Ascertaining gene flow patterns in livestock populations of developing countries: a case study in Burkina Faso goat

BACKGROUND: Introgression of Sahel livestock genes southwards in West Africa may be favoured by human activity and the increase of the duration of the dry seasons since the 1970’s. The aim of this study is to assess the gene flow patterns in Burkina Faso goat and to ascertain the most likely factors influencing geographic patterns of genetic variation in the Burkina Faso goat population. RESULTS: A total of 520 goat were sampled in 23 different locations of Burkina Faso and genotyped for a set of 19 microsatellites. Although overall differentiation is poor (FST = 0.067 ± 0.003), the goat population of Burkina Faso is far from being homogeneous. Barrier analysis pointed out the existence of: a) genetic discontinuities in the Central and Southeast Burkina Faso; and b) genetic differences within the goat sampled in the Sahel or the Sudan areas of Burkina Faso. Principal component analysis and admixture proportion scores were computed for each population sampled and used to construct interpolation maps. Furthermore, Population Graph analysis revealed that the Sahel and the Sudan environmental areas of Burkina Faso were connected through a significant number of extended edges, which would be consistent with the hypothesis of long-distance dispersal. Genetic variation of Burkina Faso goat followed a geographic-related pattern. This pattern of variation is likely to be related to the presence of vectors of African animal trypanosomosis. Partial Mantel test identified the present Northern limit of trypanosome vectors as the most significant landscape boundary influencing the genetic variability of Burkina Faso goat (p = 0.008). The contribution of Sahel goat genes to the goat populations in the Northern and Eastern parts of the Sudan-Sahel area of Burkina Faso was substantial. The presence of perennial streams explains the existence of trypanosome vectors. The South half of the Nakambé river (Southern Ouagadougou) and the Mouhoun river loop determined, respectively, the Eastern and Northern limits for the expansion of Sahelian goat genes. Furthermore, results from partial Mantel test suggest that the introgression of Sahelian goat genes into Djallonké goat using human-influenced genetic corridors has a limited influence when compared to the biological boundary defined by the northern limits for the distribution of the tsetse fly. However, the genetic differences found between the goat sampled in Bobo Dioulasso and the other populations located in the Sudan area of Burkina Faso may be explained by the broad goat trade favoured by the main road of the country. CONCLUSIONS: The current analysis clearly suggests that genetic variation in Burkina Faso goat: a) follows a North to South clinal; and b) is affected by the distribution of the tsetse fly that imposes a limit to the Sahelian goat expansion due to their trypanosusceptibility. Here we show how extensive surveys on livestock populations can be useful to indirectly assess the consequences of climate change and human action in developing countries.

背景：自20世纪70年代以来，西非的萨赫勒地区家畜基因向南渐渗（introgression）可能受到人类活动以及旱季时长增加的推动。本研究旨在评估布基纳法索山羊的基因流模式，并明确影响布基纳法索山羊种群遗传变异地理格局的最可能因素。
结果：本研究在布基纳法索的23个不同采样点共收集520份山羊样本，并对19个微卫星（microsatellites）标记进行基因分型。尽管整体遗传分化程度较低（FST = 0.067 ± 0.003），但布基纳法索山羊种群并非完全均质。障碍分析（barrier analysis）结果显示存在两类遗传间断：a）布基纳法索中部与东南部的遗传分化断点；b）布基纳法索萨赫勒或苏丹区域内采样山羊的遗传差异。本研究对每个采样种群计算了主成分分析（Principal Component Analysis, PCA）得分与遗传混合比例，并据此构建了空间插值地图。此外，种群图分析（Population Graph analysis）表明，布基纳法索的萨赫勒与苏丹生态区域通过大量延伸的连接边相连，这与长距离扩散的假说相符。布基纳法索山羊的遗传变异呈现地理相关模式，该模式可能与非洲动物锥虫病的传播媒介密切相关。偏曼特尔检验（Partial Mantel Test）显示，当前锥虫传播媒介的北界是影响布基纳法索山羊遗传变异的最显著景观边界（p = 0.008）。萨赫勒山羊基因对布基纳法索苏丹-萨赫勒区域北部与东部种群的遗传贡献显著。永久溪流的存在维持了锥虫传播媒介的种群。纳坎贝河（Nakambé River，瓦加杜古南部河段）与穆洪河（Mouhoun River）弯道分别限定了萨赫勒山羊基因扩散的东界与北界。进一步的偏曼特尔检验结果表明，相较于舌蝇（tsetse fly）分布北界这一生物地理边界，通过人类活动介导的遗传廊道进行的萨赫勒山羊基因向Djallonké山羊的渐渗，其影响相对有限。不过，在博博-迪乌拉索（Bobo Dioulasso）采样的山羊种群与布基纳法索苏丹区域其他种群之间的遗传差异，或许可归因于该国主要公路推动的大规模山羊跨区域贸易。
结论：本次分析清晰表明，布基纳法索山羊的遗传变异具备两个核心特征：a）呈现由北至南的渐变地理格局；b）受舌蝇分布的显著制约——由于山羊对锥虫病易感，舌蝇的分布范围限制了萨赫勒山羊的扩张边界。本研究证实，针对家畜种群的大规模野外调查可用于间接评估气候变化与人类活动在发展中国家引发的生态与遗传效应。