Revision 1 : D - 13 - 0016 1 2 Changes in Base Composition Bias of Nuclear and Mitochondrial 3 Genes in Lice ( Insecta : Psocodea ) 4 5

While it is well known that changes in the general processes of molecular evolution have 18 occurred on a variety of timescales, the mechanisms underlying these changes are less well understood. 19 Parasitic lice (“Phthiraptera”) and their close relatives (infraorder Nanopsocetae of the insect order 20 Psocodea) are a group of insects well known for their unusual features of molecular evolution. We 21 examined changes in base composition across parasitic lice and bark lice. We identified substantial 22 differences in percent GC content between the clade comprising parasitic lice plus closely related bark 23 lice (= Nanopsocetae) versus all other bark lice. These changes occurred for both nuclear and 24 mitochondrial protein coding and ribosomal RNA genes, often in the same direction. To evaluate whether 25 correlations in base composition change also occurred within lineages, we used phylogenetically 26 controlled comparisons, and in this case few significant correlations were identified. Examining more 27 constrained sites (first/second codon positions and rRNA) revealed that, in comparison to the other bark 28 lice, the GC content of parasitic lice and close relatives tended towards 50% either up from less than 50% 29 GC or down from greater than 50% GC. In contrast, less constrained sites (third codon positions) in both 30 nuclear and mitochondrial genes showed less of a consistent change of base composition in parasitic lice 31 and very close relatives. We conclude that relaxed selection on this group of insects is a potential 32 explanation of the change in base composition for both mitochondrial and nuclear genes, which could 33 lead to nucleotide frequencies closer to random expectation (i.e., 50% GC) in the absence of any mutation 34 bias. Evidence suggests this relaxed selection arose once in the non-parasitic common ancestor of 35 Phthiraptera + Nanopsocetae and is not directly related to the evolution of the parasitism in lice. 36 37