Expression analysis of autophagy-related genes in the hard tick Haemaphysalis longicornis

Ticks are obligate hematophagous arthropods with unique life cycles characterized by relatively short feeding periods and long non-feeding periods. They ambush a suitable host animal while staying in a pasture without any food source for up to several months. To understand the molecular mechanisms underlying their exceptional viability, we focused on autophagy, a proteolysis system via the lysosomes that is induced by starvation in eukaryotes. We hypothesized that starved conditions facilitate autophagy during host-seeking periods in the life cycle of the tick. To date, homologues of five autophagy-related (ATG) genes, ATG3, ATG4, ATG6, ATG8, and ATG12, have been identified from the hard tick Haemaphysalis longicornis. We showed previously that the mRNA levels of H. longicornis ATG (HlATG) genes were higher during the non-feeding period than the feeding period in the nymphal to adult stages. In addition, the expressions of HlATG3, HlATG4, HlATG8 and HlATG12 were highest in the egg compared to the other developmental stages in the same tick. In the present study, we used real-time polymerase chain reaction to examine the expression profiles of HlATG genes in the embryonic stage, larval to nymphal stages, and in internal organs of female ticks. We found that the HlATG genes were expressed at the highest levels in developing eggs on day 0 after oviposition. The levels of HlATG4 and HlATG8 were higher during the non-feeding period than the feeding period in the larval to nymphal stages. In the adults, the unfed condition appeared to be associated with the increased expression of HlATG genes in the fat body and midgut, which are nutrient storage organs; however, the expression patterns of HlATG genes varied in other organs. These results suggest that an up-regulation of HlATG genes is not always induced in different organs of unfed female ticks. Taken together, our findings raise the new possibility that HlATG genes play distinct biological roles in eggs, unfed ticks, engorged ticks (metamorphosis), and in each organ.