Genomic architecture of the clownfish hybrid Amphiprion leucokranos.

Natural hybridisation is increasingly recognized as playing a significant role in species diversification and adaptive evolution. Amphiprion leucokranos, the naturally occurring clownfish hybrid between A. chrysopterus and A. sandaracinos, is found within the hybrid zone of the two parental species. Based on whole-genome sequencing of parental and hybrid individuals sampled in Kimbe Bay, Papua New Guinea, we found that most of the hybrids collected were first-generation hybrids, a few were first and second generation backcrosses with A. sandaracinos, and the first evidence, to our knowledge, of both an early backcross with A. chrysopterus and a second-generation hybrid in the wild, highlighting the richness and diversity of genomic architectures in this hybrid zone. The frequent backcrossing with A. sandaracinos has led to higher levels of introgression from A. chrysopterus into the A. sandaracinos genomic background, potentially allowing for adaptive introgression. We have additionally identified morphological features which could potentially allow differentiating between first generation hybrids and backcrosses. By comparing population genetic statistics of first-generation hybrids, backcrosses, parental populations within the hybrid zone and parental allopatric populations, we provide the context to evaluate population differentiation and the consequences of ongoing hybridisation. This study is the first whole-genome analysis of a clownfish hybrid population and builds upon the growing body of literature relative to the evolutionary outcomes of hybridisation in the wild and its importance in evolution.