Anopheles stephensi (Diptera: Culicidae): changes in male mating competence and reproductive system morphology associated with aging and mating

Development of the Sterile Insect Technique to control the dengue vector Aedes aegypti (Linnaeus) in Sri Lanka

BACKGROUND

The Sterile Insect Technique (SIT) is presently being tested to control dengue in several countries. SIT aims to cause the decline of the target insect population through the release of a sufficient number of sterilized male insects. This induces sterility in the female population, as females that mate with sterilized males produce no offspring. Male insects are sterilized through the use of ionizing irradiation. This study aimed to evaluate variable parameters that may affect irradiation in mosquito pupae.

METHODS

An Ae. aegypti colony was maintained under standard laboratory conditions. Male and female Ae. aegypti pupae were separated using a Fay and Morlan glass sorter and exposed to different doses of gamma radiation (40, 50, 60, 70 and 80 Gy) using a Co60 source. The effects of radiation on survival, flight ability and the reproductive capacity of Ae. aegypti were evaluated under laboratory conditions. In addition, mating competitiveness was evaluated for irradiated male Ae. aegypti mosquitoes to be used for future SIT programmes in Sri Lanka.

RESULTS

Survival of irradiated pupae was reduced by irradiation in a dose-dependent manner but it was invariably greater than 90% in control, 40, 50, 60, 70 Gy in both male and female Ae. aegypti. Irradiation didn't show any significant adverse effects on flight ability of male and female mosquitoes, which consistently exceeded 90%. A similar number of eggs per female was observed between the non-irradiated groups and the irradiated groups for both irradiated males and females. Egg hatch rates were significantly lower when an irradiation dose above 50 Gy was used as compared to 40 Gy in both males and females. Irradiation at higher doses significantly reduced male and female survival when compared to the non-irradiated Ae. aegypti mosquitoes. Competitiveness index (C) scores of sterile and non-sterile males compared with non-irradiated male mosquitoes under laboratory and semi-field conditions were 0.56 and 0.51 respectively at 50 Gy.

SIGNIFICATION

Based on the results obtained from the current study, a 50 Gy dose was selected as the optimal radiation dose for the production of sterile Ae. aegypti males for future SIT-based dengue control programmes aiming at the suppression of Ae. aegypti populations in Sri Lanka.

A testis-expressing heme peroxidase HPX12 regulates male fertility in the mosquito Anopheles stephensi

In vertebrates dysregulation of the antioxidant defense system has a detrimental impact on male fertility and reproductive physiology. However, in insects, especially mosquitoes the importance of sperm quality has been poorly studied. Since long-term storage of healthy and viable sperm earmarks male reproductive competency, we tested whether the heme peroxidase, a member of antioxidant enzyme family proteins, and abundantly expressed in the testis, also influence male fertility in the mosquito An. stephensi. Here, we show that a heme peroxidase 12 (HPX12), is an important cellular factor to protect the sperms from oxidative stress, and maintains semen quality in the male mosquito reproductive organ. We demonstrate that knockdown of the HPX12 not only impairs the sperm parameters such as motility, viability but also causes a significant down-regulation of MAG expressing transcripts such as ASTEI02706, ASTEI00744, ASTEI10266, likely encoding putative Accessory gland proteins. Mating with HPX12 knockdown male mosquitoes, resulted in ~ 50% reduction in egg-laying, coupled with diminished larval hatchability of a gravid female mosquito. Our data further outlines that increased ROS in the HPX12 mRNA depleted mosquitoes is the ultimate cause of sperm disabilities both qualitatively as well as quantitatively. Our data provide evidence that testis expressing AsHPX12 is crucial for maintaining optimal homeostasis for storing and protecting healthy sperms in the male mosquito's reproductive organs. Since, high reproductive capacity directly influences the mosquito population, manipulating male mosquito reproductive physiology could be an attractive tool to combat vector-borne diseases.

Swarming Behavior in Anopheles gambiae (sensu lato): Current Knowledge and Future Outlook

Effective management of insect disease vectors requires a detailed understanding of their ecology and behavior. In Anopheles gambiae sensu lato (s.l.) (Diptera: Culicidae) mating occurs during swarming, but knowledge of their mating behavior under natural conditions is limited. Mosquitoes mate in flight over specific landmarks, known as swarm markers, at particular locations. Swarms consist of males; the females usually approach the swarm and depart following copulation. The number of mating pairs per swarm is closely associated with swarm size. The shape and height of swarm markers vary and may depend on the environmental conditions at the swarm's location. Male-male interactions in mosquito swarms with similar levels of attractive flight activity can offer a mating advantage to some individuals. Flight tone is used by mosquitoes to recognize the other sex and choose a desirable mate. Clarifying these and other aspects of mosquito reproductive behavior can facilitate the development of population control measures that target swarming sites. This review describes what is currently known about swarming behavior in Anopheles gambiae s.l., including swarm characteristics; mating within and outside of swarms, insemination in females, and factors affecting and stimulating swarming.

New evidence of mating swarms of the malaria vector, Anopheles arabiensis in Tanzania

Background: Malaria mosquitoes form mating swarms around sunset, often at the same locations for months or years. Unfortunately, studies of Anopheles swarms are rare in East Africa, the last recorded field observations in Tanzania having been in 1983. Methods: Mosquito swarms were surveyed by trained volunteers between August-2016 and June-2017 in Ulanga district, Tanzania. Identified Anopheles swarms were sampled using sweep nets, and collected mosquitoes killed by refrigeration then identified by sex and taxa. Sub-samples were further identified by PCR, and spermatheca of females examined for mating status. Mosquito ages were estimated by observing female ovarian tracheoles and rotation of male genitalia. GPS locations, types of swarm markers, start/end times of swarming, heights above ground, mosquito counts/swarm, and copulation events were recorded. Results: A total of 216 Anopheles swarms were identified, characterized and mapped, from which 7,142 Anopheles gambiae s.l and 13 Anopheles funestus were sampled. The An. gambiae s.l were 99.6% males and 0.4% females, while the An. funestus were all males. Of all An. gambiae s.l analyzed by PCR, 86.7% were An. arabiensis, while 13.3% returned non-amplified DNA. Mean height (±SD) of swarms was 2.74±0.64m, and median duration was 20 (IQR; 15-25) minutes. Confirmed swarm markers included rice fields (25.5%), burned grounds (17.2%), banana trees (13%), brick piles (8.8%), garbage heaps (7.9%) and ant-hills (7.4%). Visual estimates of swarm sizes by the volunteers was strongly correlated to actual sizes by sweep nets (R=0.94; P=<0.001). All females examined were nulliparous and 95.6% [N=6787] of males had rotated genitalia, indicating sexual maturity. Conclusions: This is the first report of Anopheles swarms in Tanzania in more than three decades. The study demonstrates that the swarms can be identified and characterized by trained community-based volunteers, and highlights potential new interventions, for example targeted aerosol spraying of the swarms to improve malaria control.

Sugar and Multivitamin Diet Effects on The Longevity and Mating Capacity of Laboratory-Reared Male Anopheline Mosquitoes

Successful mating by male mosquitoes is dependent on several factors, with sugar feeding being particularly important. The effect of ingested vitamins on adult male mosquitoes is poorly understood. This laboratory study used 3 anopheline species, Anopheles campestris, An. dirus, and An. sawadwongporni, to study the effect of sugar and vitamins on male longevity, copulation, and fecundity. Males were fed 1 of 5 diets containing different combinations of sugar and vitamins: 10% glucose, 10% sucrose, 10% multivitamin syrup, 10% multivitamin syrup + 10% glucose, and 10% multivitamin syrup + 10% sucrose. The longevity of males was measured for a period of 15 days. Forced mating was used to simulate copulation, and fecundity was measured by counting the number of eggs oviposited and the hatch rate of larvae. The longevity of An. campestris and An. dirus was greatest when fed a diet of 10% multivitamin syrup + 10% glucose, and the longevity of An. sawadwongporni was greatest when fed a diet of 10% multivitamin syrup + 10% sucrose. The 1st mating routinely produced the most viable eggs when males were mated with several females. The diet of 10% multivitamin syrup + 10% sucrose produced numerically greater egg production and larval emergence for all 3 species, although this was not always statistically significant due to variability and small sample size. These results indicate that the addition of multivitamin syrup to sucrose may produce healthier and more fit male anophelines. This has potential implications for increasing insectary operations and improving the fitness of laboratory-reared male mosquitoes that will be released for mosquito and disease-pathogen control studies.

A New Role of the Mosquito Complement-like Cascade in Male Fertility in Anopheles gambiae

Thioester-containing protein 1 (TEP1) is a key immune factor that determines mosquito resistance to a wide range of pathogens, including malaria parasites. Here we report a new allele-specific function of TEP1 in male fertility. We demonstrate that during spermatogenesis TEP1 binds to and removes damaged cells through the same complement-like cascade that kills malaria parasites in the mosquito midgut. Further, higher fertility rates are mediated by an allele that renders the mosquito susceptible to Plasmodium. By elucidating the molecular and genetic mechanisms underlying TEP1 function in spermatogenesis, our study suggests that pleiotropic antagonism between reproduction and immunity may shape resistance of mosquito populations to malaria parasites.

The complement-related protein TEP1, which helps kill malaria parasites, also labels damaged cells for removal during mosquito spermatogenesis and promotes male fertility in the malaria vector Anopheles gambiae.

While Anopheline mosquitoes are the most efficient vectors of human malaria, they do have protective mechanisms directed against the causative parasite, Plasmodium falciparum. Their immune system targets the invading parasites through activation of the mosquito complement-like system. A central component of this system, thioester-containing protein 1 (TEP1), is a highly polymorphic gene with four allelic classes. Although one class, called R1, mediates efficient parasite elimination, the other classes render the mosquitoes susceptible to Plasmodium infections. Until now, it was not clear how or why any of these susceptible TEP1 alleles were maintained in the population. Here we discover a new role of TEP1 in male fertility. We demonstrate that mosquitoes use the same mechanism—nitration of target surfaces—to flag both damaged sperm and Plasmodium cells. Binding of TEP1 to, and removal of, the aberrant sperm is critical to preserve high fertility rates. In the absence of TEP1, accumulation of damaged sperm degrades male fertility. Surprisingly, in spite of the common mechanism of TEP1 activation, distinct alleles of TEP1 mediate efficient removal of defective sperm and killing of malaria parasites. Our results suggest that pleiotropic function in immunity and reproduction is one of the mechanisms that maintain TEP1 polymorphism in mosquito populations.

Stimulating Anopheles gambiae swarms in the laboratory: application for behavioural and fitness studies

Background

Male Anopheles mosquitoes that swarm rely in part on features of the environment including visual stimuli to locate swarms. Swarming is believed to be the primary behaviour during which mating occurs in the field, but is not a common behaviour in the laboratory. Features that stimulate male Anopheles gambiae G3 strain swarming were created in novel large indoor cages.

Methods

The following visual features were tested in all combinations to determine which were important for swarm formation. Large cages and fading ceiling lights at dusk alone did not stimulate swarming while a dark foreground and contrasting illuminated background with a contrasting landmark stimulated and localized swarm formation during artificial twilight. Given the need to test transgenic strains in as natural a setting as possible, in this study it was investigated whether induced swarm behaviour and cage size would affect relative mating performance of wild-type and transgenic β2Ppo1 and β2Ppo2 A. gambiae sexually sterile males.

Results

Even using a mosquito colony that has been in laboratory culture for 39 years, swarming behaviour was induced by this novel arrangement. The presence of swarming stimuli was associated with an increase in insemination frequency from 74.3 to 97.7% in large cages. Transgenic males showed a lower competitiveness in large cages compared to small cages regardless of the presence of swarming stimuli.

Conclusions

The results of the present study are discussed in view of the progressive evaluation of genetically modified A. gambiae strains and the potential applications of reproducing swarms in controlled conditions to dissect the mating behaviour of this species and the mechanisms controlling it.

Participation of irradiated Anopheles arabiensis males in swarms following field release in Sudan

Background

The success of the sterile insect technique (SIT) depends the release of large numbers of sterile males, which are able to compete for mates with the wild male population within the target area. Unfortunately, the processes of colonisation, mass production and irradiation may reduce the competitiveness of sterile males through genetic selection, loss of natural traits and somatic damage. In this context, the capacity of released sterile Anopheles arabiensis males to survive, disperse and participate in swarms at occurring at varying distances from the release site was studied using mark-release-recapture (MRR) techniques.

Methods

In order to assess their participation in swarms, irradiated and marked laboratory-reared male mosquitoes were released 50, 100 or 200 m from the known site of a large swarm on three consecutive nights. Males were collected from this large swarm on subsequent nights. Over the three days a total of 8,100 males were released. Mean distance travelled (MDT), daily probability of survival and estimated population size were calculated from the recapture data. An effect of male age at the time of release on these parameters was observed.

Results

Five per cent of the males released over three days were recaptured. In two-, three- and four-day-old males, MDT was 118, 178 and 170 m, and the daily survival probability 0.95, 0.90 and 0.75, respectively. From the recapture data on the first day following each release, the Lincoln index gives an estimation of 32,546 males in the natural population.

Discussion

Sterile An. arabiensis males released into the field were able to find and participate in existing swarms, and possibly even initiate swarms. The survival probability decreased with the age of male on release but the swarm participation and the distance travelled by older males seemed higher than for younger males. The inclusion of a pre-release period may thus be beneficial to male competitiveness and increase the attractiveness of adult sexing techniques, such as blood spiking.

Effect of temperature and larval density on Aedes polynesiensis (Diptera: Culicidae) laboratory rearing productivity and male characteristics

Aedes polynesiensis Marks (Diptera: Culicidae) larvae were reared to adulthood in the laboratory under a range of temperatures and larval densities. We studied the effect of these variables on several life table parameters of relevance to male-release-based vector control strategies including: larval survivorship, developmental time to pupation, male to female ratio, male pupae yield, adult male size and survival. The range of tested rearing temperatures (20, 25, 27, and 30 °C) and larval densities (50, 100, 200, and 400 larvae/L) was selected within the conditions allowing larval growth and survival. Larval survivorship was the highest when larvae were reared at 200 larvae/L for all temperatures except 20 °C. Male to female ratio was male biased at all temperatures and densities. Time to pupation decreased with increasing temperatures. Larval density and temperature influenced the proportion of males pupating on first day of pupation with 43-47% of total male pupae produced at 25 °C. No significant differences in mean wing length were observed between male mosquitoes reared in the laboratory (except at 20 and 30 °C for some densities) and field collected males. Altogether, the study allowed the identification of rearing conditions delivering high male yield with essentially no female contamination, adequate adult male size and survival. Ae. polynesiensis thus appears particularly amenable to biological and mechanical sex separation offering good prospects for Ae. polynesiensis population suppression trials that rely on the production and release of large numbers of incompatible or sterile males.

Differential effects of inbreeding and selection on male reproductive phenotype associated with the colonization and laboratory maintenance of Anopheles gambiae

Background

Effective mating between laboratory-reared males and wild females is paramount to the success of vector control strategies aiming to decrease disease transmission via the release of sterile or genetically modified male mosquitoes. However mosquito colonization and laboratory maintenance have the potential to negatively affect male genotypic and phenotypic quality through inbreeding and selection, which in turn can decrease male mating competitiveness in the field. To date, very little is known about the impact of those evolutionary forces on the reproductive biology of mosquito colonies and how they ultimately affect male reproductive fitness.

Methods

Here several male reproductive physiological traits likely to be affected by inbreeding and selection following colonization and laboratory rearing were examined. Sperm length, and accessory gland and testes size were compared in male progeny from field-collected females and laboratory strains of Anopheles gambiae sensu stricto colonized from one to over 25 years ago. These traits were also compared in the parental and sequentially derived, genetically modified strains produced using a two-phase genetic transformation system. Finally, genetic crosses were performed between strains in order to distinguish the effects of inbreeding and selection on reproductive traits.

Results

Sperm length was found to steadily decrease with the age of mosquito colonies but was recovered in refreshed strains and crosses between inbred strains therefore incriminating inbreeding costs. In contrast, testes size progressively increased with colony age, whilst accessory gland size quickly decreased in males from colonies of all ages. The lack of heterosis in response to crossing and strain refreshing in the latter two reproductive traits suggests selection for insectary conditions.

Conclusions

These results show that inbreeding and selection differentially affect reproductive traits in laboratory strains overtime and that heterotic ‘supermales’ could be used to rescue some male reproductive characteristics. Further experiments are needed to establish the exact relationship between sperm length, accessory gland and testes size, and male reproductive success in the laboratory and field settings.

Reproductive strategies of Aedes albopictus (Diptera: Culicidae) and implications for the sterile insect technique

Male insects are expected to optimize their reproductive strategy according to the availability of sperm or other ejaculatory materials, and to the availability and reproductive status of females. Here, we investigated the reproductive strategy and sperm management of male and virgin female Aedes albopictus, a mosquito vector of chikungunya and dengue viruses. The dynamics of semen transfer to the female bursa inseminalis and spermathecae were observed. Double-mating experiments were conducted to study the effect of time lapsed or an oviposition event between two copulations on the likelihood of a female double-insemination and the use of sperm for egg fertilization; untreated fertile males and radio-sterilised males were used for this purpose. Multiple inseminations and therefore the possibility of sperm competition were limited to matings closely spaced in time. When two males consecutively mated the same female within a 40 min interval, in ca. 15% of the cases did both males sire progeny. When the intervals between the copulations were longer, all progeny over several gonotrophic cycles were offspring of the first male. The mating behavior of males was examined during a rapid sequence of copulations. Male Ae. albopictus were parceling sperm allocation over several matings; however they would also attempt to copulate with females irrespective of the available sperm supply or accessory gland secretion material. During each mating, they transferred large quantities of sperm that was not stored for egg fertilization, and they attempted to copulate with mated females with a low probability of transferring their genes to the next generation. The outcomes of this study provided in addition some essential insights with respect to the sterile insect technique (SIT) as a vector control method.

Transgenic Anopheles stephensi coexpressing single-chain antibodies resist Plasmodium falciparum development

Anopheles stephensi mosquitoes expressing m1C3, m4B7, or m2A10 single-chain antibodies (scFvs) have significantly lower levels of infection compared to controls when challenged with Plasmodium falciparum, a human malaria pathogen. These scFvs are derived from antibodies specific to a parasite chitinase, the 25 kDa protein and the circumsporozoite protein, respectively. Transgenes comprising m2A10 in combination with either m1C3 or m4B7 were inserted into previously-characterized mosquito chromosomal "docking" sites using site-specific recombination. Transgene expression was evaluated at four different genomic locations and a docking site that permitted tissue- and sex-specific expression was researched further. Fitness studies of docking site and dual scFv transgene strains detected only one significant fitness cost: adult docking-site males displayed a late-onset reduction in survival. The m4B7/m2A10 mosquitoes challenged with P. falciparum had few or no sporozoites, the parasite stage infective to humans, in three of four experiments. No sporozoites were detected in m1C3/m2A10 mosquitoes in challenge experiments when both genes were induced at developmentally relevant times. These studies support the conclusion that expression of a single copy of a dual scFv transgene can completely inhibit parasite development without imposing a fitness cost on the mosquito.

Function and composition of male accessory gland secretions in Anopheles gambiae: a comparison with other insect vectors of infectious diseases

Human malaria, a major public health burden in tropical and subtropical countries, is transmitted exclusively by the bite of a female Anopheles mosquito. Malaria control strategies aimed at inducing sexual sterility in natural vector populations are an attractive alternative to the use of insecticides. However, despite their importance as disease vectors, limited information is available on the molecular mechanisms regulating fertility in Anopheles mosquitoes. In the major malaria vector, An. gambiae, the full complement of sperm and seminal fluid required for a female's lifelong egg production is obtained from a single mating event. This single mating has important consequences for the physiology and behavior of An. gambiae females: in particular, they become refractory to further insemination, and they start laying eggs. In other insects including Drosophila, similar post-copulatory changes are induced by seminal proteins secreted by the male accessory glands and transferred to the female during mating. In this review, we analyze the current state of knowledge on the function and characterization of male seminal proteins in An. gambiae, and provide a comparative assessment of the role of these male reproductive factors in other mosquito vectors of human disease in which female post-copulatory behavior has been studied. Knowledge of the factors and mechanisms regulating fertility in An. gambiae and other vectors can help the design of novel control strategies to fight the spread of disease.

Laboratory selection for an accelerated mosquito sexual development rate

Background

Separating males and females at the early adult stage did not ensure the virginity of females of Anopheles arabiensis (Dongola laboratory strain), whereas two years earlier this method had been successful. In most mosquito species, newly emerged males and females are not able to mate successfully. For anopheline species, a period of 24 h post-emergence is generally required for the completion of sexual maturation, which in males includes a 180° rotation of the genitalia. In this study, the possibility of an unusually shortened sexual maturity period in the laboratory-reared colony was investigated.

Methods

The effect of two different sex-separation methods on the virginity of females was tested: females separated as pupae or less than 16 h post-emergence were mated with males subjected to various doses of radiation. T-tests were performed to compare the two sex-separation methods. The rate of genitalia rotation was compared for laboratory-reared and wild males collected as pupae in Dongola, Sudan, and analysed by Z-tests. Spermatheca dissections were performed on females mated with laboratory-reared males to determine their insemination status.

Results

When the sex-separation was performed when adults were less than 16 h post-emergence, expected sterility was never reached for females mated with radio-sterilized males. Expected sterility was accomplished only when sexes were separated at the pupal stage. Observation of genitalia rotation showed that some males from the laboratory strain Dongola were able to successfully mate only 11 h after emergence and 42% of the males had already completed rotation. A small proportion of the same age females were inseminated. Wild males showed a much slower genitalia rotation rate. At 17 h post-emergence, 96% of the laboratory-reared males had completed genitalia rotation whereas none of the wild males had.

Conclusion

This colony has been cultured in the laboratory for over one hundred generations, and now has accelerated sexual maturation when compared with the wild strain. This outcome demonstrates the kinds of selection that can be expected during insect colonization and maintenance, particularly when generations are non-overlapping and similar-age males must compete for mates.

Reproduction-longevity trade-off in Anopheles gambiae (Diptera: Culicidae)

Reduced survival and future reproduction due to of current reproduction is a trade-off known as the cost of reproduction. Surprisingly, only a few studies have assessed the cost of reproduction in arthropod disease vectors, despite its effect on longevity, and thus on vectorial capacity. We evaluated the cost of reproduction on survival of Anopheles gambiae Giles by comparing mosquitoes that were denied exposure to the other sex, hereafter named virgins, and those that were allowed exposure to the other sex and mating, hereafter named mated. Merely 6 d of exposure to females with mating activity reduced male survival from a median of 17 d in virgins to 15 d in mated, indicating that male mating cost is substantial. The increase in mortality of mated males began several days after the exposure to females ended, indicating that mating is not associated with immediate mortality risk. Notably, body size was negatively correlated with male mortality in mated males, but not in virgins. The rate of insemination declined after 4 d of exposure to females, indicating that male mating capacity is limited and further supporting the hypothesis that mating is costly for males. Consistent with previous studies, female survival on sugar alone (median=16 d) was shorter than on blood and sugar (median=19 d), regardless if she was mated or virgin. Overall, survival of mated females was lower than that of virgins on a diet of blood and sugar, but no difference was found on a diet of sugar only. However, the cost of reproduction in females remains ambiguous because the difference in survival between virgin and mated females was driven by the difference between virgin (median=19 d) and uninseminated females exposed to males (median=17 d), rather than between virgin and inseminated females (median=19 d). Accordingly, sperm and seminal fluid, egg development, and oviposition have negligible cost in terms of female survival. Only exposure to males without insemination decreased female survival. Nonetheless, if exposure to males under natural conditions is also associated with reduced survival, it might explain why females remain monogamous.

Male mating biology

Before sterile mass-reared mosquitoes are released in an attempt to control local populations, many facets of male mating biology need to be elucidated. Large knowledge gaps exist in how both sexes meet in space and time, the correlation of male size and mating success and in which arenas matings are successful. Previous failures in mosquito sterile insect technique (SIT) projects have been linked to poor knowledge of local mating behaviours or the selection of deleterious phenotypes during colonisation and long-term mass rearing. Careful selection of mating characteristics must be combined with intensive field trials to ensure phenotypic characters are not antagonistic to longevity, dispersal, or mating behaviours in released males. Success has been achieved, even when colonised vectors were less competitive, due in part to extensive field trials to ensure mating compatibility and effective dispersal. The study of male mating biology in other dipterans has improved the success of operational SIT programmes. Contributing factors include inter-sexual selection, pheromone based attraction, the ability to detect alterations in local mating behaviours, and the effects of long-term colonisation on mating competitiveness. Although great strides have been made in other SIT programmes, this knowledge may not be germane to anophelines, and this has led to a recent increase in research in this area.

Radiation biology of mosquitoes

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.

Analysis of expression in the Anopheles gambiae developing testes reveals rapidly evolving lineage-specific genes in mosquitoes

Background

Male mosquitoes do not feed on blood and are not involved in delivery of pathogens to humans. Consequently, they are seldom the subjects of research, which results in a very poor understanding of their biology. To gain insights into male developmental processes we sought to identify genes transcribed exclusively in the reproductive tissues of male Anopheles gambiae pupae.

Results

Using a cDNA subtraction strategy, five male-specifically or highly male-biased expressed genes were isolated, four of which remain unannotated in the An. gambiae genome. Spatial and temporal expression patterns suggest that each of these genes is involved in the mid-late stages of spermatogenesis. Their sequences are rapidly evolving; however, two genes possess clear homologs in a wide range of taxa and one of these probably acts in a sperm motility control mechanism conserved in many organisms, including humans. The other three genes have no match to sequences from non-mosquito taxa, thus can be regarded as orphans. RNA in situ hybridization demonstrated that one of the orphans is transcribed in spermatids, which suggests its involvement in sperm maturation. Two other orphans have unknown functions. Expression analysis of orthologs of all five genes indicated that male-biased transcription was not conserved in the majority of cases in Aedes and Culex.

Conclusion

Discovery of testis-expressed orphan genes in mosquitoes opens new prospects for the development of innovative control methods. The orphan encoded proteins may represent unique targets of selective anti-mosquito sterilizing agents that will not affect non-target organisms.

Effect of sugar on male Anopheles gambiae mating performance, as modified by temperature, space, and body size

Background

Anopheles gambiae plant-sugar feeding was thought to be rare and physiologically optional. Unlike adult females, males have no alternative source of energy and soon die with only water, yet they might be competent to inseminate all females within their brief lifespan. This study was designed to detect sugar's effect, if any, on male performance.

Methods

Males with and without 20% sucrose were evaluated at two body sizes and two temperatures, 23° and 27°C. Survival was recorded twice daily, and sexual behaviour was recorded each night after adult emergence. Insemination at a 2:1 male:female ratio was examined in three cage sizes, including walk-in mesocosms.

Results

Without sugar, males of both sizes lived longer at 23° than 27°C, and large males lived longer at each temperature. Survival of large males at low temperature averaged 3.7 days, small males at high temperature, 1.9 days. With sugar, males in all four treatments suffered minimal mortality. With sugar, in small cages, large males at 27°C matured most rapidly. A few erected fibrillae and inseminated females on night 1. On night 2, maximal proportions erected fibrillae and swarmed, and over one-third of females became inseminated. Small sugar-fed males at 23°C matured most slowly but had achieved nearly maximal levels of swarming by night 3. By night 5, small males had inseminated more than half the females, and large males had inseminated nearly all of them. Without sugar, large males progressed similarly during the first two nights. On night 3, however, the proportion erecting fibrillae and swarming declined precipitously at 27°C, and to a lesser degree at 23°C. Cumulative insemination never reached high levels. Small males never achieved high levels of fibrillar erection or swarming and inseminated few females, even at 23°C. In larger cages and under more semi-natural conditions, regardless of body size, without sugar male insemination capacity was virtually nonexistent.

Conclusion

Under some conditions, a limited number of sugar-deprived males can survive long enough to inseminate females. However, in nature males that cannot obtain sugar at frequent intervals will not be competitive with those that can, suggesting that male performance is closely tied to plant communities.

Sperm quantity and size variation in un-irradiated and irradiated males of the malaria mosquito Anopheles arabiensis Patton

Anopheles mosquitoes are important candidates for genetic control strategies. However, little is known about sperm quality and quantity as determinants of male reproductive success. In this study, sperm quantity and length variation were assessed in testes of un-irradiated and irradiated Anopheles arabiensis. Male reproductive organs were dissected for sperm and an estimate of the total number of spermatozoa was made. Sperm lengths were measured using imaging software. The effects of irradiation were evaluated for males exposed in the pupal or adult stage to a full (120 Gy) or partially sterilising dose (70 Gy). Sperm length variation in the laboratory strain was compared to the distribution observed in wild males. We also determined the size distribution of sperm lengths in spermathecae of inseminated females compared to those observed in male testes. Sperm quantity increased with age, and 12-day-old males had significantly more sperm in their testes (8214+/-467) than males aged 3 days (5022+/-375). Mosquitoes irradiated in the pupal stage had significantly fewer sperm (2982+/-125) than un-irradiated males (4950+/-848) although for adult stage irradiation similar amounts of sperm were observed compared to un-irradiated males. Sperm length variation was detected with sperm lengths ranging between <50 and 500 microm. There were no differences in sperm length distribution compared to wild males. Sperm length distributions were similar to those reported for the closely related sibling species An. gambiae s.s. There was no major effect of irradiation on the distribution of sperm lengths in the testes, with the exception that pupal irradiation resulted in a significant increase in sperm numbers in the category of 100-200 microm. Sperm length distributions in spermathecae were different to those measured directly from sperm in the testes and harboured less cells of the smaller (<100-200 microm), and more cells of the larger category (300-400 microm). The finding that testes of pupal irradiated males produce fewer and smaller sperm in comparison to un-irradiated and adult irradiated testes are discussed in the context of genetic control strategies, in particular the Sterile Insect Technique (SIT).

Intra-specific variation of sperm length in the malaria vector Anopheles gambiae: males with shorter sperm have higher reproductive success

Background

Intra-specific variation in sperm length influences male reproductive success in several species of insects. In males of the malaria vector Anopheles gambiae, sperm length is highly variable but the significance of this variation is unknown. Understanding what determines the reproductive success of male mosquitoes is critical for controlling malaria, and in particular for replacing natural populations with transgenic, malaria-resistant mosquitoes.

Methods

A laboratory population of A. gambiae males was tested for intra-specific variation in sperm length. A full-sib quantitative genetic design was used to test for a genetic component of sperm length in A. gambiae males and estimate its heritability. This study also tested for a relationship between sperm length and male reproductive success in A. gambiae. Male reproductive success was measured as the proportions of inseminated and ovipositing females.

Results

There was intra-specific variation of sperm length in A. gambiae. There was no significant genetic variation in sperm length and its heritability was low (h² = 0.18) compared to other insects. Sperm length was correlated with male body size (measured as wing length). Males with short sperm had significantly higher reproductive success than males with long sperm and this was independent of body size.

Conclusion

This is the first study to demonstrate intra-specific variation in sperm length in A. gambiae and that males with short sperm have higher reproductive success. That sperm length influences female oviposition is important for any strategy considering the release of transgenic males.

Comparison of male reproductive success in malaria-refractory and susceptible strains of Anopheles gambiae

Background

In female mosquitoes that transmit malaria, the benefits of being refractory to the Plasmodium parasite are balanced by the immunity costs in the absence of infection. Male mosquitoes, however, gain no advantage from being refractory to blood-transmitted parasites, so that any costs associated with an enhanced immune system in the males limit the evolution of female refractoriness and has practical implications for the release of transgenic males.

Methods

Aspects of the male cost of carrying Plasmodium-refractory genes were estimated by comparing the males' immune response and reproductive success among strains of Anopheles gambiae that had been selected for refractoriness or extreme susceptibility to the rodent malaria parasite, Plasmodium yoelii nigeriensis. The refractory males had a stronger melanization response than males from the susceptible line. Four traits were used as correlates of a male's reproductive success: the proportion of females that were inseminated by a fixed number of males in a cage within a fixed time frame, the proportion of females with motile sperm in their spermathecae, the proportion of ovipositing females, and the mean number of eggs per batch.

Results

Although there were significant differences among groups of males in sperm motility and oviposition success, these differences in male reproductive success were not associated with the refractory or susceptible male genotypes. Contrary to expectation, females mated to early emerging refractory males laid significantly more eggs per batch than females mated to later emerging susceptible males. Sperm motility and oviposition success were strongly correlated suggesting that variation in sperm motility influences female oviposition and ultimately male reproductive success.

Conclusion

An increased melanization response in male A. gambiae does not diminish male reproductive success under the experimental protocol used in this study. That refractory males induced ovipositing females to lay more eggs than susceptible males is an interesting result for any strategy considering the release of transgenic males. That sperm motility influences female oviposition is also important for the release of transgenic males.

Nature beats nurture: a case study of the physiological fitness of free-living and laboratory-reared male Anopheles gambiae s.l

Laboratory experimentation forms the basis for most of our knowledge of the biology of many organisms, in particular insects. However, the accuracy with which laboratory-derived estimates of insect life history and behaviour can predict their fitness and population dynamics in the wild is rarely validated. Such comparison is especially important in cases where laboratory-derived information is used to formulate and implement strategies for the genetic control of insects in nature. We have conducted a comparative study of the reproductive potential and life history of male Anopheles gambiae Gilies sensu lato mosquitoes from both standardized laboratory conditions and from natural field settings. We measured three indirect indicators of male mosquito fitness: energetic reserves, body size and survival, in a bid to determine whether the demographics and energetic limitations of wild males can be correctly predicted from their laboratory counterparts. Crucially, the body size and lipid reserves of wild males were substantially greater than those reared under standard laboratory conditions. We caution that the energetic limitations of insects as identified in the laboratory may underestimate their resilience in the wild, and discuss the implications of this phenomenon with respect to vector-borne disease control programmes based on genetic control of mosquitoes.

Genetic variation of male reproductive success in a laboratory population of Anopheles gambiae

Background

For Anopheline mosquitoes, the vectors of human malaria, genetic variation in male reproductive success can have important consequences for any control strategy based on the release of transgenic or sterile males.

Methods

A quantitative genetics approach was used to test whether there was a genetic component to variation in male reproductive success in a laboratory population of Anopheles gambiae. Swarms of full sibling brothers were mated with a fixed number of females and their reproductive success was measured as (1) proportion of ovipositing females, (2) proportion of ovipositing females that produced larvae, (3) proportion of females that produced larvae, (4) number of eggs laid per female, (5) number of larvae per ovipositing female and (6) number of larvae per female.

Results

The proportion of ovipositing females (trait 1) and the proportion of ovipositing females that produced larvae (trait 2) differed among full sib families, suggesting a genetic basis of mating success. In contrast, the other measures of male reproductive success showed little variation due to the full sib families, as their variation are probably mostly due to differences among females. While age at emergence and wing length of the males were also heritable, they were not associated with reproductive success. Larger females produced more eggs, but males did not prefer such partners.

Conclusion

The first study to quantify genetic variation for male reproductive success in A. gambiae found that while the initial stages of male reproduction (i.e. the proportion of ovipositing females and the proportion of ovipositing females that produced larvae) had a genetic basis, the overall reproductive success (i.e. the mean number of larvae per female) did not.

Age and body size influence male sperm capacity of the dengue vector Aedes aegypti (Diptera: Culicidae)

Understanding mosquito mating biology is essential for studies of mosquito behavior, gene flow, population structure, and genetic control. In the current study, we examine the effect of age and body size on spermatozoa number in two laboratory strains of the dengue vector, Aedes aegypti (L.), Thailand and Rockefeller (ROCK), and in wild-collected mosquitoes from Thailand. Body size was a major predictor of total spermatozoa number, with significantly greater sperm numbers in large (2.27-mm wing length) versus small males (1.85-mm wing length) within the same age group. Total sperm capacity also varied by male age. Spermatozoa numbers in virgin Ae. aegypti males increased significantly up to 10 d after emergence and then leveled off until 20 d. Significant variations in sperm number were detected among Ae. aegypti strains, with wild-collected mosquitoes having the greatest total number of sperm. Our study provides the first evidence of spermatogenesis in adult mosquitoes and indicates high rates of spermatogenesis in male mosquitoes up to 10 d of age (3.3 degree-days). Our results emphasize the potential role of body size and age on the mating capacity of this important vector of dengue and yellow fever viruses.

A reliable morphological method to assess the age of male Anopheles gambiae

BACKGROUND

Release of genetically-modified (GM) or sterile male mosquitoes for malaria control is hampered by inability to assess the age and mating history of free-living male Anopheles.

METHODS

Age and mating-related changes in the reproductive system of male Anopheles gambiae were quantified and used to fit predictive statistical models. These models, based on numbers of spermatocysts, relative size of sperm reservoir and presence/absence of a clear area around the accessory gland, were evaluated using an independent sample of mosquitoes whose status was blinded during the experiment.

RESULTS

The number of spermatocysts in male testes decreased with age, and the relative size of their sperm reservoir increased. The presence of a clear area around accessory glands was also linked to age and mating status. A quantitative model was able to categorize males from the blind trial into age groups of young (< or = 4 days) and old (> 4 days) with an overall efficiency of 89%. Using the parameters of this model, a simple table was compiled that can be used to predict male age. In contrast, mating history could not be reliably assessed as virgins could not be distinguished from mated males.

CONCLUSION

Simple assessment of a few morphological traits which are easily collected in the field allows accurate age-grading of male An. gambiae. This simple, yet robust, model enables evaluation of demographic patterns and mortality in wild and released males in populations targeted by GM or sterile male-based control programmes.

Switchover to the mated state by spermathecal activation in female Anopheles gambiae mosquitoes

Both Aedes aegypti and Anopheles gambiae mosquitoes undergo physiological and behavioral changes after the females mate, but unlike Ae. aegypti, the mated state in An. gambiae is not attained through the action of male accessory gland substances. Experiments in which the spermathecae of mated females were manipulated suggest that a spermatheca filled with sperm is responsible for triggering oviposition behavior. An estimated 48% of the previously mated An. gambiae females mated again when the interval between encounters with males was less than 24h. Unlike male Ae. aegypti that can inseminate up to seven females after their testes are removed, An. gambiae do not store sperm in the vas deferens and without testes would be unable to inseminate any females.

Male size does not affect mating success (of Anopheles gambiae in São Tomé)

For malaria control, the utility of transgenic vector Anopheles mosquitoes (Diptera: Culicidae) refractory to Plasmodium transmission, will depend on their interbreeding with the wild vector population. In many species, larger males are more successful in obtaining mates. In São Tomé island, we determined that size did not affect mating success of male Anopheles gambiae Giles sensu stricto, the main malaria vector in tropical Africa. Also we showed that larval intraspecific competition is probably insignificant in this population of An. gambiae. Thus, the potential success of transgenic An. gambiae is unlikely to be affected by size selection under field conditions.

Anopheles culicifacies: effects of age on the male reproductive system and mating ability of virgin adult mosquitoes

Under controlled laboratory conditions of 28-30 degrees C and 16:8 L:D photoperiod, an attempt was made to develop an age-grading technique for Anopheles culicifacies males. Mating activity was maximal when females were 5-12 days old males were 5-7 days old. The numbers of total and mature spermatocysts declined significantly with age, and the proportion of the testes occupied by the sperm reservoir increased as virgin males grew older. Mating resulted in the loss of spermatozoa and accessory gland substance from the reproductive system. Loss of mating ability of older virgin males seemed to be age-related, because the reproductive system contained ample supplies of accessory gland substance and spermatozoa. Morphological changes of the reproductive system, due to mating and age, were used to infer the age and reproductive history of unknown males in a laboratory evaluation.