Radiation-induced sterility for pupal and adult stages of the malaria mosquito Anopheles arabiensis

Malaria Journal BioMed Central Open Access Research Radiation-induced sterility for pupal and adult stages of the malaria mosquito Anopheles arabiensis Michelle EH Helinski1, Andrew G Parker1 and Bart GJ Knols*1,2 Address: 1Entomology Unit, FAO/IAEA Agriculture and Biotechnology Laboratory, International Atomic Energy Agency (IAEA), A-2444 Seibersdorf, Austria and 2Laboratory of Entomology, Wageningen University and Research Center, P.O. Box 8031, 6700 EH Wageningen, The Netherlands Email: Michelle EH Helinski - ; Andrew G Parker - ; Bart GJ Knols* - * Corresponding author Published: 15 May 2006 Malaria Journal 2006, 5:41 doi:10.1186/1475-2875-5-41 Received: 23 February 2006 Accepted: 15 May 2006 This article is available from: http://www.malariajournal.com/content/5/1/41 © 2006 Helinski et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract Background: In the context of the Sterile Insect Technique (SIT), radiation-induced sterility in the malaria mosquito Anopheles arabiensis Patton (Diptera: Culicidae) was studied. Male mosquitoes were exposed to gamma rays in the pupal or adult stage and dose-sterility curves were determined. Methods: Pupae were irradiated shortly before emergence (at 22–26 hrs of age), and adults <24 hrs post emergence. Doses tested ranged between 0 and 100 Gy. The effects of irradiation on adult emergence, male survival, induced sterility and insemination capability were evaluated. Emergence and insemination data were analysed using independent t-tests against the control. Correlation analyses were performed for insemination rate and dose and insemination and fecundity. Male survival was analysed using Kaplan-Meier survival analyses. Finally, the calculated residual fertility values were inverse-normal transformed and linear regression analyses performed. Results: Irradiation of pupae, for all doses tested, had no effect on adult emergence. Survival curves of males irradiated as pupae or adults were similar or even slightly higher than nonirradiated males. Overall, adults appeared to be slightly more susceptible to irradiation, although no significant differences for individual doses were observed. In the pupal stage, a significant negative correlation was found between insemination and dose, but the correlation-coefficient was associated with less than 25% of the total variation. A review of the literature indicated that An. arabiensis is more radiation resistant than other anopheline mosquitoes. Conclusion: The optimal dose for male insects to be released in an SIT programme depends on their level of sterility and competitiveness. The use of semi-sterilizing doses to produce more competitive insects is discussed. The most convenient developmental stage for mosquito irradiation on a mass-scale are pupae, but pupal irradiation resulted in a lower insemination rate at the highest dose compared to adult irradiation. On the basis of this study, a suitable dose range that includes semi-sterilizing doses is identified to initiate competitiveness experiments for males irradiated at both developmental stages. Background In the 21st century, anopheline mosquitoes remain the most deadly insects in the world. Malaria is still widely spread; it is estimated that currently 3.2 billion people live Page 1 of 10 (page number not for citation purposes) Malaria Journal 2006, 5:41 http://www.malariajournal.com/content/5/1/41 in areas at risk of malaria transmission [1]. Estimated economic growth reduction in endemically affected countries is high, and contemporary control methods are not always effective due in part to widespread resistance of the mosquitoes to insecticides and Plasmodium parasites to commonly used drugs. The Sterile Insect Technique (SIT), successfully applied against a number of pest species [2], has been evaluated against Anopheles albimanus Wiedemann in the 1970s with encouraging results [3,4]. Over the last years, a renewed interest in SIT for malaria vectors has led to a 5-year feasibility study to investigate all aspects of an SIT programme including sexing, mass production, sterilisation, and release methodology [5,6]. The project initially focuses on the African malaria vector Anopheles arabiensis Patton. have been published for An. albimanus [17], An. pharoensis Theobald [18,19] and An. stephensi Liston [7,20]. An. arabiensis has been studied in the light of genetic sexing systems [21] and small-scale irradiation studies [22] but no dose-response curve exists. Previous work has indicated that substantial inter-species variation in radiation sensitivity is present [22], justifying the need for a dose-sterility curve for An. arabiensis. In mosquitoes, both the pupal and the adult stage can be irradiated. Pupal irradiation is easier to perform, but there is evidence of a reduced competitiveness when male pupae are irradiated at high doses compared to adult irradiation [23]. The objective of this study was to determine the dose-sterility curves for the pupal and adult stages of male An. arabiensis and define a suitable dose range to initiate competitiveness experiments. The SIT relies on the sterilisation of insects by chemosterilisation [4,7,8], irradiation [2] or modern biotechnological approaches [9-11]. Modern biotechnological approaches based on transgenic organisms are promising but at an early stage of development and no legal framework yet exists to facilitate the introduction of such organisms in the wild [12,13]. Sterilisation by irradiation or chemosterilants has not been researched extensively for the last 30 years with mosquitoes. Promising results were obtained with chemosterilants in terms of the level of sterility induced and competitiveness present [8] but these have the disadvantage of being mutagenic agents. They thus present a potential hazard to humans during the treatment process and non-target organisms if residues persist in released individuals [8]. Even though the actual amount of residue released in the environment was in fact very low due to careful rinsing of the pupae [14], concerns raised about the possible negative effects on the environment if large numbers of treated insects were to be released [15,16] resulted in a disuse of chemosterilants for mosquito control. Although it would be worthwhile to identify additional compounds with chemosterilant properties, it remains doubtful if the currently available ones will be acceptable for use in future genetic control programmes. Methods Sterilisation by irradiation remains the most practical way to sterilise the insects at present, and it has been argued that radiation sterilisation should also be used to introduce the first transgenic organisms in the wild [3]. Determining the optimal (...truncated)