Department of Botany, University of British Columbia, Vancouver, BC, Canada, V6T 1Z4; Biodiversity Research Centre, University of British Columbia, Vancouver, BC, Canada, V6T 1Z4.
Mol Ecol. 2014 Oct;23(19):4737-56. doi: 10.1111/mec.12837. Epub 2014 Jul 15.
Willows (Salix: Salicaceae) form a major ecological component of Holarctic floras and consequently are an obvious target for a DNA-based identification system. We surveyed two to seven plastid genome regions (~3.8 kb; ~3% of the genome) from 71 Salix species across all five subgenera, to assess their performance as DNA barcode markers. Although Salix has a relatively high level of interspecific hybridization, this may not sufficiently explain the near complete failure of barcoding that we observed: only one species had a unique barcode. We recovered 39 unique haplotypes, from more than 500 specimens, that could be partitioned into six major haplotype groups. A unique variant of group I (haplotype 1*) was shared by 53 species in three of five Salix subgenera. This unusual pattern of haplotype sharing across infrageneric taxa is suggestive of either a massive nonrandom coalescence failure (incomplete lineage sorting), or of repeated plastid capture events, possibly including a historical selective sweep of haplotype 1* across taxonomic sections. The former is unlikely as molecular dating indicates that haplotype 1* originated recently and is nested in the oldest major haplotype group in the genus. Further, we detected significant non-neutrality in the frequency spectrum of mutations in group I, but not outside group I, and demonstrated a striking absence of geographical (isolation by distance) effects in the haplotype distributions of this group. The most likely explanation for the patterns we observed involves recent repeated plastid capture events, aided by widespread hybridization and long-range seed dispersal, but primarily propelled by one or more trans-species selective sweeps.
柳树（柳属：杨柳科）构成了全北温带植物区系的主要生态组成部分，因此是基于 DNA 的鉴定系统的明显目标。我们调查了来自五个亚属的 71 种柳树中的两个到七个质体基因组区域（3.8 kb；基因组的3%），以评估它们作为 DNA 条形码标记的性能。尽管柳树具有较高水平的种间杂交，但这可能不足以解释我们观察到的条形码几乎完全失败的现象：只有一个物种具有独特的条形码。我们从 500 多个标本中回收了 39 个独特的单倍型，可以将其分为六个主要的单倍型组。组 I 的一个独特变体（单倍型 1*）在五个柳属亚属中的三个亚属中的 53 个物种中共享。这种非同寻常的种间单倍型共享模式表明，要么是大规模的非随机合并失败（不完全谱系分选），要么是重复的质体捕获事件，可能包括单倍型 1在分类群中的历史选择性清扫。前者不太可能，因为分子年代测定表明单倍型 1起源于近期，并且嵌套在属中最古老的主要单倍型组中。此外，我们在组 I 中的突变频率谱中检测到显著的非中性，但在组 I 之外没有检测到，并且在该组的单倍型分布中没有明显的地理（隔离距离）效应。对于我们观察到的模式，最可能的解释是最近发生了多次质体捕获事件，这些事件得到了广泛的杂交和长距离种子传播的帮助，但主要是由一个或多个跨物种的选择性清扫推动的。
