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Otsuchi's tsunami-bred sticklebacks shed most of their marine-species genome in about ten generations
Researchers in Japan report that a stickleback population hybridized by the 2011 tsunami purged most of its foreign genome within about 10 generations. It is a rate measured in the wild after habitat upheaval, though it comes from one population living in pools that suited one parent.
The Scientist · Science desk
What happened
- The tsunami and its backwash are thought to have carried marine Japan Sea sticklebacks and upstream freshwater threespine sticklebacks into new freshwater pools in Otsuchi's town centre.
- In 2012, a year after the tsunami, 38% of the sticklebacks sampled from the Otsuchi population were hybrids.
- Genome regions holding major barrier genes, for freshwater adaptation, sea migration, mate choice and hybrid male sterility, lost Japan Sea ancestry fastest.
- Those major barrier loci could not account for the genome-wide loss of Japan Sea ancestry on their own, according to the study.
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Why it matters
- capability Hybridization modellers now have a purge rate measured in a wild population with a known start, on a question the release says had been poorly understood.
- decision Anyone assessing hybridization after habitat disturbance has a documented case where a large early hybrid share was followed within a decade by one parent's genome reclaiming the population.
- constraint Because freshwater adaptation genes cleared fastest in freshwater pools, the Otsuchi rate is tied to that habitat and cannot be applied to hybrids stranded in brackish or marine water without new data.
- precedent The authors' next test, whether a few strong barriers plus many weak incompatibilities is a general rule, requires other disturbed hybrid zones followed from the first generation.
According to the Research Organization of Information and Systems, which issued the release, few studies have tracked a wild population's genomes from immediately after two species hybridize through about 10 generations [13]. Otsuchi, in Iwate Prefecture, offered a start date. The 2011 tsunami there was more than 10 metres high [1]. A team from the National Institute of Genetics, Hokkaido University and seven other Japanese institutions then followed the town's hybrid population for nine years [3].
The fish breed on a cycle of about one year [6], so each year of sampling is close to one generation. By 2020 the population's genome was almost entirely that of the freshwater threespine stickleback [5]. The release does not say how many fish were sampled each year. Without that denominator, a reader cannot put an error bar on any year's hybrid share.
The direction of the purge matches the setting. The tsunami left the two species in newly formed freshwater pools [2], and the genome that won out belonged to the freshwater species [5]. Freshwater adaptation was among the traits whose genome regions lost Japan Sea ancestry fastest [7]. I think part of the barrier at Otsuchi was the habitat itself: hybrids carrying marine alleles were living in water that suited the other parent. The species boundary held here in a specific sense, with one parent's genome taking back a single population [5]. "First, we did not expect genomic regions associated with major reproductive barriers to be purged so rapidly," said Takuya Hosoki and Jun Kitano [10].
The case for many weak incompatibilities rests on individual-based simulations [9]. A simulation that reproduces the decline shows that small genetic conflicts scattered across the genome are enough to explain it. Naming the genes involved is separate work. The pair also pointed to how long the loss kept going: "Second, we were surprised that most of the foreign genome continued to disappear over subsequent generations," they said [11].
The thing this doesn't tell you is how far the rate generalises. The evidence comes from a single population [3]. The authors put that question first on their own list. "An important next question is whether this combination of a few strong reproductive barriers and many weak genetic incompatibilities represents a general mechanism by which species boundaries are maintained after hybridization," they said [12]. The paper appears in Nature Ecology & Evolution [14].
What to watch
- The paper's per-year sample sizes and ancestry curves, to show how precise the 38% hybrid share and the 2020 result are.
- Follow-up work that maps the weak incompatibilities inferred from simulation onto specific genes.
- Studies of other hybrid zones created by habitat disturbance, testing whether the strong-plus-weak barrier pattern repeats.