Identifying the sex chromosome and sex determination genes in the cattle tick, Rhipicephalus (Boophilus) microplus.

Rhipicephalus (Boophilus) microplus is globally one of the most economically important ectoparasites of cattle costing the agriculture industry billions of dollars annually. Resistance to chemical control measures has prompted the development of novel methods of control. Recent advancements in genetic control measures for human and other animal vectors have utilized sex determination research to manipulate sex ratios, which have shown promising results in mosquitoes namely Aedes aegypti and Anopheles stephensi. Here, we use R. (B.) microplus as a model to provide foundational research to allow similar avenues of investigation in ticks using R. (B.) microplus as a model. Karyotypes for R. (B.) microplus show an XX:XO sex determining system with the largest chromosome being the sex chromosome. Using flow cytometric methods, the size of the sex chromosome was estimated at 526.91 Mb. All measures to identify the sex chromosome within the cattle tick genome assembly associated sex chromosomal characteristics to two chromosomes. This discrepancy between the assembly and karyotypes of the tick led to generating a new genome assembly with a single adult male specimen. The two chromosomes in question aligned with a single scaffold within the new genome that had a length of 513.29 Mb and was the first time the sex chromosome was identified in an Ixodid genome assembly.