Morphometric Inferences on Sibling Species and Sexual Dimorphism in Neochlamisus Bebbianae Leaf Beetles: Multivariate Applications of the Thin-Plate Spline

Nominally polyphagous species of herbivorous insects sometimes are comprised of multiple morphologically similar biological species with more specialized appetites. When meristic morphological traits cannot be found to distinguish such suspected sibling species, molecular data are increasingly sough... Full description

Nominally polyphagous species of herbivorous insects sometimes are comprised of multiple morphologically similar biological species with more specialized appetites. When meristic morphological traits cannot be found to distinguish such suspected sibling species, molecular data are increasingly sought as a source of evidence. A role for morphology in distinguishing such taxa might be reclaimed, however, by recent advances in geometric morphometric methods, such as the statistical analysis of partial-warp scores from the thin-plate spline. We employed this method to detect and characterize subtle shape differences among populations and between sexes of the nominal leaf beetle species Neochlamisus bebbianae. Using the thin-plate spline, the shapes of specimens from seven beetle populations collected from five host plants in five eastern North American localities were calculated. These shapes were analyzed by MANOVA, revealing significant variation in both uniform and nonuniform components of shape among test populations. Significant sexual dimorphism in size, shape, and allometric relationships were also documented across these populations. More interestingly, our study provided evidence of sibling species where traditional taxonomic approaches have failed. Individual MANOVAs revealed significant shape variation between sympatric populations from different host plants in each of three localities. Because these sympatric shape differences were significant when adjusted for size, they cannot be attributed to allometric consequences of size variation among test populations. Because certain beetle populations differed significantly in size and shape when reared in a common environment, these morphometric traits may have a genetic basis. Together, these results are consistent with an earlier suggestion that N. bebbianae represents a complex of host-specific races or sibling species, a hypothesis that has received additional support from recent studies on host use traits, sex ratios, and mitochondrial DNA. In sum, these analyses demonstrate the power and utility of the thinplate spline as a morphological means of discriminating among closely related and anatomically homogeneous taxa.