Detection of hybrid Pyganodon grandis and P. lacustris (Bivalvia: Unionidae) using F- and M-type mitochondrial DNA sequences and geometric morphometrics

ABSTRACT Pyganodon grandis and P. lacustris, widespread and common species in North America, are known to co-occur across parts of their distributions. These mussels are genetically distinguishable with a sequence divergence between 9% and 13% for the mitochondrial (mt) DNA barcoding region, cytochrome c oxidase subunit I (COI), which is well beyond thresholds for distinct species. The objectives of this study were to (1) confirm the presence of two genetically distinct species (P. grandis and P. lacustris) in Font Lake and Lake Geneserath on Beaver Island in northern Lake Michigan, (2) identify hybrids using mitogenotype mismatch and (3) compare species and populations morphometrically. Of the 212 specimens sequenced, 146 (68.9%) were found to be heteroplasmic, with successful amplifications of both maternally (F-type, from mantle tissue) and paternally (M-type, from gonad tissue) inherited mt DNA sequences. Mismatches in species identity of the F- and M-type COI sequences were found in 22 specimens (15.1% of heteroplasmic specimens), with these being considered putative hybrids. Geometric morphometric analyses of shell shape reliably differentiated specimens by species (identified using COI sequences) and between lakes, correctly assigning 97.4% of homoplasmic and heteroplasmic specimens with matching mitogenomes to both species and lake of origin. The shape of putative hybrids generally resembled those of the F-type mt DNA sequence that was recovered. This study confirms that hybridization does occur between sympatric Pyganodon species. Additionally, our results reemphasize the value of modern morphometric techniques for distinguishing among species and populations with overlapping morphological variability.

Modeling cadmium uptake from water and food by the freshwater bivalve Pyganodon grandis

For this study, the unionid bivalve Pyganodon grandis was either exposed to dissolved cadmium (Cd) or fed Cd-contaminated algae ( Pseudokirchneriella subcapitata ) during short-term laboratory experiments. Cadmium accumulated largely in the digestive gland after a dietary exposure, or in the gills after an aqueous exposure; in these latter experiments, Cd accumulation from the dissolved phase increased as bivalve filtration rates increased. The results of these uptake experiments were used to parameterize a biodynamic model, which was then used to estimate the relative importance of water and food as sources of Cd for this bivalve, and to predict steady state Cd concentrations in the gills and digestive gland of native bivalves. In comparisons between the simulations and data obtained from earlier field studies on P. grandis, the model adequately predicted Cd concentrations in P. grandis gills, except in Ca-rich lakes, whereas it tended to overestimate Cd concentrations in the digestive gland. The field simulations indicate that water is the main source of Cd for both the gills (relative importance water:food::99:1) and the digestive gland (water:food::80:20). These results will facilitate the interpretation of spatial and temporal variations in Cd concentrations in free-living P. grandis, which is a promising metal biomonitor.