INTEGRATED PHENOTYPIC AND GENETIC ASSESSMENT OF PERFORMANCE TRAITS IN APIS MELLIFERA

Honey bees play an important role in modern agriculture as farm animals and crop pollinators and represent an animal model in scientific research. Since few decades, managed honey bees are facing large scale losses worldwide due to the interaction of several biotic and abiotic factors such as the spread of pathogens and parasites, the habitat loss, the use of pesticides, and the occurrence of climate changes. For years, beekeepers have controlled deadly pathogens such as Varroa destructor with acaricides (pyrethroid), but widespread chemical resistance is manifesting. Alternative management strategies could be developed to characterize and select bees with heritable traits allowing them to resist mites and diseases. Hygienic behaviour (HB) is a heritable phenotype that confers colony level resistance against brood disease, with a potential effect on the damaging parasitic mite V. destructor. Unfortunately, breeding such bees is complicated as the assays involved to characterize such phenotype are time-consuming and expensive if performed on a consistent number of colonies. Additionally, the mechanisms behind social immunity are not yet fully understood. These issues have motivated the scientific community to develop research tools that can offer insight into the causes of declining bee health as well as identify biomarkers to guide breeding programs. The studies presented in this thesis are the results of the collaboration between the Animal Genetic Group of the University of Milan and an Italian beekeeping company, which made available its animals for the majority of the studies presented in this work. The first two chapters of this thesis focus on three important aspects regarding hygienic behaviour: in-field phenotypic implementation of a method to measure HB on large testing populations, its genetic parameters estimation and developing molecular tools for the identification of potential HB genetic markers from breeding queens. Queens are the only fertile females of the colony, responsible for laying eggs and maintaining the cohesion of the colony. The quality of the queen may have an important effect on a colony’s development, productivity and survival and queen failure or loss is considered a cause of the decline of colonies worldwide. The queen quality, resulting from her genetic background, developmental condition, mating success and environment, can be assessed by some morphological measures. We investigated the genetic parameters of some traits that describe the quality of bee queens in the third chapter. Beside queen morphology, in the fourth chapter, we investigated also the morphology of the worker bees with Computed Tomography (CT). In this study, a non-invasive CT technique and image analysis approach coupled with brood manual inspection was used to clarify the relationship between honey bee pupa length and its varroa mite infestation status, developmental status and spatial position within the brood comb. The results of this chapter suggest that the CT-scan may represent a suitable non-invasive tool to investigate the morphology and developing status of honey bee brood. Finally, since honey yield is one of the major breeding goal for a beekeeping company, we investigated the genetic parameters for honey yield in a small testing population of honey bees in Northern Italy both considering the total honey yield and the single harvests of a colony within a year on data collected over a period of three years.