The population genetic structure of North American Lucilia sericata (Diptera: Calliphoridae), and the utility of genetic assignment methods for reconstruction of postmortem corpse relocation.

Lucilia sericata (Diptera: Calliphoridae) is a common urban blowfly, with a worldwide distribution. It is among the most important forensic insects, and a major veterinary pest. A previous amplified fragment length polymorphism (AFLP) survey of Phormia regina, another blowfly, found that the North American population structure results from the fact that flies trapped together at a bait are predominantly comprised of related individuals. We report here a similar population genetic pattern for L. sericata in North America based on AFLP genotypes with 249 loci. A STRUCTURE analysis found no population structure on a geographic scale, and analysis of molecular variance found a moderate amount of variation attributed to samples (adults collected at the same bait at the same time, Phi(SC)=20%, P=0.001). A Mantel test found a negligible correlation between geographic and genetic distances (R(2)=0.0063, P=0.02). The mean relative relatedness coefficient for every sample was positive (mean R=0.2486+/-0.18). Gravid females in a sample, those likely to oviposit on the same corpse, showed a pattern of relatively high relatedness similar to the total survey. Therefore, this pattern of local relatedness is likely to occur with larvae in a corpse, and if so it might support a genetic test for inferring the postmortem relocation of a corpse. This is because a larva may fall from the body at the original scene as it is moved. Connecting such a "stray" larva to the larval population in a corpse would provide powerful evidence that the corpse had been at both locations. Assignment tests resulted in a 96% success rate of assigning L. sericata individuals to their samples of origin.