Reproductive Isolation in Sympatric Populations of Pallid and Shovelnose Sturgeon

—Pallid sturgeon Scaphirhynchus albus and shovelnose sturgeon S. platorynchus are recognized morphologically as separate species. A previous genetic study with allozymes was unable to distinguish between the two species or demonstrate their reproductive isolation in regions of sympatry. Our main objective was to measure the genetic variability within and among populations of pallid and shovelnose sturgeon at the northern and southern extremes of their sympatric ranges to determine if genetic variation within the two species exhibits patterns consistent with reproductive isolation. Additionally, we examined a sample of individuals identified morphologically as hybrids of the two species to determine their genetic relationship to fish identified morphologically as pallid and shovelnose sturgeon. Data from five nuclear DNA microsatellite loci indicated that pallid and shovelnose sturgeon were genetically distinct at three sympatric localities. Pallid sturgeon from two northern populations in the upper Missouri River were genetically distinct from the southern Atchafalaya River population, suggesting that northern and southern populations are reproductively isolated. Shovelnose sturgeon from three populations were genetically indistinguishable and showed no population structure. Sturgeon identified morphologically as hybrids from the Atchafalaya River were genetically distinct from pallid sturgeon but were indistinguishable from shovelnose sturgeon. These latter results are the converse of companion results with mitochondrial DNA published elsewhere. Pallid sturgeon were federally listed as endangered in 1990, and information about their population structure and potential for introgression with shovelnose sturgeon is critical for management and recovery programs for pallid sturgeon. Based on morphological characteristics, the pallid sturgeon Scaphirhynchus albus was first described by Forbes and Richardson (1905) as a variant of shovelnose sturgeon S. platorynchus. Recognition of these species depends solely on morphological characters (Bailey and Cross 1954; Carlson et al. 1985; Keenlyne et al. 1994b), al* Corresponding author: [email protected] Received February 22, 2000; accepted October 1, 2000 though the two forms do have different geographic distributions and microhabitat preferences (Kallemeyn 1989). The shovelnose sturgeon is relatively common and widely distributed throughout the Mississippi, Missouri, Ohio, and Tennessee rivers, including many related major tributaries (Carlson et al. 1985). The pallid sturgeon is less abundant and is restricted to the main channels of the Missouri, Yellowstone, and lower Mississippi Rivers (Carlson et al. 1985; Mayden and Kuhuda 1997) where they occur sympatrically with shovD ow nl oa de d by [ M ic hi ga n St at e U ni ve rs ity ] at 0 9: 12 1 4 M ay 2 01 5