Incomplete reproductive isolation and strong transcriptomic signature of hybridization between sympatric sister species of salmon

AbstractGlobal change is altering ecosystems at an unprecedented rate. The resulting shifts in species ranges and reproductive timing are opening the potential for hybridization between closely-related species which could dramatically alter the genetic diversity, adaptive capacity, and evolutionary trajectory of interbreeding taxa. Here, we used behavioural breeding experiments, in vitro fertilization experiments, and whole-transcriptome gene expression data to assess the potential for and consequences of hybridization between Chinook and Coho salmon. We show that behavioural and gametic prezygotic barriers between socioeconomically valuable Chinook and Coho salmon are incomplete. Postzygotically, we demonstrate a clear transcriptomic response to hybridization among F1 Chinook-Coho offspring. Genes transgressively expressed within hybrids were significantly enriched with genes encoded in the nucleus but localized to the mitochindrion, suggesting a potential role for mito-nuclear incompatibilities as a postzygotic mechanism of hybrid breakdown. Chinook and Coho salmon are expected to continue to respond to climate change with shifts in migration timing and habitat use, potentiating hybridization between these species. The downstream consequences of hybridization on the future of these threatened salmon, and the ecosystems they inhabit, is unknown.摘要：全球变化正以史无前例的速率改变着各类生态系统。由此引发的物种分布范围与繁殖时序的变迁，使得近缘物种间产生杂交的可能性提升，而杂交事件可能会显著改变杂交类群的遗传多样性、适应能力与演化轨迹。本研究通过行为学繁殖实验、体外受精实验以及全转录组基因表达数据分析，评估了奇努克鲑（Chinook salmon）与银鲑（Coho salmon）之间发生杂交的潜在可能性及其后果。研究结果显示，兼具重要社会经济价值的奇努克鲑与银鲑之间，行为与配子层面的合子前生殖隔离并不完全。在合子后层面，本研究证实子一代（F1）奇努克-银鲑杂交后代存在明确的转录组响应。杂交个体中表达量超越亲本的基因显著富集于细胞核编码但定位于线粒体的基因，这提示线粒体-核不相容性可作为合子后杂交衰败的潜在作用机制。受气候变化影响，奇努克鲑与银鲑的洄游时间与栖息地利用模式预计将持续发生改变，进而进一步增大二者的杂交风险。目前，此类杂交事件对这两种受威胁鲑鱼及其栖息的生态系统未来的潜在连锁影响仍尚不明确。