Genetic study explains global spread of invasive longhorned ticks

The Asian longhorned tick, Haemaphysalis longicornis, spreads serious pathogens, including severe fever with thrombocytopenia syndrome (SFTS) virus, Rickettsia, and Theileria, and its bite can cause a meat allergy in humans. Once confined to Japan, South Korea, North Korea, China, and Russia, the ticks have expanded across Oceania, and in 2017 they were detected in the United States. Perhaps contributing to this spread is the ability of some female ticks to produce offspring without fertilization by a male: a feat known as parthenogenesis. Only parthenogenetic ticks-the vast majority of which are female-have been found outside their previously known range.

Ryo Nakao and colleagues explored the genetic structure of native and introduced H. longicornis populations using complete mitogenomes and genome-wide single-nucleotide polymorphisms (SNPs) extracted through a technique known as multiplexed inter-simple sequence repeats genotyping by sequencing (MIG-seq). The authors used samples collected from locations across Japan and in Fiji, New Caledonia, and Australia and also analyzed five laboratory strains, two sexually reproducing and three parthenogenetic.

Initially, the tree emerging from mitogenome sequences shows two major clades aligning well with reproductive strategy, suggesting that a parthenogenetic branch spread around the world. However, three male ticks were found within that largely parthenogenetic clade, complicating the picture. Moreover, SNP analysis showed gene flow between clades, suggesting that parthenogenesis evolved multiple times. According to the authors, the ability to evolve parthenogenesis may help explain the spread of this vector species.