Wednesday, 21 December 2011

Reading this will make you itch, the 1000 head louse genome project.

It’s that time of year when kids come home from school and start complaining of “the itches”; the nits are back.
I don’t know about the rest of the world but here in the UK we used to have “nit nurses” who would go from school to school checking for headlice. As a kid I remember lining up with the other kids in class and having her fingers run through my hair looking for eggs and lice. It may be strange but for me it is a lovely memory!

Nit genomics: This year when the school sent home the inevitable letter that head lice had been confirmed in my daughters class my thoughts turned to the louse genome (I am aware this is a bit nerdy). Has it been sequenced and what might we learn about the spread of nits and individual susceptibility through genomic analysis? After all the Belly Button Biodiversity project turned out to be pretty interesting didn’t it?

The nits close relative the body louse (Pediculus humanus humanus) has been recently sequenced. The body louse was sequenced using Sanger shotgun methods in 2010. 1.3M. It is a very AT rich genome. It has the smallest insect genome yet sequenced and apparently is the first animal genome to be shown to have fragmented mitochondrial mini-chromosomes.

The head louse genome is only 108Mb. As these parasites are generally quite prolific it should be possible for me to collect a reasonable number from each of my kids heads and mine and my wifes over the few weeks I am combing then out with conditioner (wet combing is as effective as any insectidcide based trewatment). I got four or five this morning from my daughter!

Ideally one would collect only larvae that have not yet started sucking blood to avoid having to sequence some of the Human genome as well (although I am not certain how much Human DNA would contaminate each louse).

With this sample it might be possible to get some idea of the population structure within a school, possibly through some molecular barcoding once we have good genes to target. Perhaps we can learn something about the spread of this organism through a community. As it is pretty harmless it should be easy to collect samples form schools allover the world. Are the head lice in Wymondham different from the lice in Waikiki, do they have lice in Waikiki?

If we could look deeper into the host could we find susceptibility loci and would screening of more susceptible individuals reduce the outbreaks we see each year? What else might we learn about this host:parasite interaction? Are different blood groups more or less affected by lice? There are so many questions we might answer.

I am not certain I will get the time to pursue this project but if there is an enterprising grad student that wants to take this on do get in touch.

Nit biology and evolution: I am writing this just because I am almost certainly never going to write about it again but I want to make sure I can explain it to my kids! Most of this comes from two papers I'd encourage you to read so see the references at the end.

Nits and lice are hemimetabolous rather than holometabolous insects. That is they develop from nymphs to adults rather than going through a larvae–pupae–adult transformation. The holometabolous strategy allows larvae and adults to occupy different ecological niches and as such has proven highly successful. However the niche occupied by nits is the same regardless of life-cycle stage. Nits are a strict Human obligate-ectoparasite and are provided with a homogenous diet (our blood) and few xenobiotic challenges. As such it appears that lice were able to reduce their genome size by losing genes associated with odorant and chemo-sensing; they have 10 fold fewer odorant genes than other insect sequenced and relatively few detoxification-enzyme encoding genes. Basically they don't need to find food or deal with harmful plant toxins.

Lice have been in contact with us for a long time and Human and Chimpanzee lice diverged at the same time as we did from our common ancestor about 25M years ago. We have been living and evolving together ever since with the body louse evolving relatively recently as we began to wear clothing. A paper by Kittler et al used a molecular clock analysis with 2 mtDNA and 2 nuclear loci across diverse human and chimpanzee lice. They saw higher diversity in African lice similar to Human diversity and estimated that body lice evolved around 70,000 years ago. They said this correlated with the time when Humans left Africa, I guess we had to wear something when we moved into Europe as it’s a whole lot colder than Africa.

Kirkness et al. Genome sequences of the human body louse and its primary endosymbiont provide insights into the permanent parasitic lifestyle. PNAS 107; 27: 2168–12173 (2010)
Kittler et al. Molecular evolution of Pediculus humanus 
and the origin of clothing. Curr Biol 13:1414–1417. (2003)

1 comment:

  1. Really i came to know through your blog that smallest insect genome yet sequenced and apparently is the first animal genome to be shown to have fragmented mitochondrial mini-chromosomes. Thanks for sharing.