A nondestructive, noninvasive genetic test for the presence/absence of the Giant Palouse earthworm (Driloleirus americanus)
The Giant Palouse earthworm (Driloleirus americanus) is the only known earthworm species native to Palouse prairie, a unique and critically endangered ecosystem of which less than 1% of the original extent remains. Little is known about D. americanus and current sampling methodologies result in varying degrees of ecosystem disturbance. Our goal was the development of a nondestructive, noninvasive test for detecting the presence or absence of D. americanus in Palouse prairie remnants. Samples of tissue and body swabs were taken from reference specimens of both D. americanus and Aporrectodea trapezoides, an exotic, invasive earthworm dominant in Palouse prairie remnants. DNA was extracted using commercial kits and the extracted DNA was amplified via PCR using cytochrome c oxidase subunit I (COI) primers. Fragment analysis was performed by capillary electrophoresis and a conserved primer set was designed for D. americanus using sequences of both D. americanus and A. trapezoides. Three field sample collection methods were tested: (1) cotton swab of burrow, (2) scrape of burrow lining, and (3) excreted casts. Samples were collected from five Palouse prairie remnants. DNA was extracted, amplified with the primer set for D. americanus, and analyzed by capillary electrophoresis.
Initial results were mixed. Swabs were not successful, but scrapes and casts were. Of the five Palouse prairie remnants tested, three tested positive for the presence of D. americanus. This included one site which had no previous record of the organism. Because DNA is susceptible to degradation by air and light, shed cells in burrow linings and excreted casts may be more protected. This may explain why a simple swab of burrow linings was not successful in field testing. However, there is no practical difference among all three methods with regards to speed and ease of collection. There is the potential to develop primer sets for multiple earthworm species which would allow for an assessment of community structure. The methodology developed in this study allows for the detection of the presence or absence of D. americanus, a rare native earthworm species, in Palouse prairie remnants in a way that is both nondestructive and noninvasive.