Laboratory for Amphibian Systematics and Evolution, College of Biology & the Environment, Nanjing Forestry University, Nanjing, China.
PLoS One. 2024 Nov 25;19(11):e0314477. doi: 10.1371/journal.pone.0314477. eCollection 2024.
Hybrid zones, i.e., geographic areas where diverging lineages meet, hybridize and eventually mix their genomes, offer opportunities to understand the mechanisms behind reproductive isolation and speciation. Hybrid zones are particularly well suited to study reinforcement, i.e., the process by which selection against hybridization increases reproductive barriers, which, in anuran amphibians, is typically expressed by increased divergence in advertisement calls-the main cue to assortative mating-in parapatric ranges. Using mitochondrial barcoding (16S sequences), population genomics (thousands of SNPs) and bioacoustic analyses (four call parameters), we examine the hybrid zone between two incipient species of midwife toads (Alytes obstetricans and A. almogavarii) in southern France, with the purposes of locating their transition, measuring genetic introgression, and documenting potential signatures of reinforcement. We map range boundaries in the Eastern Pyrenees and the southwestern foothills of the Massif Central, namely along the Ariège valley and the Montagne Noire area. Similarly to another transition between these species in Spain, we found the hybrid zone to be narrow, involving geographically restricted gene flow (~20 km wide allele frequency clines) and barrier loci (i.e., loci resisting introgression), both suggestive of partial post-zygotic isolation (hybrid incompatibilities). The calls of the species overlap less inside than outside the hybrid zone, due to a reduction of their standing variation rather than a shift towards distinctive variants. While neutral causes cannot be excluded, this pattern follows the general expectations of reinforcement, yet without reproductive character displacement. Our study highlights the potential of amphibian hybrid zones to assess the genetic and behavioral drivers of reproductive isolation in statu nascendi and under various evolutionary contexts.
杂种区，即分歧谱系相遇、杂交并最终混合其基因组的地理区域，为理解生殖隔离和物种形成背后的机制提供了机会。杂种区特别适合研究强化作用，即选择对抗杂交增加生殖障碍的过程，在有尾两栖动物中，这通常表现为在并系范围内广告叫声的差异（即配对交配的主要线索）增加。我们使用线粒体条形码（16S 序列）、群体基因组学（数千个 SNP）和生物声学分析（四个叫声参数）研究了法国南部两种即将成为助产蟾蜍（Alytes obstetricans 和 A. almogavarii）之间的杂种区，目的是定位其过渡区，测量遗传渐渗，并记录强化作用的潜在特征。我们在东比利牛斯山脉和中央高原西南部的山麓地区绘制了范围边界，即沿着阿列日河谷和黑山区。与西班牙这两个物种之间的另一个过渡区类似，我们发现杂种区狭窄，涉及地理上受限制的基因流（~20 公里宽的等位基因频率渐变群）和屏障基因座（即抵抗渐渗的基因座），这都表明存在部分合子后隔离（杂种不兼容性）。在杂种区内部，物种的叫声比外部重叠得少，这是由于其遗传变异减少，而不是向独特变体的转变。虽然不能排除中性原因，但这种模式符合强化作用的一般预期，但没有生殖特征取代。我们的研究强调了两栖动物杂种区在评估生殖隔离的遗传和行为驱动因素方面的潜力，包括在形成状态和各种进化背景下。
