Browsing by Author "Ferguson, Heather M."

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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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    The Impact of Livestock on the Abundance, Resting Behaviour and Sporozoite Rate of Malaria Vectors in Southern Tanzania
    (Springer, 2015) Mayagaya, Valeriana S.; Nkwengulila, Gamba; Lyimo, Issa N; Kihonda, Japheti; Mtambala, Hassan; Ngonyani, Hassan; Russell, Tanya L.; Ferguson, Heather M.
    Increases in the coverage of long-lasting insecticidal nets (LLINs) have significantly reduced the abundance of Anopheles gambiae sensu stricto in several African settings, leaving its more zoophagic sibling species Anopheles arabiensis as the primary vector. This study investigated the impact of livestock ownership at the household level on the ecology and malaria infection rate of vectors in an area of Tanzania where An. arabiensis accounts for most malaria transmission. Mosquito vectors were collected resting inside houses, animal sheds and in outdoor resting boxes at households with and without livestock over three years in ten villages of the Kilombero Valley, Tanzania. Additionally, the abundance and sporozoite rate of vectors attempting to bite indoors at these households was assessed as an index of malaria exposure. The mean abundance of An. gambiae s.l. biting indoors was similar at houses with and without livestock. In all years but one, the relative proportion of An. arabiensis within the An. gambiae s.l. species complex was higher at households with livestock. Livestock presence had a significant impact on malaria vector feeding and resting behaviour. Anopheles arabiensis were generally found resting in cattle sheds where livestock were present, and inside houses when absent. Correspondingly, the human blood index of An. arabiensis and An. funestus s.l. was significant reduced at households with livestock, whereas that of An. gambiae s.s. was unaffected. Whilst there was some evidence that sporozoite rates within the indoor-biting An. gambiae s.l population was significantly reduced at households with livestock, the significance of this effect varied depending on how background spatial variation was accounted for. These results confirm that the presence of cattle at the household level can significantly alter the local species composition, feeding and resting behaviour of malaria vectors. However, the net impact of this livestock-associated variation in mosquito ecology on malaria exposure risk was unclear. Further investigation is required to distinguish whether the apparently lower sporozoite rates observed in An. gambiae s.l. at households with livestock is really a direct effect of cattle presence, or an indirect consequence of reduced risk within areas where livestock keepers choose to live.
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    Nature Beats Nurture: A Case Study of the Physiological Fitness of Free-Living and Laboratory-Reared Male Anopheles Gambiae S.L.
    (The Company of Biologists, 2007) Huho, B. J.; Ng’habi, K. R.; Killeen, G. F.; Nkwengulila, Gamba; Knols, B. G. J.; Ferguson, Heather M.
    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.
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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.