There is a distinct paucity of genomic data available for the class Acari (mites and ticks), and subsequently our understanding of their biology is limited. More information regarding mite genomics would greatly assist the development of novel control strategies underpinned by molecular approaches. Three astigmatid mites of particular medical and veterinary interest are the itch mite Sarcoptes scabiei (Sarcoptidae), the sheep scab mite Psoroptes ovis (Psoroptidae) and the house dust mite Dermatophagoides pteronyssinus (Pyroglyphidae).
Scabies remains a truly neglected disease, caused by the burrowing ectoparasitc "itch mite" S. scabiei. The scarcity of molecular data on S. scabiei has been due in part to very low parasite burden in most patients, and the historical lack of in vitro culture and animal models. A major advance was the creation of S. scabiei var. hominis cDNA libraries resulting in a database of ~43,000 Expressed Sequence Tags (ESTs), providing substantial molecular data for this parasite and securing a solid base for recombinant biology [1, 2]. More recently, a tractable porcine model of scabies has been established , providing large amounts of mite material for molecular studies. P. ovis is a non-burrowing, ectoparastic mite causing the highly infectious disease 'sheep scab' in sheep and cattle, responsible for major economic losses and serious welfare concerns . There is a similar dearth of understanding regarding parasite biology, but recently over 1,500 ESTs have been generated and deposited in public databases, representing the largest molecular data resource on P. ovis to date . Finally, although allergies to house-dust mites are extremely common, remarkably little genetic information is available on the causative agents, the free-living mites D. pteronyssinus, D. farinae and Euroglyphus maynei. Research efforts have mainly focused on characterisation and generation of recombinant house dust mite allergens for diagnostics and immunotherapy . However, a dataset of ~3000 ESTs is available , and the mitochondrial genome of D. pteronyssinus has recently been sequenced .
The Ixodes scapularis (black-legged tick) genome sequencing project marked the beginning of the genomics era for the field of acarology. As previously observed, there appears to be no patterns regarding genome size in the Acari. Flow cytometry based genome estimates indicated a large haploid genome for all Ixodida with a mean of 1281 Mbp (approx. 1.31 pg) for the Argasidae and 2671 Mbp (approx. 2.73 pg) for the Ixodidae . An exciting development for mite genomics has been the publication of a preliminary genome survey for the honey-bee mite Varroa destructor. At 565 Mbp, the Varroa genome is larger than many insects, including its host Apis melifera (262 Mbp). In contrast, the two-spotted spider mite Tetranychus urticae is predicted to possess a genome of only 75 Mb (0.08 pg) , however, this estimate was recently revised upward to 90.7 Mbp . A similarly small nuclear genome of 88-90 Mbp was estimated in the phytoseiid mite Metaseiulus occidentalis, with the orbatid mite Archegozetes longisetosus genome estimated to be 150 Mbp .
Attempts to estimate the S. scabiei genome size accurately using flow cytometry have been impeded by the inability to obtain sufficient cell numbers from egg and whole-body preparations. The nuclei are very small in comparison to nuclei from mosquito cell lines, suggesting that the scabies genome may also be small (Fischer, unpublished observations). Similar issues were reported in M. occidentalis, where flow cytometry failed to resolve genome size, with estimates ranging from 35-160 Mbp, depending on egg age . Due to the small size of mites, tissue dissections are difficult, hindering aquisition of homogenous cell preparations. An alternative approach utlilising quantitative-PCR (qPCR) was suggested to be particularly useful for organisms where the genome size is expected to be small and the availability of genetic material limited . Based on on a pioneering study by Wilhelm , this method has proven reliable for a number of species, including Saccharomyces cerevisiae, Xiphorphours maculaus, Homo sapiens, Musca domestica, and Drosophila melanogaster. Here, we use qPCR to estimate the genome sizes of S. scabiei, P. ovis and D. pteronyssinus, with the purpose of informing future sequencing projects. Additionally, the chromosome number of S. scabiei was determined, providing a starting point for understanding the genetic organisation and evolution for this species.