Project description:Most animals have a conserved mitochondrial genome structure composed of a single chromosome. However, some organisms have their mitochondrial genes separated on several smaller circular or linear chromosomes. Highly fragmented circular chromosomes ("minicircles") are especially prevalent in parasitic lice (Insecta: Phthiraptera), with 16 species known to have between nine and 20 mitochondrial minicircles per genome. All of these species belong to the same clade (mammalian lice), suggesting a single origin of drastic fragmentation. Nevertheless, other work indicates a lesser degree of fragmentation (2-3 chromosomes/genome) is present in some avian feather lice (Ischnocera: Philopteridae). In this study, we tested for minicircles in four species of the feather louse genus Columbicola (Philopteridae). Using whole genome shotgun sequence data, we applied three different bioinformatic approaches for assembling the Columbicola mitochondrial genome. We further confirmed these approaches by assembling the mitochondrial genome of Pediculus humanus from shotgun sequencing reads, a species known to have minicircles. Columbicola spp. genomes are highly fragmented into 15-17 minicircles between ?1,100 and ?3,100 bp in length, with 1-4 genes per minicircle. Subsequent annotation of the minicircles indicated that tRNA arrangements of minicircles varied substantially between species. These mitochondrial minicircles for species of Columbicola represent the first feather lice (Philopteridae) for which minicircles have been found in a full mitochondrial genome assembly. Combined with recent phylogenetic studies of parasitic lice, our results provide strong evidence that highly fragmented mitochondrial genomes, which are otherwise rare across the Tree of Life, evolved multiple times within parasitic lice.
Project description:Parasitic "wing lice" (Phthiraptera: Columbicola) and their dove and pigeon hosts are a well-recognized model system for coevolutionary studies at the intersection of micro- and macroevolution. Selection on lice in microevolutionary time occurs as pigeons and doves defend themselves against lice by preening. In turn, behavioral and morphological adaptations of the lice improve their ability to evade host defense. Over macroevolutionary time wing lice tend to cospeciate with their hosts; yet, some species of Columbicola have switched to new host species. Understanding the ecological and evolutionary factors that influence coadaptation and codiversification in this system will substantially improve our understanding of coevolution in general. However, further work is hampered by the lack of a robust phylogenetic framework for Columbicola spp. and their hosts. Previous attempts to resolve the phylogeny of Columbicola based on sequences from a few genes provided limited support. Here, we apply a new approach, target restricted assembly, to assemble 977 orthologous gene sequences from whole-genome sequence data generated from very small, ethanol-preserved specimens, representing up to 61 species of wing lice. Both concatenation and coalescent methods were used to estimate the species tree. These two approaches yielded consistent and well-supported trees with 90% of all relationships receiving 100% support, which is a substantial improvement over previous studies. We used this new phylogeny to show that biogeographic ranges are generally conserved within clades of Columbicola wing lice. Limited inconsistencies are probably attributable to intercontinental dispersal of hosts, and host switching by some of the lice. [aTRAM; coalescent; coevolution; concatenation; species tree.].
Project description:BACKGROUND:Many groups of insects have obligate bacterial symbionts that are vertically transmitted. Such associations are typically characterized by the presence of a monophyletic group of bacteria living in a well-defined host clade. In addition the phylogeny of the symbiotic bacteria is typically congruent with that of the host, signifying co-speciation. Here we show that bacteria living in a single genus of feather lice, Columbicola (Insecta: Phthiraptera), present an exception to this typical pattern. RESULTS:The phylogeny of Columbicola spp. symbionts revealed the presence of three candidate clades, with the most species-rich clade having a comb-like topology with very short internodes and long terminal branches. Evolutionary simulations indicate that this topology is characteristic of a process of repeated symbiont replacement over a brief time period. The two remaining candidate clades in our study exhibit high levels of nucleotide substitution, suggesting accelerated molecular evolution due to relaxed purifying selection or smaller effective population size, which is typical of many vertically transmitted insect symbionts. Representatives of the fast-evolving and slow-evolving symbiont lineages exhibit the same localization, migration, and transmission patterns in their hosts, implying direct replacement. CONCLUSIONS:Our findings suggest that repeated, independent symbiont replacements have taken place over the course of the relatively recent radiation of Columbicola spp. These results are compatible with the notion that lice and other insects have the capability to acquire novel symbionts through the domestication of progenitor strains residing in their local environment.