The role of oxygen depletion and subsequent radioprotective effects during irradiation of mosquito pupae in water

Abstract

Background: Radiation induced sterility is the basis of the Sterile Insect Technique, by which a target insect pest population is suppressed by releasing artificially reared sterile males of the pest species in overflooding numbers over a target site. In order for the sterile males to be of high biological quality, effective standard irradiation protocols are required. Following studies investigating the effects of mosquito pupae irradiation in water versus in air, there is a need to investigate the oxy-regulatory behavior of mosquito pupae in water to better understand the consequences of irradiation in hypoxic versus normoxic conditions. Methods: Pupae of Aedes aegypti, Ae. albopictus, and Anopheles arabiensis were submerged in water inside airtight 2 ml glass vials at a density of 100 pupae/ml and the dissolved oxygen (DO) levels in the water were measured and plotted over time. In addition, male pupae of Ae. aegypti (aged 40-44 h), Ae. albopictus (aged 40-44 h) and An. arabiensis (aged 20-24 h) were irradiated in a gammacell220 at increasing doses in either hypoxic (water with < 0.5% O 2 content) or normoxic (in air) conditions. The males were then mated to virgin females and resulting eggs were checked for induced sterility. Results: All three species depleted the water of DO to levels under 0.5% within 30 minutes, with An. arabiensis consuming oxygen the fastest at under 10 minutes. Following irradiation, the protective effect of hypoxia was observed across species and doses (P < 0.0001), increasing at higher doses. This effect was most pronounced in An. arabiensis. Conclusions: The consumption of dissolved oxygen by pupae submerged in water was significantly different between species, indicating that their oxy-regulatory capacity seems to have possibly evolved according to their preferred breeding site characteristics. This needs to be considered when sterilizing male mosquitoes at pupal stage in water. Depending on species, their DO consumption rates and their density, irradiation doses needed to achieve full sterility may vary significantly. Further assessments are required to ascertain optimal conditions in terms of ambient atmosphere during pupal irradiation to produce competitive sterile males, and temperature and density dependent effects are expected.

Effects of Chilling and Anoxia on the Irradiation Dose-Response in Adult Aedes Mosquitoes

“…Our study also showed that hypoxia protects from O 2 effects during adult irradiation as was seen in mosquito pupae ( Yamada et al, 2019 , 2020 ), but may come with its own negative effects. Anoxia had high radioprotective effects, with up to a 14-fold increase in residual fertility compared to males irradiated with the same dose in normoxia.…”

“…For mass irradiation at the pupal stage, the pupae would need to be placed in sufficient water within the irradiation canister to provide buoyancy to avoid the pupae at the bottom being crushed. However, this creates a hypoxic environment as pupae submerged in water continue to respirate through their cuticle and quickly deplete the surrounding water of dissolved oxygen ( Yamada et al, 2020 ). As hypoxia reduces irradiation effects, the irradiation of pupae in water results in differential levels of sterility within the sample ( Yamada et al, 2020 ), therefore this method for irradiation cannot be reliable unless, again, the full cohort is significantly overdosed.…”

“…However, this creates a hypoxic environment as pupae submerged in water continue to respirate through their cuticle and quickly deplete the surrounding water of dissolved oxygen ( Yamada et al, 2020 ). As hypoxia reduces irradiation effects, the irradiation of pupae in water results in differential levels of sterility within the sample ( Yamada et al, 2020 ), therefore this method for irradiation cannot be reliable unless, again, the full cohort is significantly overdosed. Apart from quality costs of over-dosing, pupae exposed to hypoxia suffer additional stress and loss in quality.…”

“…More recently, a series of experiments to assess the impacts of several biological and physical factors (e.g., strain geographical origin, pupal age pupal size, atmospheric conditions) on dose-reponse in mosquitoes were conducted ( Yamada et al, 2019 ; Yamada et al, 2020 ) with the aim to develop standardized protocols for the irradiation of mosquito pupae ( FAO/IAEA, 2019 ). However; standardizing irradiation protocols for pupae is difficult, especially in practical terms in large-scale SIT programmes for the following reasons: Pupal age is an important factor that significantly impacts dose-response ( Balestrino et al, 2010 ; Yamada et al, 2019 ).…”

Radiation dose-rate is a neglected critical parameter in dose–response of insects

“…Several factors that affect dose–response in mosquito species currently targeted by the SIT (namely Anopheles arabiensis, Aedes aegypti, and Ae. albopictus ) have been investigated at the Insect Pest Control Laboratory IPCL of the Joint FAO/IAEA Centre of Nuclear Techniques in Food and Agriculture, such as handling methods, life stage, pupal age, strain origin, ambient temperature, and atmosphere during irradiation 18 , 19 . However, additional factors that affect dose response are suspected and require further scrutiny.…”