No abstract

Asian carps are classified as either bighead carp Hypophthalmichthys nobilis or silver carp H. molitrix by multiple presumptively diagnostic morphological characteristics; however, hybrids pose a dilemma. Fish sharing the morphological characteristics of both species were observed in an Illinois River backwater (Calhoun County, Illinois) approximately 5 mi (8 km) upriver from the confluence with the Mississippi River as well as in two locations in Pool 26 of the Mississippi River (Madison County, Illinois). Biopsied tissues from individuals exhibiting mixed morphological features were analyzed at four diagnostic allozyme loci (ADH‐1*, sMDH‐A*, CK‐A*, and sSOD‐1*) via starch gel electrophoresis. This comparison revealed a high percentage of hybridization (22.5%) from an indiscriminate sample of 120 fish. Moreover, an unexpected percentage (12.5%) of individuals identified in the wild as either parental bighead carp or silver carp by gill raker morphology were genetically identified as hybrids. Finally, two levels of hybridization were detected, first‐generation hybrids (F1) and post‐F1 hybrids, revealing the onset of extensive introgression and the potential for a hybrid swarm. Variation in the amplified COII domain of mitochondrial DNA indicated a strong directional bias of hybrids (88%) containing silver carp maternal lineages. Morphologically, F1 hybrids were often identifiable (88%) by the presence of twisted gill rakers, but post‐F1 hybrids were difficult to identify with any appreciable certainty. This result creates concern where taxonomic assignment is critical for management or monitoring, warranting a more extensive and intensive examination of this phenomenon in North American waters. Finally, prior observations in aquaculture have shown reduced jumping behavior, fitness, and condition of fish resulting from post‐F1 matings between these species. This is the first confirmed presence of wild post‐F1 individuals of Asian carps in the United States and, although further monitoring is needed, a hybrid swarm may ultimately decrease invasion success as introgression continues.

We investigated the geographic distribution of the mitochondrial DNA (mtDNA) diversity of American shad from 15 North American rivers in 1992 with the intent of assessing sampling efficiency for future mixed-stock analysis. We observed 116 haplotypes among the 988 individuals assayed. Because no single or group of haplotypes completely discriminated river stocks or regional complexes, we investigated haplotype frequencies as stock descriptors. Analysis of four unique indices of haplotype divergence indicated that including rather than suppressing restriction site heteroplasmy increased resolution; however, the final results were not overwhelmingly dependent on this choice. A redundancy of variation among restriction enzymes diminished information returns rapidly after considering the six best enzymes, caused by physical linkage of restriction sites on the mtDNA molecule. Stock discriminatory power was tested by computing allocation efficiencies of mtDNA characters. When each individual was temporarily removed from the data set and reallocated to the various candidate populations on the basis of haplotypic similarity, 28% of the reallocations were correct, a fourfold increase over random success. We demonstrate that although the specific stock identity of individuals cannot be confidently established, the haplotypic arrays from baseline stocks can support stock identification and mixed-stock analysis for shad because rivers support stock-specific haplotype frequencies.

Hybridization among conspecifics in native and introduced habitats has important implications for biological invasions in new ecosystems. Bighead (Hypophthalmichthys nobilis) and silver carp (H. molitrix) are genetically isolated and occur in sympatry within their native range. Following their introduction to North America, however, introgressant hybrids have been reported throughout their expanded range within the Mississippi River Basin (MRB). The extent of introgression, both spatially and generationally, is largely unknown. Therefore, we examined mixed-species populations from across the MRB to characterize the extent of interspecific gene flow. We assayed 2798 individuals from nine locations with a suite of species-diagnostic SNPs (57 nuclear and one mitochondrial). Forty-four per cent (n = 1244) of individuals displayed hybrid genotypes. Moreover, the composition of hybrid genotypes varied among locations and represented complex hybrid swarms with multiple generations of gene flow. Introgressive hybrids were identified from all locations, were bidirectional and followed a bimodal distribution consisting primarily of parental or parental-like genotypes and phenotypes. All described hybrid categories were present among individuals from 1999 to 2008, with parents and later-generation backcrosses representing the largest proportion of individuals among years. Our mitochondrial SNP (COII), tested on a subset of 730 individuals, revealed a silver carp maternal bias in 13 of 21 (62%) F1 hybrids, in all silver carp backcrosses, and maintained throughout many of the bighead carp backcrosses. The application of this suite of diagnostic markers and the spatial coverage permits a deeper examination of the complexity in hybrid swarms between two invasive, introduced species.

Bighead carp (Hypophthalmichthys nobilis) and silver carp (H. molitrix) are invasive species and listed as US federally injurious species under the Lacy Act. They have established populations in much of the Mississippi River Basin (MRB; Mississippi, Illinois, and Missouri rivers) and are capable of producing fertile hybrids and complex introgression. Characterizing the composition of this admixture requires a large set of high-quality, evolutionarily conserved, diagnostic genetic markers to aid in the identification and management of these species in the midst of morphological ambiguity. Restriction site-associated DNA (RAD) sequencing of 45 barcoded bighead and silver carp from the United States and China produced reads that were aligned to the silver carp transcriptome yielded 261 candidate single nucleotide polymorphisms (SNPs) with fixed allelic differences between the two species. We selected the highest quality 112 SNP loci for validation using 194 putative pure-species and F1 hybrids from the MRB and putative bighead carp and silver carp pure species from China (Amur, Pearl and Yangtze rivers). Fifty SNPs were omitted due to design/amplification failure or lack of diagnostic utility. A total of 57 species-diagnostic SNPs conserved between carp species in US and Chinese rivers were identified; 32 were annotated to functional gene loci. Twenty-seven of the 181 (15%) putative pure species were identified as hybrid backcrosses after validation, including three backcrosses from the Amur River, where hybridization has not been documented previously. The 57 SNPs identified through RAD sequencing provide a diagnostic tool to detect population admixture and to identify hybrid and pure-species Asian carps in the United States and China.