Genome analysis of major tick and mite vectors of human pathogens

Cate A. Hill, Tick and Mite Genomes Consortium
Broad Institute

Ticks and mites (subphylum Chelicerata: class Acari) transmit a greater variety of human and animal pathogens than any other arthropod vector. Tick- and mite-borne diseases are global health problems caused by a variety of bacterial, viral and protozoan pathogens, which are responsible for significant morbidity and thousands of human deaths annually. The incidence of many tick-borne diseases is increasing worldwide - many are emerging zoonoses or exotic diseases that could be introduced to the U.S. The Acari are a diverse and basal group within the phylum Arthropoda, comprised of two lineages; the Acariformes or chigger mites and the Parasitiformes which includes the ticks (suborder Ixodida) and other medically important mites. Despite their impact on human health, little is known about the biology and genetic basis of vector competence in the Acari. The NIH funded Ixodes scapularis (Lyme disease tick) assembly represents the only available genome sequence of a medically important species within this entire diverse lineage. Unfortunately, application of this resource is limited by low sequence coverage and fragmentation of the assembly.

Here we propose immediate sequencing of the mite vector of scrub typhus, Leptotrombidium deliense, and additional sequencing of I. scapularis to expand genomic resources for this group (summarized in Table 1). These projects will provide the anchor for additional sampling of species at increasing evolutionary distance. We also identify six members of the Ixodida (Dermacentor variabilis, Amblyomma americanum, I. pacificus, I. ricinus, I. persulcatus and Ornithodorus turicata) that are considered high priority sequencing targets by the tick and mite research communities. The haploid genome size of these ticks is expected to exceed 1 Gbp. Recognizing the inherent challenges associated with de novo sequencing and assembly of such genomes and the likely rapid advances anticipated in genome sequencing and assembly technology, we propose extensive transcriptome sequencing of these species to position them for anticipated genome sequencing efforts. Should sequencing of L. deliense and I. scapularis prove successful, we identify two additional species of Ixodida (D. variabilis and O. turicata) as candidates for genome sequencing efforts.

Identifying new strategies to control tick and mite vectors and the pathogens they transmit is a central theme of tick and mite research programs worldwide. The genome sequence data needed to accomplish these goals are not yet in hand. This proposal represents the cooperative efforts of the international tick and mite research community to develop the critical resources to facilitate comprehensive genomic studies across this important, yet largely neglected group of arthropod vectors. It outlines our long term plan for implementing genomic research in major evolutionary groups and key vector species of the Acari, and identifies the steps the community will take to achieve this goal.