The World Vegetable Center, Taiwan 1 Introduction: key issues relating to the genetic diversity of tomatoes 2 Taxonomy and mating system of tomato and its wild relatives 3 Conservation of tomato genetic resources worldwide 4 Policies affecting access to plant genetic resources 5 Issues affecting the exchange and use of plant genetic resources 6 Phytosanitary requirements for the exchange of plant genetic resources 7 Phytosanitary measures in practice: the case of solanaceous viroids 8 Ways to strengthen sharing of crop genetic resources 9 Summary and future trends 10 Where to look for further information 11 References
1 Introduction: key issues relating to the genetic diversity of tomatoes Tomato is the most important vegetable crop grown worldwide in terms of production value, ranking 8th among all food and agricultural commodities in 2012 (FAOSTAT, 2015). In 2012 global tomato production touched 161.8 million metric tons, with a production value of $59.1 billion. The top five producer countries were China (50 million metric tons; 30.9% of world production); India (17.5 million metric tons; 10.8%); USA (13.2 million metric tons; 8.2%); Turkey (11.4 million metric tons; 7.0%) and Egypt (8.6 million metric tons; 5.3%). During the last 20 years (1993–2012) the tomato production area expanded from 3 to 4.8 million hectares (60% increase), yield increased from 25.8 to 33.7 t/ha (30.6% increase) and total production doubled from 77.9 to 161.8 million metric tons (FAOSTAT, 2015). Compared to other vegetables, especially leafy vegetables, tomatoes are not considered a nutrient-dense food source (Keatinge et al., 2011). However, because of the relatively large quantities consumed, tomatoes make a substantial nutritional contribution to the human diet. In the United States, the third largest producer of this crop, tomato is the fourth most popular fresh-market vegetable behind potatoes, lettuce and onions (USDAERS, 2013). Tomato fruits contain considerable quantities of ß-carotene, a provitamin http://dx.doi.org/10.19103/AS.2016.0007.02 © Burleigh Dodds Science Publishing Limited, 2016. All rights reserved.
Ensuring the genetic diversity of tomato
A carotenoid and ascorbic acid. Apart from their value as a provitamin and vitamin, respectively, ß-carotene and ascorbic acid also function as antioxidants (Hanson et al., 2004). Tomato consumption has been shown to protect DNA from oxidative damage that may lead to cancer (Ellinger et al., 2006). Farmers’ varieties, landraces and crop wild relatives are an inestimable source for vegetable improvement to tackle both biotic and abiotic stresses. This is increasingly important as pronounced climatic changes occur at the global level. Exposure of tomatoes and other vegetable crops to more extreme and erratic weather patterns is likely to lead to reduced productivity and quality (de la Peña et al., 2011). Moreover, climate change is fostering the spread of pathogens and the evolution of new strains of insect pests and fungal and bacterial diseases. Climate change scenarios predict increased average temperatures and more frequent heat and drought spells for many regions of the world (Dai, 2013; Christensen et al., 2007; Tebaldi et al., 2006). Elevated temperatures reduce fruit set, induce excessive evaporation and speed up plant development with subsequent reductions in crop yield (Battisti and Naylor, 2009). This is of special relevance for many agricultural and horticultural crops in the tropics, currently already grown close to their thermal limit (Berry et al., 2014). The exploitation of genetic diversity, to develop stress-tolerant crops to both biotic and abiotic stresses, is of strategic importance to combat the negative impact of climate change on crop production (Kissoudis et al., 2015; de la Peña et al., 2011). Crop wild relatives are often well adapted to marginal environments and can withstand biotic and abiotic stresses better than elite varieties. Since ancient times, they have served as the basis for crop domestication and improvement. Today, crop wild relatives that are threatened in the wild and are only partially conserved in gene banks are being rediscovered as essential resources for crop improvement programmes (Ebert and Schafleitner, 2015; Maxted and Kell, 2009). Tomato is a model crop in research and breeding. Given its relatively small genome size, diploid genetics, short reproduction period and great diversity of genetic resources, the tomato genome has been selected as one of the model genomes for the Solanaceae family. International genome sequencing efforts led to the publishing of the tomato genome in 2012 (Mueller, 2013; The Tomato Genomic Consortium, 2012). Accessibility and use of crop wild relatives for crop improvement is especially important in tomatoes, a crop where the cultigen contains less than 5% of the genetic diversity of its wild relatives (Miller and Tanksley, 1990). An enormous number of biotic and abiotic stress-tolerance traits have already been studied in the pool of wild relatives and extensively used in tomato breeding (Ebert and Schafleitner, 2015; Razdan and Mattoo, 2007) and genetic engineering (Fatima et al., 2008). Phytonutrients found in tomato and its wild relatives have also generated interest for use in breeding due to their potential health benefits (Caicedo and Peralta, 2013). Sustainable conservation and easy access to crop genetic resources are key to successful crop improvement and adaptation to climate change. Tomato germplasm resources are well represented in ex situ collections at the global level. Currently, there are three international agreements/protocols in place that define ownership and regulate access and benefit sharing of plant genetic resources (PGR): The Convention on Biological Diversity (CBD), The International Treaty on Plant Genetic Resources for Food and Agriculture (ITPGRFA) and the Nagoya Protocol on Access to Genetic Resources and the Fair and Equitable Sharing of Benefits Arising from their Utilization (NP). Since the implementation of these agreements, germplasm exchange and new acquisitions have become more cumbersome. Increasingly stricter phytosanitary requirements delay and restrict germplasm acquisition and distribution. © Burleigh Dodds Science Publishing Limited, 2016. All rights reserved.
Ensuring the genetic diversity of tomato3
2 Taxonomy and mating system of tomato and its wild relatives Tomato, pepper and potato are the most well-known and widely cultivated members of the Solanaceae family, also called Nightshades, which comprises 105 genera and 2030 accepted species (The Plant List, 2016). All three of these crops originated in the New World – in Central and South America (Knapp, 2002). Despite the enormous number and global distribution of the genera and species included in this family, cytogenetically it is a very conservative family as most taxa are characterized by a basic chromosome number of n 12 (Chiarini et al., 2010). The genus Solanum comprises 3411 scientific plant names of species rank, of which 747 are accepted species names and a further 473 are scientific plant names of infraspecific rank (The Plant List, 2016). Botanically, tomatoes (Solanum lycopersicum L.) are classified as berries. They originated in the South American Andes, ranging from northern Chile in the south, through Bolivia, Peru to Ecuador and Colombia in the north (Bai and Lindhout, 2007; Grubben and Denton, 2004). S. lycopersicum is divided into two botanical varieties: the cherry tomato (S. lycopersicum var. cerasiforme (Dunal) Spooner, G. J. Anderson and R. K. Jansen (SLC) and S. lycopersicum var. lycopersicum (SLL) (Blanca et al., 2015). SLC is native to the Andean region of Ecuador and Peru (Blanca et al., 2015), but is also found in Mesoamerica in semi-wild state and has been thought to be the direct ancestor of cultivated tomato (Tanksley, 2004). Modern cultivars appear to be closely related to a cherry tomato-like cultivar grown widely in Mexico and throughout Central America at the time of discovery by the Spanish (Rick, 1995). Both Mesoamerica (Jenkins, 1948) and Ecuador and northern Peru have been proposed as the centre of domestication of tomato. The latter would coincide with the centre of origin and genetic diversity of Solanum pimpinellifolium (SP), the closest wild ancestor of cultivated tomato (Peralta et al., 2008). Based on recent genomic studies, Blanca et al. (2015, 2012) now propose a two-step domestication process. In a first step, early farmers in the Andean region of Ecuador and Peru are believed to have made selections from SP or primitive forms of SLC resulting in domesticated forms of SLC. The second step took place in Mesoamerica and consisted of further selection within the pre-domesticated SLC after their migration from Ecuador and Peru. Subsequently, the Spaniards took tomato plants from Mesoamerica to Spain and Italy and from there they reached the rest of the world. Genetic studies confirmed that European traditional varieties originated from Mesoamerica (Blanca et al., 2012). There has been much debate on the generic status of tomato (Ebert and Schafleitner, 2015) going back to the sixteenth century when the crop was introduced by the Spaniards into Europe. The genus name of tomato has changed several times, from Solanum to Lycopersicon and back again to Solanum. Today, tomatoes are formally classified under the genus Solanum sect. Lycopersicon. This classification is based on evidence derived from phylogenetic studies by using DNA sequences and more in-depth studies of plant morphology and distribution of the species (Peralta et al., 2006). The relatively small section Lycopersicon in the genus Solanum contains the domesticated tomato (Solanum lycopersicum L.) and 12 crop wild relatives (see Table 1; Peralta et al., 2008; Bai and Lindhout, 2007). The environments under which these wild tomato species are grown range from sea level to 3340 m altitude, from arid to rainy climate and from the Andean Highlands to the coast of the Galapagos Islands, where S. cheesmaniae and S. galapagense originated. The wild species are often found in © Burleigh Dodds Science Publishing Limited, 2016. All rights reserved.
This book chapter describes key issues regarding genetic diversity of tomatoes, including taxonomy and mating system. Figures on global ex s...