Gene duplication and genome evolution in the Ixodidae

Hard ticks (Ixodidae) are obligatory hematophagous ectoparasites of worldwide medical and veterinary importance. Ixodid ticks have large genomes (1 to >7 Gbp) compared to those of other arthropods, but little is known about their genome structure and organization. We hypothesize that the evolution of genome size and chromosome number in Ixodid ticks is associated with an extensive accumulation of DNA through major duplication events and proliferation of repetitive sequences. To investigate the role of duplications events on genome size, a bioinformatics approach was used to identify putatively duplicated genes (paralogs) from expressed sequence tag sequences (ESTs) from four species of hard ticks, including the prostriate tick Ixodes scapularis (Lyme disease tick) and the metastriate ticks Amblyomma variegatum (tropical bont tick), Rhipicephalus microplus (southern cattle tick) and R. appendiculatus (brown ear tick). Among these ticks, 2-10% of EST sequences represent putatively duplicated genes, and ~25% of duplicated genes are under positive selection pressure based on ratios of non-synonymous to synonymous nucleotide substitution rates. Many genes under positive selection are associated with tick immunity, blood feeding and detoxification that may include important targets for vaccines or acaricides. Analysis of synonymous substitution rates suggests that two and three large gene duplication events occurred recently (<10-15 MYA) within the prostriate and metastriate tick lineages, respectively. Putative functions were assigned to paralogs ESTs using Gene Ontology (GO) searches and Blast2GO software. This research marks the first genome-wide analysis of gene duplication in ticks and provides insight towards an understanding of genome evolution within the Ixodidae.