Natural variations in egg hatching behaviour and selection of novel slowhatching strains for male anopheline deployment

Ottih, Emmanuel Chinweuba

Abstract

Genetic Mosquito mass release is a promising tool in the suppression of wild Anopheles gambiae sensu lato mosquitoes as part of malaria control in Africa. This tool requires a plethora number of eggs and a strain with good fitness. Thus, the success greatly depends on how the strain can survive and successfully introgress with the wild (target) population. Given the geographical distribution of Anopheles gambiae complex in sub-Sahara Africa, this technique may also require shipment of eggs from egg production facilities to egg rearing and deployment locations. Because colonised mosquito strains are inadvertently selected for early hatching, due to the simplified rearing protocols in insectaries, shipment of eggs of the prolonged established strains is therefore constrained particularly to remote locations. The production of novel late hatchers with good fitness should be readily available for this purpose to compensate for the early time of hatching of Anopheles eggs. It was hypothesised that egg hatching time was under genetic control and the study was designed to test that hypothesis.
Using prolonged and recently colonised strains of three sibling species of Anopheles gambiae complex: An. gambiae s.s, (Kisumu strain 44 years old); An. arabiensis, (Senn strain 50 years old and Rufisque strain 2 years old) and An. coluzzii, (Mopti strain 16 years old and VK strain 1 year), egg-hatching biology was explored to determine variation in egg hatching distribution of the five strains colonised at different times and effect of the time of colonisation on hatch rate. The effect of simulating rainfall by splashing water of different temperatures (Still-non agitation at 26 C, 4 C spray, and 15 C spray) on eggs every day for 7 days was explored. It was observed that Anopheles eggs colonised at different times exhibited distinct hatching patterns in relation to water treatments. No eggs hatched within 24 hours (day 0) post oviposition in both agitated and non-agitated groups. However, on the 2nd day (day1), the hatch rate started increasing, with the older strains exhibiting outstanding higher hatch rates compared to their recently colonised counterparts. Overall, the older strains (Senn, Kisumu, and Mopti) hatched within days 1 and 2. In the five strains, hatching spread up to a span of 5 days with only the recently colonised Rufisque and VK strains hatching on the 7th day (day 6)- the last day of the experiment. Across all water conditions, the older strains had highest hatching success compared to their recently colonised counterparts.
The recently colonised VK strain was subjected to an evolutionary study by employing a bidirectional (artificial) selection to investigate if egg-hatching time has a genetic component that could be selected and passed down from the parents to their offspring over many generations. Fast (Early1 and Early2) and Slow (Late1 and Late2) hatching strains were established from a parental strain. At the end of the study, the eggs of the Early hatchers hatched significantly faster than their parental strain, while Late hatchers on the other hand took a significantly longer time to hatch than the parental strain in the second trial at generation 17. The fitness of both the selected (Early1, Late1, Early2, and Late2) strains and the longestablished unselected (Mopti) strain was assessed to determine if selection has negative impacts on mating success, fertility, and adult survival. Focusing on the late-selected strains, we observed significant improvement in all parameters tested compared to the well-established Mopti, which indicated a potential loss of fitness in their adult survival. There were no significant differences in mating success and fertility between Mopti and Late2. However, adults of the selected late hatching strains, Late1 and Late2 survived significantly longer compared to the long-established Mopti. The variations observed in these studies suggest that both genetic and environmental factors could be responsible for early or late hatching patterns in Anopheles eggs. Evidence from the study suggests that there are genes encoding egg hatch time as a heritable trait. Understanding these factors would help to improve egg hatching management and relieve constraints in Anopheles mass maintenance and egg shipment for male anopheline deployment.

Citation

Ottih, E. C. Natural variations in egg hatching behaviour and selection of novel slowhatching strains for male anopheline deployment. (Thesis). Keele University. https://keele-repository.worktribe.com/output/598866

• Thesis Type: Thesis
• Deposit Date: Oct 12, 2023
• Public URL: https://keele-repository.worktribe.com/output/598866
• Additional Information: Embargo on access until 1 October 2028 - The thesis is due for publication, or the author is actively seeking to publish this material.
• Award Date: 2023-10