Population structure, genetic variability and gene flow of the bean leaf beetle, Cerotoma trifurcata (forster) in the midwestern US and its morphological analysis

Genetic variability and gene flow within and among bean leaf beetle subpopulations in the Midwest-US were investigated using amplified fragment length polymorphism (AFLP). Bean leaf beetle (BLB) is a common pest of soybean in the Midwest. However, there are currently no studies on the genetic variability of bean leaf beetle which forms part of the key information useful for designing effective pest management programs. The AMOVA results in this study indicate that the majority of the genetic variation was from within subpopulations and only a small amount of the total variation was attributed to the variation among the subpopulations. The relative measure of genetic differentiation between subpopulations (GST) for the entire BLB population indicated that the majority of genetic variation is found within the BLB subpopulations, further supporting the AMOVA results. The average gene flow among all the BLB subpopulations was high. There was no indication of correlation of geographical and genetic distance for all the BLB subpopulations. Even the five regions when looked at separately did not show any positive correlation between genetic and geographic distances. A UPGMA dendrogram constructed from the genetic distance matrix did not reveal well defined BLB samples grouping based on their geographic areas. For temporal variation among BLB subpopulations, the subpopulations from both Nebraska and Ohio were found not to differ genetically relative to year of sampling. Therefore, it can be concluded that there is no significant genetic difference among the bean leaf beetle subpopulations in the Midwest-US; the BLB is a panmictic population in the Midwest-US. This study also looked at both the morphological differences and the genetic variation among the four color forms in BLB population using AFLP. Bean leaf beetle exhibits a relatively large amount of morphological variation in terms of color but less is known about its genetic structure and gene flow. This study revealed low amounts of the total genetic variation among the four BLB color morphs samples which imply low genetic variation among the color morphs. This has been supported by the high gene flow recorded in this study. The pattern of genetic variation is very similar to that observed for the majority of the morphological traits examined. This similarity suggests that the four different color morphs are not genetically or reproductively distinct from each other. For the morphological data, the red with no spots and the green with no spots samples showed differences in weight, elytra-length and body-length when compared to the red with spots sample. The elytra-length was also significantly different between green with spots and red with spots. In addition, green with spots was also significantly different to the red with no spots and the green with no spots samples in terms of elytra-length. All the other color morphological characters were not significantly different among the color morphs. This study also evaluated canonical analysis as a method for grouping and distinguishing four BLB color morphs by morphometric parameters. Canonical analysis based on average morphometric characters enabled the examination of the grouping and morphometric relationships of these color morphs. It has been revealed that canonical variable 1 and 2, which accounted for 98% of the total variation were able to distinctly group and separate the BLB color morphs. The two canonical variables suggested that the assemblages of four BLB color morphs were different in their assemblage and composition relative to the measured parameters. Therefore, it can be concluded that this is just a single interbreeding BLB population showing differences in color and other morphological characters. Therefore, it can be concluded that this is a single interbreeding, population showing differences in color and other morphological characters