Data from: Long-distance dispersal suppresses introgression of local alleles during range expansions

During range expansions, even low levels of interbreeding can lead to massive introgression of local alleles into an invaders genome. Nonetheless, this pattern is not always observed in human populations. For instance, European Americans in North America are barely introgressed by Amerindian genes in spite of known contact and admixture. With coalescent spatially explicit simulations we examined the impact of long distance dispersal (LDD) events on introgression of local alleles into the invading population using a set of different demographic scenarios applicable to a diverse range of natural populations and species. More specifically, we consider two distinct LDD models: one where LDD events originate in the range core and targets only the expansion front and a second one where LDD events can occur from any area to any other. We find that LDD generally prevents introgression, but that LDD events specifically targeting the expansion front are most efficient in suppressing introgression. This is likely due to the fact that LDD allows for the presence of a larger number of invader alleles at the wave front, where effective population size is thus increased and local introgressed alleles are rapidly outnumbered. We postulate that the documented settlement of pioneers directly on the wave front in North America has contributed to low levels of Amerindian admixture observed in European Americans and that this phenomenon may well explain the lack of introgression after a range expansion in natural populations without the need to evoke other mechanisms such as natural selection.

在种群范围扩张过程中，即使仅发生低水平的杂交，也可能导致本地等位基因通过基因渐渗（introgression）大量渗入入侵种群的基因组。然而，这一规律在人类种群中并非普遍存在。例如，尽管北美欧洲裔美国人与美洲原住民存在已知的接触与基因混合，但其基因组几乎未被美洲原住民基因渐渗。
本研究采用空间显式溯祖模拟（coalescent spatially explicit simulations），针对适用于多样自然种群与物种的多组不同种群人口统计学情景，探究了长距离扩散（long distance dispersal, LDD）事件对本地等位基因渐渗入入侵种群的影响。具体而言，我们考量了两类截然不同的LDD模型：一类的LDD事件起源于种群分布核心区，仅作用于扩张波前；另一类的LDD事件可发生于任意区域之间。
研究结果表明，长距离扩散总体上会抑制基因渐渗，而专门针对扩张波前的LDD事件对基因渐渗的抑制效果最为显著。这一现象的潜在机制为：长距离扩散可使扩张波前处的入侵等位基因数量增多，进而提升该区域的有效种群大小，使本地渐渗等位基因迅速被淹没。
我们据此推测，北美地区的拓殖者直接定居于扩张波前这一已被文献记录的现象，是导致欧洲裔美国人中美洲原住民基因混合水平较低的重要原因；且该现象或可解释自然种群在范围扩张后未出现基因渐渗的现象，无需借助自然选择等其他机制。