Evolution of the egg glue protein in great ape lice inferred from its sequence comparisons across five louse types

Humans host three types of parasitic louse: the conspecific head and body lice differing in habitat (scalp and clothing), and the pubic louse which is a distinct species adapted to coarser body hair. Chimpanzees and gorillas host only a single louse species respectively related to the human head/body lice and the pubic louse. Louse nit sheath proteins (LNSPs) are the egg glue of human head (or body) that function to secure the eggs to the hair (or clothing). In this study, comparisons of protein sequences at various regions of LNSP1/2 from five types of lice infesting the great apes were done, which showed species-dependent variations. Estimations of the Unit Evolutionary Period (UEP, time required for 1% difference in amino acid sequence) performed using the known times of divergence from their common ancestors suggested that both the LNSP1 (0.37 MY (million years)) and the LNSP2 (0.48 MY) evolved ~3-fold faster than the cytochrome c of these louse species (with UEP of 1.3 MY). Furthermore, the observation inferred from the dN/dS analysis that evolutionary pressure acts differently on various regions of LNSP1/2 may be related to the homologous LNSP1 and LNSP2 having differing detailed functionalities (desiccation resistance and glue platform formation, respectively).