This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: How are species boundaries maintained in the face of environmental disturbance? This is not only a fundamental question in evolutionary biology but also an increasingly important one as human activities and climate change alter habitats and bring previously separated species into contact.
As mechanisms preventing gene exchange between populations, known as reproductive isolation, accumulate, populations can eventually diverge into separate species. When reproductive isolation is incomplete, however, environmental changes can bring closely related species into contact and lead to hybridization. For species boundaries to persist despite extensive hybridization, many of the genomic regions introduced from another species must be eliminated.
Yet how rapidly this process occurs in nature, and which reproductive isolation mechanisms contribute to the removal of foreign genomic regions, have remained poorly understood. The tsunami triggered by the Great East Japan Earthquake in 2011 dramatically altered habitats along the Pacific coast of northeastern Japan. Otsuchi Town in Iwate Prefecture was struck by a tsunami exceeding 10 meters (33 feet) in height.
The tsunami and subsequent backwash are thought to have brought the marine Japan Sea stickleback, Gasterosteus nipponicus, from the sea and a freshwater population of the threespine stickleback, Gasterosteus aculeatus, from upstream into newly formed freshwater habitats in the town center. This brought the two closely related fish species into contact and resulted in interspecific hybridization. A research team comprising scientists from the National Institute of Genetics (NIG), Hokkaido University, Gifu Kyoritsu University, Kyoto University, Ishinomaki Senshu University, Nagoya University, Keio University, The University of Tokyo, and Tokyo University of Marine Science and Technology tracked this hybrid population for nine years.
In 2012, 38% of the individuals sampled were hybrids. Over the following years, however, genomic regions derived from G. nipponicus rapidly declined across the genome, and by 2020 the population had returned to an almost entirely G. aculeatus genomic composition. Because the generation time of this population is approximately one year, most of the G. nipponicus-derived genome was eliminated within about 10 generations.
The study is published in the journal Nature Ecology & Evolution. Genomic regions containing major reproductive isolation loci associated with freshwater adaptation, migration to the sea, mate choice, and hybrid male sterility showed particularly rapid declines in G. nipponicus ancestry. However, these major loci alone could not explain the genome-wide removal observed over 10 generations.
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