Hybridization between two bitterling fish species in their sympatric range and a river where one species is native and the other is introduced

Host size matters for reproduction: Evolution of spawning preference and female reproductive phenotypes in mussel-symbiotic freshwater bitterling fishes

Bitterling fishes evolve an idiosyncratic symbiosis with freshwater mussels, in which they are obligated to spawn in the gills of mussels for reproduction. In recent years, freshwater mussel populations have been drastically diminishing, due to accelerating anthropogenic impacts, which can be large threats to the risk of bitterling's extinction cascade (i.e. 'coextinction'). The host mussel size may be an important factor driving the adaptation and evolution of bitterling's reproductive phenotypes. Here we examined the host size preference and morphological adaptation of female bitterling to the host size from 17 localities at the Han River in Korea. Using our developed molecular-based species identification for bitterling's eggs/larvae inside the mussels, we further determined the spawning patterns of seven bitterling species. Mean length of spawned mussels (N = 453) was significantly larger than that of unspawned mussels (N = 1814), suggesting that bitterling prefers to use larger hosts as a spawning ground. Spawning probability was clearly greater as mussel size increases. Results of our reciprocal transplant experiments do provide some evidence supporting the 'bitterling's larger host preference' hypothesis. Interspecific competition appeared to be intense as two fish species often spawned eggs in the same mussel individuals simultaneously. Longer ovipositor and more elongated egg may evolve in females of Tanakia signifer in response to larger host environments. The observed bitterling's spawning preference for large-sized mussels may evolve perhaps because of the fitness advantage in relation to the offspring survival. Our findings further inform on the development of effective conservation and management strategy for the endangered bitterling fishes.

Mating of escaped domestic pigs with wild boar and possibility of their offspring migration after the Fukushima Daiichi Nuclear Power Plant accident

The 2011 Tohoku earthquake drastically changed human activities in some regions of Fukushima Prefecture, Japan. The subsequent tsunami damage and radioactive pollution from the Fukushima Daiichi nuclear power plant resulted in the evacuation of humans, and abandonment of agricultural lands, allowing population expansion of wildlife into areas formally inhabited by domesticated livestock. Unintentional escape of domesticated pigs into wildlife inhabited environments also occurred. In this study, we tested the possibility of introgression between wild boar and domesticated pigs in Fukushima and neighboring prefectures. We analyzed mitochondrial DNA sequences of 338 wild boar collected from populations in the Tohoku region between 2006 and 2018. Although most boar exhibited Asian boar mitochondrial haplotypes, 18 boar, phenotypically identified as wild boar, had a European domesticated pig haplotype. Frequencies of this haplotype have remained stable since first detection in 2015. This result infers ongoing genetic pollution in wild boar populations from released domesticated pigs. In 2018, this haplotype was detected outside of evacuated areas, suggesting migration and successful adaptation. The natural and anthropocentric disasters at Fukushima gave us the rare opportunity to study introgression processes of domestic genes into populations of wild boar. The present findings suggest a need for additional genetic monitoring to document the dispersal of domestic genes within wild boar stock.

Hybridization between an endangered freshwater fish and an introduced congeneric species and consequent genetic introgression

Artificial transplantation of organisms and consequent invasive hybridization can lead to the extinction of native species. In Matsuyama, Japan, a native bitterling fish, Tanakia lanceolata, is known to form hybrids with another bitterling species, T. limbata, which was recently introduced from western Kyushu, Japan. These bitterlings spawn in the gills of two freshwater unionid species, Pronodularia japanensis and Nodularia douglasiae nipponensis, which have rapidly declined on the Matsuyama Plain in the past 30 years. To gauge the effect of invasive hybridization, we determined the genetic introgression between T. lanceolata and T. limbata and analyzed the morphology of these species and their hybrids to infer their niche overlap. We collected adult individuals of Tanakia spp. and genotyped them based on six microsatellite loci and mitochondrial cytochrome b sequences. We analyzed their meristic characters and body shapes by geometric morphometrics. We found that 10.9% of all individuals collected were hybrids. Whereas T. lanceolata were more densely distributed downstream and T. limbata were distributed upstream, their hybrids were widely distributed, covering the entire range of native T. lanceolata. The body height and anal fin length of T. limbata were greater than those of T. lanceolata, but their hybrids were highly morphologically variable, covering both parental morphs, and were widely distributed in the habitats of both parental species. Hybridization has occurred in both directions, but introduced T. limbata females and native T. lanceolata males are more likely to have crossed. This study shows that invasive hybridization with the introduced T. limbata is a potential threat to the native population of T. lanceolata via genetic introgression and replacement of its niche in streams.