Informal "seed" systems and the management of gene flow in traditional agroecosystems: the case of cassava in Cauca, Colombia

Modelling cassava production and pest management under biotic and abiotic constraints

We summarise modelling studies of the most economically important cassava diseases and arthropods, highlighting research gaps where modelling can contribute to the better management of these in the areas of surveillance, control, and host-pest dynamics understanding the effects of climate change and future challenges in modelling. For over 30 years, experimental and theoretical studies have sought to better understand the epidemiology of cassava diseases and arthropods that affect production and lead to considerable yield loss, to detect and control them more effectively. In this review, we consider the contribution of modelling studies to that understanding. We summarise studies of the most economically important cassava pests, including cassava mosaic disease, cassava brown streak disease, the cassava mealybug, and the cassava green mite. We focus on conceptual models of system dynamics rather than statistical methods. Through our analysis we identified areas where modelling has contributed and areas where modelling can improve and further contribute. Firstly, we identify research challenges in the modelling developed for the surveillance, detection and control of cassava pests, and propose approaches to overcome these. We then look at the contributions that modelling has accomplished in the understanding of the interaction and dynamics of cassava and its' pests, highlighting success stories and areas where improvement is needed. Thirdly, we look at the possibility that novel modelling applications can achieve to provide insights into the impacts and uncertainties of climate change. Finally, we identify research gaps, challenges, and opportunities where modelling can develop and contribute for the management of cassava pests, highlighting the recent advances in understanding molecular mechanisms of plant defence.

Selective signatures and high genome-wide diversity in traditional Brazilian manioc (Manihot esculenta Crantz) varieties

Knowledge about genetic diversity is essential to promote effective use and conservation of crops, because it enables farmers to adapt their crops to specific needs and is the raw material for breeding. Manioc (Manihot esculenta ssp. esculenta) is one of the world's major food crops and has the potential to help achieve food security in the context of on-going climate changes. We evaluated single nucleotide polymorphisms in traditional Brazilian manioc varieties conserved in the gene bank of the Luiz de Queiroz College of Agriculture, University of São Paulo. We assessed genome-wide diversity and identified selective signatures contrasting varieties from different biomes with samples of manioc's wild ancestor M. esculenta ssp. flabellifolia. We identified signatures of selection putatively associated with resistance genes, plant development and response to abiotic stresses that might have been important for the crop's domestication and diversification resulting from cultivation in different environments. Additionally, high neutral genetic diversity within groups of varieties from different biomes and low genetic divergence among biomes reflect the complexity of manioc's evolutionary dynamics under traditional cultivation. Our results exemplify how smallholder practices contribute to conserve manioc's genetic resources, maintaining variation of potential adaptive significance and high levels of neutral genetic diversity.

Farmers' Sources and Varieties of Cassava Planting Materials in Coastal Kenya

Cassava (Manihot esculenta Crantz) grows in diverse agro-ecological zones. In Kenya, it is widely cultivated in Western and Coastal regions. It is mainly grown for food and nutrition security and excess roots are sold to generate income for the farm households. Its productivity per unit of the land area is high compared to maize and wheat which are staple crops in the country. However, scarcity of cassava planting materials and pests and diseases limit production in these regions. This study aimed at revealing the sources and varieties of cassava planting materials used by farmers and other farmers' practices in coastal Kenya. Four focus group discussions (FGD) and a survey were conducted in 2018, using a semi-structured questionnaire targeting 250 farmers. The data collected on the sources of planting materials, preferred varieties and the practices employed by the farmers in cassava production, was analyzed using descriptive statistics. Majority of farmers (83%) interviewed indicated that they recycled planting materials from the previous crop while some 67% respondents obtained the planting material from their neighbors. Kenya Agricultural and Livestock Research Organization's (KALRO) and the local markets were reported as sources of planting materials by 11 and 5% farmer respondents, respectively. The only formal seed source reported was KALRO. The rest, own seed, neighbors, and the local markets, were informal seed sources. Piecemeal harvesting practiced by 98% of the farmers favored planting material recycling. Farmers dedicated a small proportion of their land (mean 0.2 ha) to cassava production as reported by 60 and 72.8% of farmers from Taita Taveta and Kilifi, respectively. Slightly above half (56%) of the farmers in Kilifi dedicated slightly more land to cassava, planting between 100 and 4,000 cuttings (2.5% of an acre up to a full acre) compared to 87% farmers from Taita Taveta who planted 100 cuttings or less (which is about 2.5% of an acre or less). A majority (81.1%) of farmers in Taita Taveta planted local cassava varieties compared to Kilifi's 57.8%. Slightly above half of the farmer respondents reported Kibandameno as the preferred variety followed by Tajirika as the second preferred variety as reported by 18% farmers. Kibandameno was preferred for its sweet taste by 75.6% farmers while Tajirika was preferred by 52.4% farmers because of the high yielding capacity. Nearly all farmer respondents, in Taita Taveta County obtained the planting material from informal seed sources, except a negligible number, who reported buying their planting material from KALRO, a formal seed source, far from their locality. Farmers sourcing cuttings from a formal seed source such as those from Kilifi County were more likely to use a tractor for land preparation compared to those who sourced planting materials informally who more likely had scarce knowledge on cassava production and the value of cassava. Therefore, interventions to establish a sustainable healthy cassava planting materials seed system are needed to address the systemic constraint and help develop a viable cassava value chain.

Network Analysis of Seed Flow, a Traditional Method for Conserving Tartary Buckwheat (Fagopyrum tataricum) Landraces in Liangshan, Southwest China

The Yi ethnic group in Liangshan Prefecture, Sichuan Province in Southwest China have cultivated Tartary buckwheat (Fagopyrum tataricum) for at least a thousand years. Tartary buckwheat landraces are maintained through their traditional seed system. Field work and social network methodologies were used to analyze the seed sources and their flows, and to create a seed network map. Self-saving, exchanging with neighbors and relatives, and purchasing from the market were the main means farmers used to save and exchange Tartary buckwheat seeds. The flow of seed within villages was higher than between villages. Wedding dowry was an important pathway for seed flow among all of the villages. Of the 13 Tartary buckwheat landraces maintained, four landraces were exchanged frequently. The seed exchange network structure was affected by the number of Tartary buckwheat landraces, the age of nodal households, geographic environment, culture, and cultural groups. Nodal households play an important role in the conservation and on-farm management of Tartary buckwheat landraces.

Diversity in 113 cowpea [Vigna unguiculata (L) Walp] accessions assessed with 458 SNP markers

Single Nucleotide Polymorphism (SNP) markers were used in characterization of 113 cowpea accessions comprising of 108 from Ghana and 5 from abroad. Leaf tissues from plants cultivated at the University of Ghana were genotyped at KBioscience in the United Kingdom. Data was generated for 477 SNPs, out of which 458 revealed polymorphism. The results were used to analyze genetic dissimilarity among the accessions using Darwin 5 software. The markers discriminated among all of the cowpea accessions and the dissimilarity values which ranged from 0.006 to 0.63 were used for factorial plot. Unexpected high levels of heterozygosity were observed on some of the accessions. Accessions known to be closely related clustered together in a dendrogram drawn with WPGMA method. A maximum length sub-tree which comprised of 48 core accessions was constructed. The software package structure was used to separate accessions into three groups, and the programme correctly identified varieties that were known hybrids. The hybrids were those accessions with numerous heterozygous loci. The structure plot showed closely related accessions with similar genome patterns. The SNP markers were more efficient in discriminating among the cowpea germplasm than morphological, seed protein polymorphism and simple sequence repeat studies reported earlier on the same collection.

New genes in traditional seed systems: diffusion, detectability and persistence of transgenes in a maize metapopulation

Gene flow of transgenes into non-target populations is an important biosafety concern. The case of genetically modified (GM) maize in Mexico has been of particular interest because of the country’s status as center of origin and landrace diversity. In contrast to maize in the U.S. and Europe, Mexican landraces form part of an evolving metapopulation in which new genes are subject to evolutionary processes of drift, gene flow and selection. Although these processes are affected by seed management and particularly seed flow, there has been little study into the population genetics of transgenes under traditional seed management. Here, we combine recently compiled data on seed management practices with a spatially explicit population genetic model to evaluate the importance of seed flow as a determinant of the long-term fate of transgenes in traditional seed systems. Seed flow between farmers leads to a much wider diffusion of transgenes than expected by pollen movement alone, but a predominance of seed replacement over seed mixing lowers the probability of detection due to a relative lack of homogenization in spatial frequencies. We find that in spite of the spatial complexities of the modeled system, persistence probabilities under positive selection are estimated quite well by existing theory. Our results have important implications concerning the feasibility of long term transgene monitoring and control in traditional seed systems.