Abstract
In the face of high proportions of yearly colony losses, queen health and fecundity has been a major focus of industry and research. Much of the reproductive quality of the queen, though, is a function of the mating success and quality of the drones (males). Many environmental factors can negatively impact drone semen quality, but little is known about factors that impact the drones’ ability to successfully mate and deliver that semen, or how widely drones vary. In our study, we observed the daily variation in honey bee drone reproductive quality over time, along with a number of morphological traits. Drones were reared in cages in bank colonies, and 20 individuals were dissected and measured daily. The number of viable spermatozoa in the seminal vesicles was zero at emergence and reached an average maximum of 7.39 ± 0.19 million around 20 days of life. Decline in spermatozoa count occurred after day 30, though viability was constant throughout life, when controlling for count. Older drones had smaller wet weights, head widths, and wing lengths. We predict that this is likely due to sampling bias due to a differential lifespan among larger, more reproductively developed drones. Our study shows that drones are more highly variable than previously suggested and that they have a significant variation in reproductive physiology as a function of age.
Highlights
No drones had semen present in their seminal vesicles on the first day of life. This percentage increased to about 50% of the drones by day 3, with spermatozoa transfer to the Principal components varied over age, both within the trimmed dataset and across the entire age range
This percentage increased to about 50% of the drones by day 3, with spermatozoa transfer to the seminal vesicles begun in all drones by day 6
Our results suggest that a major mode of reproductive senescence is more complicated than previously suggested and, all things being equal, semen from older drones is unlikely to be less viable than that from younger drones
Summary
Colony losses have continued in subsequent years, with US beekeepers reporting an average of 40.5% of their colonies lost every year [6,7]. These trends of increased mortality occur more widely in other insect pollinator taxa [8,9,10]. The number of managed colonies of honey bees has largely remained stable over the past decade, the population is sustained by the continual need of beekeepers to replace colonies and a corresponding increase in their management costs [4]. This, combined with increased human reliance on pollinator-dependent crops [11], has the potential to lead to increased economic hardship across all levels of food production [12,13]
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