Role of gene duplication in genome size and evolution of hard ticks

Hard ticks (Ixodidae) are obligatory hematophagous ectoparasites of worldwide medical and veterinary importance. Ticks transmit a greater variety of animal pathogens than any other arthropod and are second only to mosquitoes as vectors of human pathogens. Hard ticks have large genomes ranging from 1 to >7 Gbp, but the composition and organization of these genomes are largely unknown. We hypothesize that segmental or whole-genome duplications, in addition to a proliferation of repetitive DNA, have occurred in multiple tick lineages that can account for genome size and chromosome number observed in Ixodid ticks. A bioinformatics approach was used to identify duplicated genes (paralogs) using EST sequences from four species of hard ticks, namely the prostriate tick Ixodes scapularis, and the metastriate ticks Amblyomma variegatum ( tropical bont tick), Rhipicephalus microplus (southern cattle tick) and Rhipicephalus appendiculatus (brown ear tick). Between 2-10% of the ESTs were duplicated, representing putative gene or segmental duplication events. Based on the ratio of non-synonymous to synonymous nucleotide substitution rates, approximately 25% of these duplications appear to be under positive selection, which indicates they may be evolving novel functions. Synonymous substitution rates in the paralogs will be used to estimate the timing of duplication events in each tick, which is important for understanding genome evolution within the Ixodidae. Cytogenetic analyses using fluorescent in situ hybridization have been developed in I. scapularis and R. microplus that will be used to validate putative duplications and determine their role in tick genome composition and evolution.