Development of a food web for prairie grasshoppers using DNA barcoding
The construction of food webs is particularly challenging for animal species that are easily disturbed, highly mobile, and broadly generalist. One approach is to combine field observations with data collected through DNA barcoding. This molecular technique can be used to identify species from unknown tissue samples by comparing a DNA sequence to a DNA barcode "library" of potential matches from the field site or habitat to identify the source. Studies have used barcoding to describe the diets of several species of herbivorous mammals by barcoding the plant material in the feces. More recently, the diets of specialized tropical beetles have been determined by grinding up the beetles and extracting the plant DNA. In this study, we are building a feeding network for prairie grasshoppers, using a non-destructive approach that involves the collection of feces from live grasshoppers. At three blackland prairie sites in eastern Mississippi, grasshoppers were captured, identified, and held in test tubes until they produced a fecal pellet. They were then released, unharmed. Plant DNA was extracted from the frass, and the DNA was amplified through PCR using primers for the rbcL gene region. After the PCR product was sequenced, the sequences were aligned and manually edited.
Our samples came from 17 species of grasshoppers in 13 genera. From 187 initial samples, 56 yielded high quality, bi-directional sequences (with three additional uni-directional sequences). Of these, 35 were homozygous, consisting of plant DNA from a single species. These were compared to a pre-existing DNA barcode library for prairie plants from Mississippi and Alabama. Twelve samples were identified to a single plant species with greater than 99.5% pairwise identity, while 16 were identified to genus. The remaining seven samples were likely derived from plant species not in the barcode library. Twenty-four samples appeared to contain plant DNA from more than one species. Of the identified plants, sixteen taxa are represented, from 14 genera and seven plant families. The broad diets of many of the grasshopper species are evident. For example, in the six samples analyzed from Amblytropidia mysteca, four plant species were represented from three plant families. While we are currently working on ways to improve the recovery of DNA from samples, this technique allows for the construction of a basic feeding network that can then complemented and expanded by field methods.