Informal “Seed” systems and the management of gene flow in traditional agroecosystems: the case of cassava in Cauca, Colombia

Our ability to manage gene flow within traditional agroecosystems and their repercussions requires understanding the biology of crops, including farming practices' role in crop ecology. That these practices' effects on crop population genetics have not been quantified bespeaks lack of an appropriate analytical framework. We use a model that construes seed-management practices as part of a crop's demography to describe the dynamics of cassava (Manihot esculenta Crantz) in Cauca, Colombia. We quantify several management practices for cassava—th e first estimates of their kind for a vegetatively-propagated crop—describe their demographic repercussions, and compare them to those of maize, a sexually-reproduced grain crop. We discuss the implications for gene flow, the conservation of cassava diversity, and the biosafety of vegetatively-propagated crops in centers of diversity. Cassava populations are surprisingly open and dynamic: farmers exchange germplasm across localities, particularly improved varieties, and distribute it among neighbors at extremely high rates vis-à-vis maize. This implies that a large portion of cassava populations consists of non-local germplasm, often grown in mixed stands with local varieties. Gene flow from this germplasm into local seed banks and gene pools via pollen has been documented, but its extent remains uncertain. In sum, cassava's biology and vegetative propagation might facilitate pre-release confinement of genetically-modified varieties, as expected, but simultaneously contribute to their diffusion across traditional agroecosystems if released. Genetically-modified cassava is unlikely to displace landraces or compromise their diversity; but rapid diffusion of improved germplasm and subsequent incorporation into cassava landraces, seed banks or wild populations could obstruct the tracking and eradication of deleterious transgenes. Attempts to regulate traditional farming practices to reduce the risks could compromise cassava populations' adaptive potential and ultimately prove ineffectual.我们若要管控传统农业生态系统内的基因流（gene flow）及其影响，就必须先理解作物的生物学特性，其中也包括耕作实践在作物生态学中所扮演的角色。当前尚未有针对这些耕作实践对作物种群遗传学影响的量化研究，这恰恰凸显了合适分析框架的缺失。本研究采用将种子管理实践视作作物种群统计学组成部分的模型，对哥伦比亚考卡省的木薯（*Manihot esculenta* Crantz）种群动态进行刻画。我们对木薯的多项管理实践进行了量化——这是针对无性繁殖作物的同类研究中首次完成此类估算——阐明了这些实践对种群统计学的影响，并将其与有性繁殖谷类作物玉米的相关情况进行了对比。本研究还探讨了上述发现对基因流、木薯多样性保护，以及多样性中心内无性繁殖作物生物安全的启示。 木薯种群展现出出人意料的开放性与动态性：农户会在不同地区间交换种质资源（germplasm），尤其是改良品种，且相较于玉米，农户向邻里分发种质的频率极高。这意味着木薯种群中有相当大的比例由非本地种质构成，这些种质常与本地品种（landraces）混种。已有研究记录了这类种质通过花粉向本地种子库（seed bank）和基因库（gene pool）发生的基因流，但其发生规模仍未明确。 综上，正如预期，木薯的生物学特性与无性繁殖方式或许有助于实现转基因品种（genetically-modified varieties）的释放前隔离，但如果完成释放，这些品种也可能借助这些特性在传统农业生态系统中快速扩散。转基因木薯不太可能取代地方品种或损害其多样性；但改良种质的快速扩散，以及随后这些种质融入木薯地方品种、种子库或野生种群的过程，可能会阻碍有害转基因的追踪与清除工作。若试图通过管控传统耕作实践来降低相关风险，反而可能损害木薯种群的适应潜力，最终也难以达成预期效果。