Browsing by Author "Ng'habi, Kija R."

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    Effect of Larval Crowding on Mating Competitiveness of Anopheles Gambiae Mosquitoes
    (BioMed Central, 2005) Ng'habi, Kija R.; John, Bernadette; Nkwengulila, Gamba; Knols, Bart G. J.; Killeen, Gerry F.; Ferguson, Heather M.
    The success of sterile or transgenic Anopheles for malaria control depends on their mating competitiveness within wild populations. Current evidence suggests that transgenic mosquitoes have reduced fitness. One means of compensating for this fitness deficit would be to identify environmental conditions that increase their mating competitiveness, and incorporate them into laboratory rearing regimes. Anopheles gambiae larvae were allocated to three crowding treatments with the same food input per larva. Emerged males were competed against one another for access to females, and their corresponding longevity and energetic reserves measured. Males from the low-crowding treatment were much more likely to acquire the first mating. They won the first female approximately 11 times more often than those from the highcrowding treatment (Odds ratio = 11.17) and four times more often than those from the mediumcrowding treatment (Odds ratio = 3.51). However, there was no overall difference in the total number of matings acquired by males from different treatments (p = 0.08). The survival of males from the low crowding treatment was lower than those from other treatments. The body size and teneral reserves of adult males did not differ between crowding treatments, but larger males were more likely to acquire mates than small individuals. Larval crowding and body size have strong, independent effects on the mating competitiveness of adult male An. gambiae. Thus manipulation of larval crowding during mass rearing could provide a simple technique for boosting the competitiveness of sterile or transgenic male mosquitoes prior to release
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    A Reliable Morphological Method to Assess the Age of Male Anopheles Gambiae
    (BioMed Central, 2006) Huho, Bernadette J.; Ng'habi, Kija R.; Killeen, Gerry F.; Nkwengulila, Gamba; Knols, Bart G. J.; Ferguson, Heather M.
    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. 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 experimen 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 (≤ 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. 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.
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    A Reliable Morphological Method to Assess the Age of Male Anopheles Gambiae.
    (BioMed Central, 2006) Huho, Bernadette J.; Ng'habi, Kija R.; Killeen, Gerry F.; Nkwengulila, Gamba; Knols, Bart G. J.; Ferguson, Heather M.
    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. 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. 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 (≤ 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. 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.
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    Sexual Selection in Mosquito Swarms: May the Best Man Lose?
    (Elsevier, 2008) Ng'habi, Kija R.; Huho, Bernadette J.; Nkwengulila, Gamba; Killeen, Gerry F.; Knols, Bart G. J.; Ferguson, Heather M.
    One of the greatest paradoxes in evolutionary biology is the continued maintenance of genetic variation for phenotypic traits that appear to confer strong fitness advantages. Of these traits, body size is perhaps the one that has been most consistently linked to increased longevity and reproductive success in males. We investigated two hypotheses for how events occurring during mating in the mosquito Anopheles gambiae could impede directional selection on male body size: (1) whether male copulation probability is random with respect to body size in aerial swarms, and (2) whether large males are less competitive during mating than smaller, shorter-lived rivals. By manipulation of larval nutritional conditions (low, intermediate and high food allocation), we generated cohorts of male A. gambiae mosquitoes that differed in adult body size and energy reserves (body size and energy reserves being positively correlated with larval nutrition). When competing against one another in aerial swarms, males from the intermediate food treatment were six and two times more successful at acquiring mates than those from the high and low food treatments, respectively. The median survival of males from this most sexually competitive group was approximately 13% lower than that of the larger males with high larval nutrition. We conclude that phenotypic determinants of long-term survival and mating success may not be correlated in this system, and thus that stabilizing selection as well as environmental condition-dependent expression of traits could account for the maintenance of variation in male body size in this species and in other swarming insects.