Radiation biology of mosquitoes

Volume 8 Supplement 2 Development of the sterile insect technique for African malaria vectors Review Open Access Radiation biology of mosquitoes Michelle EH Helinski1, 2, Andrew G Parker1 and Bart GJ Knols3Email author Malaria Journal20098 (Suppl 2) :S6 https://doi.org/10.1186/1475-2875-8-S2-S6 ©  Helinski et al; licensee BioMed Central Ltd. 2009 Published: 16 November 2009 Abstract There is currently renewed interest in assessing the feasibility of the sterile insect technique (SIT) to control African malaria vectors in designated areas. The SIT relies on the sterilization of males before mass release, with sterilization currently being achieved through the use of ionizing radiation. This paper reviews previous work on radiation sterilization of Anopheles mosquitoes. In general, the pupal stage was irradiated due to ease of handling compared to the adult stage. The dose-response curve between the induced sterility and log (dose) was shown to be sigmoid, and there was a marked species difference in radiation sensitivity. Mating competitiveness studies have generally been performed under laboratory conditions. The competitiveness of males irradiated at high doses was relatively poor, but with increasing ratios of sterile males, egg hatch could be lowered effectively. Males irradiated as pupae had a lower competitiveness compared to males irradiated as adults, but the use of partially-sterilizing doses has not been studied extensively. Methods to reduce somatic damage during the irradiation process as well as the use of other agents or techniques to induce sterility are discussed. It is concluded that the optimal radiation dose chosen for insects that are to be released during an SIT programme should ensure a balance between induced sterility of males and their field competitiveness, with competitiveness being determined under (semi-) field conditions. Self-contained 60Co research irradiators remain the most practical irradiators but these are likely to be replaced in the future by a new generation of high output X ray irradiators. Keywords Virgin FemalePupal StageSterile Insect TechniqueWild MaleLufenuron Background The sterile insect technique (SIT) for mosquitoes includes the mass production, sex separation, sterilization and release of sterile males. Contemporary methods available to induce sterility in the released insects are ionizing radiation or chemosterilization. Chemosterilants were used experimentally and in field trials in the 1960-70s against mosquitoes [1, 2] but they were mutagenic, and thus presented a potential hazard to humans during the treatment process (but see [3]). Their use was discontinued after concerns were raised about the effect of residues in the environment and on non-target organisms, particularly when large numbers of treated insects were released [4]. These concerns were based mainly on the findings of one, so far un-replicated, study that found that spiders fed on a diet of only chemosterilized mosquitoes subsequently became sterile [5]. Although the amount of residue released in the environment was very low, due to the careful rinsing of pupae [6], ionizing radiation has become the principal technique for sterilization, even though it has been reported to reduce competitiveness of the males more than chemosterilization [2, 7]. However, successful SIT programmes for the elimination of the New World screwworm Cochliomyia hominivorax from the USA, Central America [8] and Libya [9] and the tsetse fly Glossina austensi from Zanzibar [10] relied on radiation-sterilized insects, as well as the ongoing SIT programmes against the Mediterranean fruit fly Ceratitis capitata from Central and Latin America [11]. A variety of novel sterilization methods based on transgenesis are currently under development [12, 13] and are discussed in detail in [14]. However, for mosquitoes, many of these technologies are still in the experimental phase and little regulatory framework exists for the introduction of transgenic mosquitoes into the wild [15]. The aim of this paper is to give an overview of irradiation studies performed on anopheline mosquitoes, together with some information from other insects. No attempt is made to review all the available literature on anopheline irradiation but rather to set a baseline for future work on this subject. Introduction to irradiation When biological material is irradiated, molecular bonds are broken, ions created, and free radicals formed. The free radicals attack further molecular bonds, and when DNA is damaged it can lead to the formation of dominant lethal mutations in the germ cells [16, 17]. Damage to somatic cells also occurs, especially in cells undergoing mitosis. In general, damage to the germ and somatic cells increases with dose and somatic damage decreases when irradiated later in development of the insect as the number of cells undergoing division decreases. As field competitiveness is a crucial parameter, it is important to minimize the adverse effects of irradiation. Although it is generally believed that the released males need to be fully sterile, it has been suggested that more sterility can be introduced into the field population using lower radiation doses but with more competitive insects [18, 19]. Moreover, reduced competitiveness can be partly overcome by increasing the ratio of sterile-to-wild insects [20]. Radiation source and dosimetry For the irradiation of insects, gamma rays are usually used due to their high energy and penetration. The most common sources of gamma rays are the radioisotopes 60Co and 137Cs as both have a long half-life and emit high-energy gamma rays. 60Co is more easily manufactured and is therefore more often used. In conventional self-shielded irradiators (e.g. the Gammacell 220®, MDS Nordion, Ottawa, Canada, Figure 1), the sample chamber is surrounded by several rods or "pencils" of the isotope. The dose rate of the cell is determined by the activity of the source and the absorbed dose delivered to the insects is controlled by adjusting the exposure time [21]. The sample chamber volume of this machine is 3.7 L. The dose rate distribution in the chamber is not uniform and accordingly, insects receive different dose rates when placed at different positions in the chamber with the dose rate being most uniform towards the centre of the chamber. Besides gamma rays, X rays and accelerated electron beams can also be used to irradiate insects. X rays of appropriate energy have similar penetration as gamma rays, and they have been used in a number of studies on Anopheles irradiation [22–25], but the use of electron beams has not been reported. Figure 1 Cobalt 60 irradiator. The Gammacell 220 ® (MDS Nordion, Ottawa, Canada), an example of a conventional self-shielded irradiator. In the irradiate position the sample chamber is surrounded by several rods or "pencils" of the isotope. The dose rate of the cell is determined by the activ (...truncated)