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Study Finds Fragmented Mitochondrial Genomes in Sucking Lice

NEW YORK (GenomeWeb News) – Rather than carrying one circular mitochondrial chromosome, the sucking louse Pediculus humanus — and at least a few other blood-feeding lice — have developed fragmented mt genomes consisting of mini-circular chromosomes, according to a new study scheduled to appear online in Genome Research this week.

Scientists from the University of Queensland and the J. Craig Venter Institute sequenced and characterized the mt genome of the human body louse Pediculus humanus. They found that the sucking lice mt genome wasn't found on one chromosome, as in chewing lice. Instead, sucking louse mtDNA was spread out over 18 circular mini-chromosomes that appear to have recombined with one another — a pattern the team detected in four other sucking lice species.

"It appears that blood-feeding has co-evolved with mini-circular mt chromosomes in the lineage leading to sucking lice," co-senior author Stephen Barker, a parasitology and molecular biology researcher at the University of Queensland, and his colleagues wrote.

Lice have been classified into four different subgroups. Three of these include "chewing lice" that gnaw on feathers, hair, and other epidermal materials while the fourth subgroup consists of "sucking lice," which feed on blood alone. Human head, body, and pubic lice all fall into the sucking lice category.

Previous studies suggest that the three different chewing lice species evaluated have 37 mt genes on a circular chromosome that's 14,700 to 15,600 base pairs in size. For the latest project, the researchers focused on the human body louse P. humanus, using whole genome shotgun sequencing to sequence the creature's nuclear and mt genomes.

Although P. humanus had all 37 mt genes described for chewing lice, the researchers found that they did not all fall on a lone mt chromosome. Instead, body louse mt sequences fell into 18 different contigs, each containing a distinct set of mt genes and non-coding DNA.

Their subsequent experiments and analyses suggest each of the louse's 18 mini-circular chromosomes is about 3,000 to 4,000 bases long and houses between one and three mt genes, along with non-coding sequences. Much of the non-coding mtDNA was extremely polymorphic, even when comparing different clones representing the same mini-chromosome.

For instance, repetitive regions of non-coding mtDNA had variable sequences, length, and copy number. Even so, the team speculated that these sequences likely differ more from one louse to the next than within the same louse.

Interestingly, when the researchers assessed four other sucking lice species — the human head louse, human crab louse, chimpanzee louse, and langur louse — they found that these organisms also appear to have mini-circular mtDNA chromosomes. The human head louse had a dozen or more, the human crab louse had at least nine, the chimpanzee louse had around five, and the langur louse had one or more.

On the other hand, the researchers noted, there is no evidence so far suggesting chewing lice have fragmented mtDNA based on previous studies of wallaby, pigeon, screamer, and other chewing lice mtDNA.

That, in turn, suggests mt genome fragmentation from a single mt chromosome may have evolved hand-in-hand with blood feeding in sucking lice, the researchers suggested. They speculated that recombination between the mini-chromosomes, which appears to occur, could have spurred sequence variation and adaptation to new hosts, environments, and feeding strategies.

Even so, they noted, more research is needed to understand this evolution, its effects on mtDNA expression, and role in adaptation to blood feeding, if any.

"Increased sequence variation in mt genes may have been an advantage to the sucking lice when they were adapting to a new diet, blood, and new hosts, the eutherian mammals," Barker and his co-authors wrote. "It remains to be elucidated, however, how changes in life-history traits, such as feeding style, may interact with changes in mt genomic structure."

In a statement released last night, Brown University researcher David Rand, who studies mtDNA evolution and was not involved in the study, said the new research raises interesting new questions about the nature of mtDNA evolution, potential gene expression changes associated with fragmented mt genomes, why this mt pattern evolved in sucking lice, and whether other creatures have similar mt genome organization.

"As new sequencing tools allow researchers to move away from single-gene studies to whole genome analyses of mtDNA, we may see that the sucking lice story is more common than we expected," Rand noted.

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