Lousy little secret for blood suckers

Most animals have a single circular mitochondrial chromosome, but for some reason, the species of lice than infest human heads, underarms and pubic regions have 18.

The anomaly may have arisen along with their blood-sucking behaviour.

The genome of the body louse, Pediculus humanus, has recently been sequenced. Two researchers from the University of Queensland, Renfu Shao and Stephen Barker, along with Ewen Kirkness from the J. Craig Venter Institute in the US, have conducted a detailed analysis of the mitochondrial genome of the louse and found something very odd.

The team had previously attempted to amplify the entire mitochondrial genomes of sucking lice by techniques like PCR, but had failed, so they suspected there might be something different about the blood-suckers.

They were right. While some protists – algae, ciliates and flagellates – have multiple mitochondrial chromosomes, all other eukaryotes have a single, circular mt chromosome of about 16 kilobases long, containing 37 genes.

The louse, on the other hand, has 18 mini-circular chromosomes with the 37 genes parceled among them, with each chromosome about three to four kb long and containing between one and three genes.

Interestingly, other species of sucking lice – known as the Anoplura suborder of the Phthiraptera order of insects – also have this strange chromosomal structure. However, the species of lice who chew – the Amblycera, Ischnocera and Rhyncophthirina suborders – have the normal structure.

Chewing lice normally feed on hair and feathers, while sucking lice feed on blood.

While they are not sure why this strange structure has developed – it is the first known in an animal species, and any advantages of it are unknown – it seems to have co-evolved with blood-feeding.

The team says it will be interesting to explore the relationship between these characteristics. The research is published today in Genome Research.