After a tsunami, stickleback hybrids quickly lost much of their foreign DNA

A nine-year study found that DNA from a marine stickleback was removed from most of a newly formed hybrid population within a few generations.

Editorial illustration — not from the study.

On 11 March 2011, a tsunami struck Japan’s Tohoku region, creating new stickleback habitats and bringing freshwater Gasterosteus aculeatus into hybridization with the marine Gasterosteus nipponicus. Researchers followed the resulting population genetically for nine years.

Within the first few generations, G. nipponicus alleles—alternative forms of genes—were removed from major regions involved in keeping the species apart. Marine alleles were later purged across most of the genome, although they remained at several locations.

How the hybrid DNA changed

The hybrid population underwent rapid genome-wide purging: G. nipponicus alleles declined across most of the genome within a few generations. The losses included the ancestral-X and neo-X chromosomes, regions involved in freshwater survival, differences in migration and habitat choice, sexual isolation and hybrid sterility. Several marine alleles persisted at specific loci. The researchers say these observations provide direct evidence from a natural population that rapid removal of introgressed DNA can allow a species’ identity to persist after extensive hybridization.

Why species identity persisted

The study shows in real time how a population can retain its species identity after substantial mixing with another species. In particular, the rapid loss of marine alleles from regions involved in reproductive isolation suggests that these parts of the genome can be removed early in the process. Understanding how this genome-wide purging occurs is important for explaining how barriers between species are maintained and how speciation progresses toward completion.

Evidence and caveats

The evidence comes from nine years of genomic monitoring of a tsunami-induced hybrid stickleback population in Japan, rather than from a laboratory experiment. The abstract does not report the sample sizes, the exact timing of each sampling event or the genetic mechanisms responsible for the purging. It describes one natural population formed after one tsunami, so the findings do not by themselves show how often the same pattern occurs in other species or settings.

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