Abstract
We examined age-related biochemical and histological changes in the fat bodies and hemolymph of Osmia rufa males and females. We analysed solitary bees during diapause, in October and in April; as well as the flying insects following diapause, in May and June. The trophocyte sizes, as well as the numbers of lipid droplets were the greatest at the beginning of diapause. Subsequently, they decreased along with age. Triglyceride and glucose concentrations systematically decreased in fat body cells but increased in the hemolymph from October to June. Concentrations/activities of (enzymatic and non-enzymatic) antioxidant and proteolytic systems, as well as phenoloxidase, aspartate aminotransferase, alanine aminotransferase and alkaline phosphatase levels were constant during the diapause, usually lower in the males than the females. Prior to the diapause/overwintering, the concentrations/activities of all the compounds were higher in the fat bodies than in the hemolymph. Later in the spring and in the summer, they increased in the hemolymph and on the body surfaces, while decreasing in the fat bodies. The global DNA methylation levels increased with age. Higher levels were always observed in the males than in the females. The study will promote better understanding of bee evolution and will be useful for the protection and management of solitary bees, with benefits to the environment and agriculture.
Highlights
There are two reasons to study age-related biochemical and histological changes in Osmia rufa L. (O. bicornis; red mason bee), its biochemical defence characteristics.(1) The appearance of insect eusociality has been one of the major transitions in evolution [1]
Microscopic images of the fat bodies were similar in both sexes of O. rufa in October and April (Fig 1)
It is mainly involved in the hormonal control of the storage and mobilization of energy reserves that are crucial for the reproductive anabolic processes and, above all, for the group overwintering capabilities [46,47]
Summary
There are two reasons to study age-related biochemical and histological changes in Osmia rufa L. (1) The appearance of insect eusociality has been one of the major transitions in evolution [1]. The eusocial bee female castes (queens and workers) and males (drones) exhibit different physiological bases of life span, resistance, senescence, etc. That in order to fully understand the biochemical evolution of eusociality, it is necessary to compare age-related biochemical changes in eusocial and solitary bees. There is a significant gap in the knowledge about age-related biochemical processes in solitary bees, hindering such comparisons, which we decided to fill with this work. We used the solitary O. rufa bee, which only has fertile females and males differing in body size and physiology. Neither age polyethism nor female castes are present in it. Males and females at the diapause stage after the start of overwintering (in October), those just after the diapause/overwintering and at the beginning of the active life stage (in April) and flying, fully active insects (males and females in May, and only females in June) were collected for our analyses
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