Genetic distinction of pallid, shovelnose, and Alabama sturgeon: Emerging species and the US Endangered Species Act

The sturgeon genus Scaphirhynchus consists of three recognized species. Pallid and shovelnose sturgeon “S. albus and S. platorynchus, respectively” are sympatric in the Missouri and lower Mississippi Rivers of the central United States. The Alabama sturgeon “S. suttkusi” is endemic to the nearby Mobile River drainage and is isolated geographically from the other two species. Pallid sturgeon and the extremely rare Alabama sturgeon are listed as endangered under the US Endangered Species Act “ESA”. In contrast, shovelnose sturgeon are relatively common and are not listed. Despite these taxonomies and morphological evidence, some biologists have questioned the genetic and taxonomic distinctions of the three species, thus raising doubts concerning the validity of protecting pallid and Alabama sturgeon under the ESA. To investigate these questions, we compared a 436 base-pair sequence of the mitochondrial DNA “mtDNA” control region among the three species. We observed 16 mtDNA haplotypes defined by 27 single base-pair substitutions “transitions” and one single base-pair insertion/deletion “indel” among 78 individuals examined. The maximum sequence divergence among those haplotypes “2.06%” was less than values usually observed between fish species. However, Alabama sturgeon “n = 3” were distinguished from the other two taxa “n = 75” by a unique base-pair substitution and haplotype, and pallid and shovelnose sturgeon at their northern range of natural sympatry “upper Missouri River” did not share any haplotypes. On the other hand, only frequency differences among shared haplotypes distinguished “P < 0.01” pallid and shovelnose sturgeon at their southern range of natural sympatry “Atchafalaya River”, and genetic distances between northern and southern localities for each species were nearly as large as the distances between species. These latter results are consistent with several hypotheses, including reports “based on morphology” of putative natural hybrids in the Atchafalaya River but not in the upper Missouri River. Overall, these mtDNA results indicate significant reproductive isolation between pallid and shovelnose sturgeon in areas of natural sympatry, and recent evolutionary divergence of Alabama sturgeon. These mtDNA results provide the first molecular genetic evidence for distinguishing the three Scaphirhynchus species and, coupled with morphological and biogeographic data, indicate that pallid and Alabama sturgeon should be evaluated as distinct species under the ESA.