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

Abstract

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.

Figure 1

Distribution of sperm length (mm) in relation to colony age and genetic refreshing. Twenty mature sperm cells were measured in 134 An. gambiae s.s. Field Mopti males and males from the Mopti 2008, Mopti 2008 refreshed 2009, Mopti 2003, Mopti 2003 refreshed 2008, KIL, and Kisumu colonies.

Figure 2

Mean sperm length and testes and accessory gland sizes in relation to colony age and genetic refreshing. A) The mean sperm length (20 cells per male) was measured in male progeny of field-collected Mopti, in Mopti strains of various age and in very old strains, and in genetically refreshed colonies. B) The mean testes (light grey) and accessory glands (dark grey) size corrected for body size (±95% CI) measured in the same strains. Bars labeled with different letters were significantly different (Tukey P < 0.05).

Figure 3

Mean sperm length and testes and accessory gland sizes in relation to genetic transformation. A) The mean sperm length (20 cells per male) was measured in male progeny of field-collected Mopti, and the KIL parental line of the transgenic phase-1 EE and EVida3 strains. B) The mean testes (light grey) and accessory glands (dark grey ) size corrected for body size (±95% CI) measured in the same strains. Bars labeled with different letters were significantly different (Tukey P < 0.05).

Figure 4

Mean sperm length and testes and accessory gland sizes in relation to heterosis. A) The mean sperm length (20 cells per male) was measured in male progeny of field-collected Mopti, the older KIL, Kisumu and Mopti 2003 strains and heterotic males crosses between the later strains. B) The mean testes (light grey) and accessory glands (dark grey) size corrected by wing length² (±95% CI) measured in the same strains. Bars labeled with different letters were significantly different (Tukey P < 0.05).

Figure 5

Relationship between male body size and reproductive traits. A) The mean sperm length (20 cells per male), and B) mean testes and C) accessory gland sizes are shown in relation to male body size measured as wing length. Equations of significant allometric relationships are indicated in their linear and exponential form (in brackets). There was no significant allometric relationship between mean sperm length and male body size.