Selection for Earlier Flowering Crop Associated with Climatic Variations in the Sahel

Climate changes will have an impact on food production and will require costly adaptive responses. Adapting to a changing environment will be particularly challenging in sub-Saharan Africa where climate change is expected to have a major impact. However, one important phenomenon that is often overlooked and is poorly documented is the ability of agro-systems to rapidly adapt to environmental variations. Such an adaptation could proceed by the adoption of new varieties or by the adaptation of varieties to a changing environment. In this study, we analyzed these two processes in one of the driest agro-ecosystems in Africa, the Sahel. We performed a detailed study in Niger where pearl millet is the main crop and covers 65% of the cultivated area. To assess how the agro-system is responding to recent recurrent drought, we analyzed samples of pearl millet landraces collected in the same villages in 1976 and 2003 throughout the entire cultivated area of Niger. We studied phenological and morphological differences in the 1976 and 2003 collections by comparing them over three cropping seasons in a common garden experiment. We found no major changes in the main cultivated varieties or in their genetic diversity. However, we observed a significant shift in adaptive traits. Compared to the 1976 samples, samples collected in 2003 displayed a shorter lifecycle, and a reduction in plant and spike size. We also found that an early flowering allele at the PHYC locus increased in frequency between 1976 and 2003. The increase exceeded the effect of drift and sampling, suggesting a direct effect of selection for earliness on this gene. We conclude that recurrent drought can lead to selection for earlier flowering in a major Sahelian crop. Surprisingly, these results suggest that diffusion of crop varieties is not the main driver of short term adaptation to climatic variation.气候变化将对粮食生产造成影响，并需要投入高昂成本实施适应性应对措施。在气候变化预计将产生重大影响的撒哈拉以南非洲地区，适应不断变化的环境尤为困难。然而，一个常被忽视且鲜有研究记载的重要现象，是农作系统快速适应环境变化的能力。此类适应性调整可通过两种路径完成：一是引种新品种，二是让现有品种适应变化的环境。本研究以非洲最干旱的农业生态系统之一萨赫勒地区为研究场景，对上述两类适应路径展开了分析。我们在尼日尔开展了针对性详细研究：当地主栽作物为珍珠粟（pearl millet），其种植面积覆盖全国总耕地的65%。为评估农作系统对近期频发干旱的响应机制，我们分析了1976年与2003年在尼日尔全境所有耕地区域的同一村庄中采集的珍珠粟农家品种（landraces）样本。我们通过持续三个种植季的同质园实验（common garden experiment），对1976年与2003年采集的样本的物候与形态特征差异开展了对比分析。研究结果显示，主栽品种及其遗传多样性并未出现显著变化，但我们观测到适应性性状发生了显著偏移：相较于1976年的样本，2003年采集的样本生育周期更短，植株及穗部尺寸均有所缩小。此外，我们还观测到，1976年至2003年间，PHYC基因座上的早花等位基因（allele）频率有所上升。该频率增幅超出了遗传漂变与取样误差所能带来的影响范围，表明该基因确实受到了早花性状定向选择的直接作用。综上，频发干旱可促使萨赫勒地区主要作物朝着早花方向产生适应性选择。令人意外的是，本研究结果显示，作物品种的扩散并非短期适应气候变异的主要驱动因子。