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.
The introduction of non-native organisms into a habitat can bring about the extinction of related native species through competition and hybridization [1,2]. In rivers in western North America, for example, native cutthroat trout breed with introduced rainbow trout and face local extinctions through competition and displacement by the resulting hybrids, although the two parent species seldom mix with each other in naturally coexisting ranges [3,4]. In Japanese freshwater systems, invasive hybrids of alien and native cyprinid species are causing the decline of the native species [5,6].
Bitterling fishes of the family Cyprinidae inhabit rivers, agricultural ditches, and small ponds, and breed by depositing eggs in the gills of freshwater bivalves [7,8]. In Japan, 15 of the 16 native bitterling species and subspecies are listed on the Japanese Red List, facing extinction crises due to multiple stresses, the most critical of which are habitat loss from urbanization, river improvement, and the consequent decline of unionid bivalve populations [9,10,11,12]. The introduction of alien species is another factor negatively impacting native species. The rosy bitterling Rhodeus ocellatus ocellatus was introduced to Japanese waters from China in 1942 ; consequent competition for habitat and breeding sites (unionid species) along with invasive hybridization caused the extinction of most populations of native R. ocellatus kurumeus . In western Japan, Tanakia limbata has been artificially transplanted to some rivers outside its native range, where it generates invasive hybrids with the native congeneric species T. lanceolata . Tanakia limbata was introduced to Ehime Prefecture in the 1970s  and now competes with two other bitterlings there, the native T. lanceolata and the exotic R. ocellatus ocellatus, for the use of the same unionid species as breeding substrata . Populations of the unionid mussels, Pronodularia japanensis and Nodularia douglasiae nipponensis, which serve as spawning sites for bitterlings on the Matsuyama Plain, have decreased rapidly over the past 30 years because of habitat degradation . Consequently, T. lanceolata populations have also decreased in the past two decades . The decline of suitable breeding substrata may also force bitterlings to engage in group spawning by multiple males and females instead of pair spawning, a behavior that has been observed in the bitterling Rhodeus ocellatus . In group spawning, different bitterling species may spawn simultaneously and hybridize incidentally with each other .
Tanakia lanceolata and T. limbata diverged about 10 million years ago . They produce fertile hybrids , but the survival rate of these hybrids in the post-larval stage is lower than that of purebred fish, suggesting some degree of postzygotic isolation . Prezygotic isolation has also likely been established through sexual isolation between hybridizing fishes because of a decline in parental fitness of that breed with other species [23,24]. In fact, T. lanceolata and T. limbata are somewhat segregated by habitat and prefer different breeding substrates within a river: T. lanceolata prefers faster-flowing waters and spawns mainly on unionids in the thalweg section, whereas T. limbata lives and spawns near the banks of rivers, where the flow is weaker . Male T. limbata will defend a unionid breeding territory against T. lanceolata as well as other T. limbata males [26,27].
The fitness of hybrid individuals determines the consequences of hybridization, (i.e. whether it enhances speciation by adaptive radiation through transgressive segregation [28,29], causes the extinction of either parent, or leads to the fusion of the parental species [30,31]). In theory, the fitness of hybrids is strongly affected by the phylogenetic distance between its two parent species and the variety of ecological niches that are available , but it is necessary to accumulate more empirical studies about the phylogenetic distances between hybridizing species, the traits and niche preference of hybrid individuals, and the consequences of hybridization to construct a general rule about hybrid fitness.
To this end, we classified individuals as T. lanceolata, T. limbata, or their hybrid using microsatellite nuclear DNA markers and mitochondrial cytochrome b sequences. We also analyzed the direction of hybridization using both nuclear and mitochondrial genes. Finally, we examined the body shapes, meristic characters, and longitudinal distribution within streams of T. lanceolata, T. limbata, and their hybrids.
Clippings (ca. 2 × 2 mm) were taken from the ethanol-preserved fins, and genomic DNA was extracted using the Wizard Genomic DNA Purification Kit following the manufacturer’s protocol (Promega, Madison, WI, USA). We amplified the cytochrome b (cyt b) region of mtDNA by PCR with the forward primer NEW-FOR, 5’-AGCCTACGAAAAACACACCC- 3’ , and the reverse primer cytb-Rev, 5’-GATCTTCGGATTACAAGACC-3’ . The reaction mixture contained 6.05 μL of sterile distilled water, 1.0 μL of 10× ImmoBuffer (Bioline, London, UK), 1.0 μL of dNTP mix (10 mM), 0.3 μL of MgCl2 (50 mM), 0.3 μL of each primer (10 μM), 0.05 μL of BIOTAQ HS DNA polymerase (5U/μL, Bioline), and 1.0 μL of DNA template. The reaction protocol consisted of an initial denaturation at 95°C for 10 min; this was followed by 30 cycles of 95°C for 60 s, 58°C for 60 s, and 72°C for 90 s and a final extension at 72°C for 7 min. We obtained sequences for 48 of the 211 collected individuals as follows: PCR products were purified using polyethylene glycol following the protocol of  and subjected to direct cycle sequencing with BigDye Terminator version 3.1 (Thermo Fisher Scientific, MA, USA) using the PCR primers. Sequencing followed the ABI recommended protocol. Labeled fragments were sequenced using an ABI 3130 Genetic Analyzer (Thermo Fisher Scientific). Sequences were stored under the accession numbers AB907119–31, AB907133–5, AB907145–8, and AB920289–91. Following sequencing, we defined the mitochondrial genotype of each individual as T. lanceolata or T. limbata [15,18].
The remaining 163 individuals were genotyped by PCR-restriction fragment length polymorphism (PCR-RFLP) using the restriction enzyme HhaI (TaKaRa Bio, Shiga, Japan), which cuts the cyt b fragments of T. lanceolata and T. limbata in different locations . HhaI produced fragments of 91, 218, 252 and 369 bps for T. lanceolata, and fragments of 18, 109, 252 and 821 bps for T. limbata (S1 Fig). We incubated 5.0 μL of the PCR products with 0.2 μL of HhaI (10 U/μL), 3.8 μL of sterile distilled water, and 1.0 μL of 10× M buffer overnight. We conducted gel electrophoresis with the reaction products for 40 min at 100 V on 3% agarose gel (NIPPON Genetic, Tokyo, Japan) and genotyped individual fish according to the resulting fragment patterns.
We used six microsatellite loci as markers following our preliminary study : RC363 and RC317A, which were reported from Rhodeus ocellatus ssp. , and Rser02, Rser03, Rser07, and Rser10, which were reported from R. sericeus . The target sequences were amplified by PCR following the method of , and each microsatellite primer was labeled with one of four fluorescent dyes: RC363 and Rser07 were labeled with PET, RC317A and Rser03 with FAM, Rser02 with NED, and Rser10 with VIC. The reaction mixture contained 3.15 μL of sterile distilled water, 0.5 μL of 10× ImmoBuffer (Bioline), 0.5 μL of dNTP mix (10 mM), 0.15 μL of MgCl2 (50 mM), 0.02 μL of forward primer (10 μM), 0.08 μL of reverse primer (10 μM), 0.08 μL of fluorescent dye label (10 μM), 0.025 μL of BIOTAQ HS DNA polymerase (5U/μL, Bioline), and 0.5 μL of DNA template. The reaction consisted of an initial denaturation at 95°C for 10 min; followed by 30 cycles of 95°C for 30 s, 60°C for 45 s, and 72°C for 30 s; 10 cycles of 95°C for 30 s, 53°C for 45 s, and 72°C for 45 s; and a final extension at 72°C for 10 min. We determined the lengths of fragments using an ABI 310 Genetic Analyzer (Thermo Fisher Scientific) and Peak Scanner software version 2.0 (Thermo Fisher Scientific). The number of alleles, expected heterozygosity (He), and observed heterozygosity (Ho) were calculated using Genepop on the Web [39,40]. Deviations from Hardy–Weinberg equilibrium (HWE) were also tested using Genepop on the Web [39,40]. Null alleles and their frequencies were determined using Micro-Checker version 2.2.3 . The population genetic structure was analyzed using Structure version 2.3.4  with the following parameters: 100,000 burn-in; 1,000,000 Markov chain Monte Carlo steps; admixture model with independent allele frequencies; and five replicates of each simulation from K = 1 to 10 genetic clusters. The optimum K value calculated by Structure Harvester version 0.6.94  was 2 (S2 Fig). As the resultant 2 populations corresponded well with the mitochondrial genotypes and morphological characters, we defined the population with a cyt b sequence and morphology of T. lanceolata as purebred T. lanceolata, and the other population, which possessed the T. limbata morphology and cyt b sequence, was defined as purebred T. limbata. We defined hybrids as those with more than 20% of their genetic material matching both purebred type .
We used NewHybrids  to assign each individual to one of six genotypic classes based on the posterior probabilities: the two parental species (P0, P1), first-generation hybrid (F1), second-generation hybrid (F2), backcross of F1 with P0, and backcross of F1 with P1. We used 0.5 as a threshold value for the posterior probability (q) with their assignment following . Parameters of NewHybrids were set as follows: without individual or allele frequency prior information; Jeffreys-like prior for both mixing proportions and allele frequencies; 25,000 sweeps of burn-in; and 100,000 iterations of the Markov chain Monte Carlo. To assess the efficiency and accuracy of genotype assignment by NewHybrids, we conducted a simulation method using HYBRIDLAB version 1.0 [44,47]. A total of 100 simulated hybrids of each hybrid class were generated based on the allelic frequencies of 81 T. lanceolata individuals and 68 T. limbata that were identified as purebreds in the previous Structure analysis, with a cut-off value of 0.9 for the ancestry coefficient. This dataset was analyzed by NewHybrids to assign each individual to a genotypic class with q = 0.5 to estimate the accuracy and efficiency of the assignment.
In terms of the longitudinal distribution of bitterlings in the study streams, T. lanceolata were found primarily downstream, whereas T. limbata preferred upstream waters. In the Harai River in central Japan, where these two bitterlings are sympatric, their habitat is segregated within the river: T. limbata is distributed near the banks, where water is slow flowing and shallower, and T. lanceolata is widely distributed in the middle of the river . In another sympatric locality in northern Kyushu, T. limbata are found upstream of T. lanceolata . We found similar habitat segregation in the three streams in this study, with T. limbata distributed primarily upstream, where water flow was slower. Morphometric analysis showed that T. limbata had greater body height than did T. lanceolata, which is consistent with previous results given by the direct measurement of the body depth of Tanakia spp. in the Harai River . Body height correlates negatively with swimming speed in cyprinid fishes . The taller T. limbata experiences more drag than T. lanceolata and thus prefers habitats with slower-flowing water. Hybrid individuals with a T. limbata-like body shape may also prefer slower flow, but hybrids with an intermediate body shape may expand their distribution to habitats with faster water flow. The body shape of hybrids can be intermediate between parental species, as in the hybrids of another pair of cyprinid fishes, Rutilus rutilus and Abramis brama, in an Irish lake , but this is far from the only possibility. Hybrids can have a wide range of body shapes extending to the morphs of both parental species and sometimes exceeding the range of their parent species (through transgressive segregation), as in African cichlids . Hybrids in this study had a wide range of both meristic characters and body shapes, covering the morphs of both parental species, and were distributed both upstream and downstream. Kunichi, Koyori, and Nagaodani streams are inhabited by the unionids Nodularia douglasiae nipponensis, Pronodularia japanensis, and Sinanodonta spp. However, the ranges of distribution of these unionids, as well as their population densities, have rapidly declined in the past 30 years. Only a small P. japanensis population remains in the middle reaches where the habitats of T. lanceolata and T. limbata overlap . Therefore, in the breeding season (from March to July for T. lanceolata and from March to September for T. limbata), adult fishes gather in the midstream for reproduction [15,52]. Thus, the habitat segregation observed between T. lanceolata and T. limbata seems to be caused by different preferences for habitat based on different body shapes, and annual migration.
In this system, it is difficult to accurately distinguish genetic classes among hybrids based on six microsatellite markers, but a high rate of F2 hybrids, 19 of 23 hybrid individuals (82.6%), was detected. This result suggests that the genetic introgression proceeds from this invasive hybridization. Although some bitterling fish species prefer different freshwater mussel species [25,55], T. lanceolata and T. limbata choose the same species of unionid  and sometimes spawn their eggs simultaneously in the same individual mussels. Thus, the rapid decline of the unionid population in their habitats causes overcrowding of bitterling fishes and accelerates competition for breeding substrata. Bitterling males are known to use alternative reproductive tactics depending on the availability of females and the breeding substratum (mussels); they may engage in pair spawning on a mussel territory, sneaking toward a pair, or group spawning with multiple males [19,56,57,58]. When the density of bitterlings is much higher than that of unionids, territoriality collapses, and sneaking and group spawning become dominant [19,59,60]. In both of these strategies, different bitterling species spawn simultaneously and may incidentally hybridize with each other [1,61]. In fact, T. lanceolata and T. limbata have been observed breeding simultaneously on the same Pronodularia japanensis at our study site (Matsuba, Ouchi, Hata, personal observations). The limited availability of suitable breeding substrata due to the rapid decline of mussel populations and the concentration of the remaining unionid population in the midstream, where the ranges of the two bitterling species overlap, increases the frequency of encounters between native T. lanceolata and T. limbata. Hybridization occurred in both directions but was skewed toward female T. limbata–male T. lanceolata crosses. This bias is likely due to the rarity of introduced species during the colonizing period; in group spawning, T. lanceolata eggs might be encountered and fertilized more frequently by sperm of the more common T. lanceolata.
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