Scientists have monitored the phenomenon for nine years, documenting how mechanisms of reproductive isolation can maintain genetic distinction between species even after intense hybridization caused by a natural disaster. But how did this change happen? The tsunami and the subsidence of the land have profoundly modified coastal habitats, contributing to the formation of new freshwater basins in the town centre. According to the reconstruction of the scholars, the tsunami transported specimens of Gasterosteus nipponicus, a marine stickleback, from the sea, while the reflux of the water conveyed individuals of Gasterosteus aculeatus, a similar species present in fresh waters, from upstream. Before the disaster, the two species lived in different environments, but the new basin led them to share the same habitat, resulting in interbreeding.
In 2012, 38% of the individuals sampled were hybrids, that is, fish that had inherited genetic material from both species. In the following years, researchers followed the composition of their genome to verify whether this mixing was destined to persist and, from the analyses, they highlighted a progressive decrease in the genomic regions coming from the marine species. By 2020, the population's genetic makeup had once again become almost entirely attributable to G. aculeatus, the freshwater species. Since the generation time of these fish is about a year, much of the genetic contribution of G. nipponicus has been eliminated within about ten generations. The previous situation was recovered only in terms of the predominant genetic composition: after an initial phase of hybridization, the hereditary heritage of the freshwater species returned to almost completely prevail in the population studied.
As the scientists underlined, by examining the genomic regions involved in the mechanisms that limit the exchange of genes between different species, when two closely related species interbreed, a part of the genetic heritage of one can be transmitted to the descendants of the other. The permanence of these regions in subsequent generations also depends on their compatibility with the rest of the genome and on the biological characteristics of the individuals who inherit them. In the case of Otsuchi, in particular, the genetic regions coming from the marine species rapidly decreased in correspondence with important reproductive barriers. These include regions associated with freshwater adaptation, seaward migration, mate choice, and hybrid male sterility. According to the data, some of the mechanisms that keep the two species separate have contributed to reducing the genetic material introduced through interbreeding. These major barriers, however, were not sufficient to explain the observed genome-wide loss on their own.
Some simulations allowed the researchers to identify a possible additional contribution, highlighting how numerous genetic incompatibilities of relatively weak effect, distributed across different genomic regions, may have acted together with the strongest barriers. Their combined action would in fact have favored the rapid decrease in the genetic heritage of the marine species. “There were two big surprises in this study,” Hosoki and Kitano said, noting that “first, we did not expect genomic regions associated with major reproductive barriers to be eliminated so quickly, and second, we were surprised that most of the foreign genome continued to disappear in subsequent generations.”
It now remains to be established whether the combination of a few strong reproductive barriers and numerous weak genetic incompatibilities represents a general mechanism for maintaining boundaries between species. Although the simulations have suggested this possibility, the result does not demonstrate that all hybrid populations follow the same path. The study showed that the tsunami caused initially extensive genetic mixing, without leading to a stable fusion of the hereditary heritage of the two species. In the basin examined, the contribution of the marine species has progressively become residual, while that of the freshwater species has become almost exclusive again. The evolutionary effect of the tsunami-induced encounter has therefore offered new insights into the mechanisms through which species can maintain their genetic distinction even after intense gene exchange.
