A phylogeny for Driloleirus americanus, the Giant Palouse earthworm, of the Pacific Northwest
The Giant Palouse earthworm (GPE) (Driloleirus americanus) is the only known native earthworm species of the Palouse prairie and thought extinct. However, a small number of specimens were recently detected in the Palouse region. Further, a number of specimens morphologically similar to the GPE were found in the Eastern Cascades. It is well established that earthworms are important ecosystem engineers and have a profound impact on ecosystem function. Thus, the GPE may play an essential role in the Palouse prairie ecosystem. Determining the geographic distribution and role of this rare earthworm is important since Palouse prairie is one of the most endangered prairie ecosystems in North America.
Observed morphological differences between isolates from both regions and the spatial separation of habitats indicated that these earthworms may not represent the same species. To clarify the phylogenetic relationship of the two populations we applied molecular methods. We isolated total DNA from earthworm tissue originating from worms from both areas. We used primers specific to two mitochondrial loci (16SrRNA and COI) and to two nuclear genes (12SrRNA and 28SrRNA) to amplify four amplicons for each specimen. These amplicons were sequenced and the nucleotide sequences were subsequently aligned.
Preliminary results of our phylogenetic analyses indicate that the two populations from the Palouse region and the Eastern Cascades probably represent a single species rather than two distinct species. A petition to list D. americanus on the Endangered Species List was rejected partly because of a lack of information known about the species. These results help address that deficiency. The rapid disappearance of the Palouse prairie habitat and the low population density of GPE necessitated immediate work on more detailed characterization of the species. This study establishes a phylogeny and geographic distribution for a rare, native species of earthworm in the Pacific Northwest.