Hybridization is one of the major factors contributing to the emergence of highly successful parasites. Hybrid vigour can play an important role in this process, but subsequent rounds of recombination in the hybrid population may dilute its effects. Increased fitness of hybrids can, however, be frozen by asexual reproduction. Here, we identify invasion of a ‘frozen hybrid’ genotype in natural populations of Gyrodactylus turnbulli, a facultatively sexual ectoparasitic flatworm that causes significant damage to its fish host. We resequenced genomes of these parasites infecting guppies from six Trinidad and Tobago populations, and found surprisingly high discrepancy in genome‐wide nucleotide diversity between islands. The elevated heterozygosity on Tobago is maintained by predominantly clonal reproduction of hybrids formed from two diverged genomes. Hybridization has been followed by spread of the hybrids across the island, implying a selective advantage compared with native genotypes. Our results thus highlight that a single outcrossing event may be independently sufficient to cause pathogen expansion.

中文翻译：

---

孔雀鱼体外寄生虫 Gyrodactylus Turnbulli 中冷冻杂交体的扩增


杂交是导致高度成功的寄生虫出现的主要因素之一。杂交活力可以在此过程中发挥重要作用，但杂交群体中后续几轮重组可能会削弱其影响。然而，杂交品种适应性的增加可能会被无性繁殖所冻结。在这里，我们在Gyrodactylus Turnbulli的自然种群中发现了一种“冷冻杂交”基因型的入侵，这是一种兼性性外寄生扁虫，对其鱼类宿主造成严重损害。我们对感染特立尼达和多巴哥六个种群的孔雀鱼的这些寄生虫的基因组进行了重新测序，发现岛屿之间全基因组核苷酸多样性存在令人惊讶的高度差异。多巴哥岛杂合性的提高主要是通过两个不同基因组形成的杂交体的克隆繁殖来维持的。杂交后，杂交品种在全岛传播，这意味着与本地基因型相比具有选择性优势。因此，我们的结果强调，单个异型杂交事件可能足以独立地引起病原体扩张。