Changes in base composition bias of nuclear and mitochondrial genes in lice (Insecta: Psocodea)
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While it is well known that changes in the general processes of molecular evolution have occurred on a variety of timescales, the mechanisms underlying these changes are less well understood. Parasitic lice (“Phthiraptera”) and their close relatives (infraorder Nanopsocetae of the insect order Psocodea) are a group of insects well known for their unusual features of molecular evolution. We examined changes in base composition across parasitic lice and bark lice. We identified substantial differences in percent GC content between the clade comprising parasitic lice plus closely related bark lice (=Nanopsocetae) versus all other bark lice. These changes occurred for both nuclear and mitochondrial protein coding and ribosomal RNA genes, often in the same direction. To evaluate whether correlations in base composition change also occurred within lineages, we used phylogenetically controlled comparisons, and in this case few significant correlations were identified. Examining more constrained sites (first/second codon positions and rRNA) revealed that, in comparison to the other bark lice, the GC content of parasitic lice and close relatives tended towards 50 % either up from less than 50 % GC or down from greater than 50 % GC. In contrast, less constrained sites (third codon positions) in both nuclear and mitochondrial genes showed less of a consistent change of base composition in parasitic lice and very close relatives. We conclude that relaxed selection on this group of insects is a potential explanation of the change in base composition for both mitochondrial and nuclear genes, which could lead to nucleotide frequencies closer to random expectation (i.e., 50 % GC) in the absence of any mutation bias. Evidence suggests this relaxed selection arose once in the non-parasitic common ancestor of Phthiraptera + Nanopsocetae and is not directly related to the evolution of the parasitism in lice.
KeywordsMolecular evolution GC content Relaxed selection Slightly deleterious mutation
We thank C. Lienhard, E. L. Mockford, Y. Kamimura, L. Durden, and J. Klicka for specimens. We also thank Joe Felsenstein and Colin Dale for suggestions in data analyses. Thanks are also due to two anonymous reviewers for constructive comments that are helpful in improving the earlier ms. KY thanks E. Hasegawa and M. Ôhara for allowing the use of their molecular facilities. This study was supported by Japan Society for the Promotion of Science Grants 18770058 and 21770083 to KY and US National Science Foundation Grant DEB-0612938 to KPJ.
- Dowton M, Austin AD (1995) Increased genetic diversity in mitochondrial genes is correlated with the evolution of parasitism in the Hymenoptera. J Mol Evol 16:298–309Google Scholar
- Li W-H (1997) Molecular evolution. Sinauer Associates, SunderlandGoogle Scholar
- Maddison DR, Maddison WP (2005) MacClade ver 4.08. Sinauer Associates, SunderlandGoogle Scholar
- Maddison WP, Maddison DR (2010) Mesquite: a molecular system for evolutionary analysis. Ver. 2.74. http://mesquiteproject.org. Accessed 30 June 2011
- Matsuo Y (2003) Evolution of the GC content of the histone 3 gene in seven Drosophila species. Genes Genet Syst 78:309–318Google Scholar
- Midford PE, Garland Jr. T, Maddison WP (2010) PDAP package of Mesquite. Ver. 1.15. http://mesquiteproject.org/pdap_mesquite/index.html. Accessed 30 June 2011
- Rosenberg MS (ed) (2009) Sequence alignment. University of California Press, BarkleyGoogle Scholar
- SAS Institute Inc (2009) JMP, version 8. SAS Institution, CrayGoogle Scholar
- Sharp PM, Li W-H (1989) On the rate of DNA sequence evolution in Drosophila. J Mol Biol 28:398–402Google Scholar