Fragmented mitochondrial (mt) genomes have been reported for 11 species/subspecies of sucking lice (suborder Anoplura) that infest humans, chimpanzees, pigs, horses and rodents. There is substantial variation among these lice in mt karyotype: the number of minichromosomes of a species/subspecies ranges from 9 to 20; the number of genes in a minichromosome ranges from 1 to 8; arrangement of genes in a minichromosome differs between species, even in the same genus. We sequenced the mt genome of the guanaco louse, Microthoracius praelongiceps, which is from a major branch of sucking lice not sampled previously. We aim to establish the ancestral mt karyotype for sucking lice, and to understand how mt genome organization evolved in sucking lice.
The 37 mt genes of the guanaco louse are on 12 minichromosomes; each minichromosome is 2,274 to 2,940 bp in size, and has 2 to 5 genes and a non-coding region. The guanaco louse shares many features with the rodent lice in mt karyotype, more than with other sucking lice. Phylogenetic analysis of mt genome sequences, however, showed that the guanaco louse is more closely related to the human lice, chimpanzee lice, pig lice and horse lice than to the rodent lice. By analysis of shared features in mt karyotype, we infer that the most recent common ancestor of sucking lice, which lived ~75 MYA, had 11 minichromosomes; each minichromosome has 1 to 6 genes and a non-coding region. As sucking lice diverged, split of mt minichromosomes occurred many times in the lineages leading to the lice of humans, chimpanzees and rodents whereas merge of minichromosomes occurred in the lineage leading to the lice of pigs and horses, resulting in a complex and dynamic mt genome organization not seen in any other animals.