Reconstructing the Evolutionary History of Bacterial Endosymbiosis in Obligate Blood-feeding Lice (Phthiraptera: Anoplura)

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This project will begin an in-depth exploration of three types of organisms that have lived together for many millions of years, mammals, their parasitic lice, and bacteria that live within the lice. These lice survive by taking blood meals from their host, yet mammalian blood is lacking nutrients that are required for the survival of the parasitic lice. In order to complete their life cycle the lice must also get nourishment from bacteria that live as symbionts within the lice. It was previously thought that the acquisition of a single bacterial symbiont long ago enabled these lice to persist on mammal blood alone. However, preliminary work by the PI, Co-PIs, and collaborators has shown that each major group of lice has its own endosymbiont distinctly different from the others. This suggests that lice have taken on internal symbionts several times, raising the question how often have these complex three-level interactions arisen between mammals, lice, and bacteria? The investigators will use DNA sequences from mammals, lice, and bacteria to study precisely how these groups have been evolving together over the last 80 million years. Because many of the mammals being studied have good fossil records, the investigators will be able to calibrate their evolutionary trees in order to estimate the age of each mammal/louse/bacterium association. The lice being studied include parasites of humans such as the human head louse and the human clothing louse. Head lice are epidemic in developed countries because lice have evolved resistance to the insecticidal shampoos used to treat people. Clothing lice, which are less common, cause three deadly diseases that still threaten humans in underdeveloped countries. Understanding how lice interact with their mammalian hosts and their bacterial symbionts is important for treating human louse infestations. This project will train one postdoc, one PhD student, and several undergraduate students in molecular and morphological phylogenetics (building evolutionary trees) as well as cophylogenetics (the study of co-evolving organisms). It will also take advantage of the infrastructure available at participating natural history museums to develop a program using mammals, lice, and bacteria to teach basic principles of evolutionary biology.