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Open Access Highly Accessed Research article

The complexity of Rhipicephalus (Boophilus) microplus genome characterised through detailed analysis of two BAC clones

Paula M Moolhuijzen12, Ala E Lew-Tabor123, Jess A T Morgan23, Manuel Rodriguez Valle23, Daniel G Peterson5, Scot E Dowd6, Felix D Guerrero4, Matthew I Bellgard1* and Rudi Appels1

Author Affiliations

1 Centre for Comparative Genomics, Murdoch University, South St., Perth, Western Australia, 6150, Australia

2 Cooperative Research Centre for Beef Genetic Technologies, Armidale, NSW, Australia

3 Queensland Alliance for Agriculture & Food Innovation, The University of Queensland, (c/o Department of Employment, Economic Development and Innovation), Locked Mail Bag No. 4, Moorooka, QLD 4105, Australia

4 USDA-ARS, Knipling-Bushland U.S. Livestock Insects Research Laboratory, 2700 Fredericksburg Rd., Kerrville, TX 78028, USA

5 Department of Plant & Soil Sciences and Life Sciences & Biotechnology Institute, Mississippi State University, 117 Dorman Hall, Box 9555, Mississippi State, MS 39762, USA

6 Research and Testing Laboratory, 4321 Marsha Sharp Fwy, Lubbock, TX 79407, USA

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BMC Research Notes 2011, 4:254  doi:10.1186/1756-0500-4-254

Published: 22 July 2011

Abstract

Background

Rhipicephalus (Boophilus) microplus (Rmi) a major cattle ectoparasite and tick borne disease vector, impacts on animal welfare and industry productivity. In arthropod research there is an absence of a complete Chelicerate genome, which includes ticks, mites, spiders, scorpions and crustaceans. Model arthropod genomes such as Drosophila and Anopheles are too taxonomically distant for a reference in tick genomic sequence analysis. This study focuses on the de-novo assembly of two R. microplus BAC sequences from the understudied R microplus genome. Based on available R. microplus sequenced resources and comparative analysis, tick genomic structure and functional predictions identify complex gene structures and genomic targets expressed during tick-cattle interaction.

Results

In our BAC analyses we have assembled, using the correct positioning of BAC end sequences and transcript sequences, two challenging genomic regions. Cot DNA fractions compared to the BAC sequences confirmed a highly repetitive BAC sequence BM-012-E08 and a low repetitive BAC sequence BM-005-G14 which was gene rich and contained short interspersed elements (SINEs). Based directly on the BAC and Cot data comparisons, the genome wide frequency of the SINE Ruka element was estimated. Using a conservative approach to the assembly of the highly repetitive BM-012-E08, the sequence was de-convoluted into three repeat units, each unit containing an 18S, 5.8S and 28S ribosomal RNA (rRNA) encoding gene sequence (rDNA), related internal transcribed spacer and complex intergenic region.

In the low repetitive BM-005-G14, a novel gene complex was found between to 2 genes on the same strand. Nested in the second intron of a large 9 Kb papilin gene was a helicase gene. This helicase overlapped in two exonic regions with the papilin. Both these genes were shown expressed in different tick life stage important in ectoparasite interaction with the host. Tick specific sequence differences were also determined for the papilin gene and the protein binding sites of the 18S subunit in a comparison to Bos taurus.

Conclusion

In the absence of a sequenced reference genome we have assembled two complex BAC sequences, characterised novel gene structure that was confirmed by gene expression and sequencing analyses. This is the first report to provide evidence for 2 eukaryotic genes with exon regions that overlap on the same strand, the first to describe Rhipicephalinae papilin, and the first to report the complete ribosomal DNA repeated unit sequence structure for ticks. The Cot data estimation of genome wide sequence frequency means this research will underpin future efforts for genome sequencing and assembly of the R. microplus genome.