Japan's 2011 Tsunami Made Two Fish Species Interbreed, and Evolution Undid It in a Decade
Hybrids made up 38% of a stickleback population in a tsunami-made pond in Otsuchi in 2012. By 2020 the foreign species' DNA had almost vanished, giving scientists a rare real-time look at how species boundaries hold.
The tsunami that struck northeastern Japan in March 2011 did something evolutionary biologists rarely get to watch: it threw two related fish species together and made them interbreed. A nine-year study now shows that within about 10 generations, nature had almost completely sorted them back out.
The town of Otsuchi in Iwate Prefecture was hit by waves more than 10 meters, or 33 feet, high. The tsunami and the land sinking that came with the earthquake created new freshwater ponds in the town center. Researchers believe the incoming wave carried the marine Japan Sea stickleback, Gasterosteus nipponicus, in from the ocean, while the backwash brought a freshwater population of the threespine stickleback, Gasterosteus aculeatus, down from upstream. The two small fish met in the new ponds and began to hybridize.
A team from Japan's National Institute of Genetics and eight universities, including Hokkaido University, Kyoto University and the University of Tokyo, followed the population from 2012 to 2020. In 2012, 38% of the fish they sampled were hybrids. Over the following years, the stretches of DNA inherited from the marine species dropped steadily across the whole genome. By 2020 the population was almost entirely threespine stickleback again. Because these fish live about one year per generation, most of the marine species' genome was purged in roughly 10 generations. The study was published in Nature Ecology & Evolution.
The marine DNA disappeared fastest in regions of the genome that contain major "reproductive isolation" genes, the ones that keep species apart. Those included genes linked to living in fresh water, migrating to the sea, choosing a mate and causing sterility in hybrid males. But those big genes alone could not account for how thoroughly the foreign DNA was removed. Computer simulations of individual fish suggested that many small genetic incompatibilities scattered throughout the genome also helped clean out the marine ancestry.
"There were two major surprises in this study," said Takuya Hosoki and Jun Kitano, the study's lead researchers. "First, we did not expect genomic regions associated with major reproductive barriers to be purged so rapidly. Second, we were surprised that most of the foreign genome continued to disappear over subsequent generations." They said the next question is whether this mix of a few strong barriers and many weak ones is a general rule for how species stay distinct after they interbreed.
The work is one of very few studies to track a wild population's genome from the moment two species first hybridized through about 10 generations. The question matters beyond Japan, because climate change and human activity are reshaping habitats around the world and bringing once-separated species into contact. The Otsuchi sticklebacks suggest that, at least in some cases, species boundaries can snap back quickly even after heavy mixing.
