The size inequality of Apis mellifera ligustica hypopharyngeal glands along a gradient of heavy metal pollution

A number of anthropogenic activities, including farming and urbanization, have a significant impact on the environment and can produce an irreversible damage at the population level. To assess the effects of environmental stressors as metal pollution on Apis mellifera ligustica populations, we analyzed the hypopharyngeal gland size and morphology of foraging bees along a urban-suburban gradient. Foraging bees are collected from beehives located in S. Giovanni (Trieste, “urban site”) and in Domio (Trieste, “suburban site”) over two full activity periods in early summer (July). Foraging bee heads from urban site (n = 4) and from suburban site (n = 2) were fixed and embedded in Epon 812-Araldite perform morphometrical analyses of hypopharyngeal glands, the minimum and maximum diameters of the acini was measure used ImageJ program. Diameters were expressed as mean±SE and differences were assessed by nonparametric statistics. Statistical analyses were performed using R version 3.0.1 software (R Development Core Team 2013). The statistical comparison showed a smaller diameter of the acini of hypopharyngeal gland of bees from suburban site (0.080 mm ± 0.003), compared to those of urban site (0.114 mm ± 0.004). Statistical analysis showed an highly significant difference in the size of the acini between the site of Domio (Wilcoxon rank sum test, p = 1.651x10-7) compared to the site of S. Giovanni. Metal contents were recorded for foraging bees (n = 20 from each site) and measured by inductively coupled plasma-mass spectrometry (ICP-MS). Recorded elements are As, Bi, Cd, Co, Cr, Cu, Ni, Pb, Sr, V, and Zn. They are accumulated in two different rank order Zn> Cu> Sr=Bi> Ni> Cr> Pb=Co> V>Cd >As in bees from urban site and Zn> Cu> Sr >Cr >Ni>B =Co=V>Pb>As>Cd in bees from suburban site. Significantly differences in concentration of Cd, Cr and Cu were recorded in bees from the two sites (Wilcoxon rank sum test p<9.229x10-5 for Cd, p < 0.00021 for Cr and p < 0.00053 for Cu). Metal concentrations in the animal’s body reflect quantitatively pollution levels of sites. Moreover, morphological data demonstrate that metal pollutions in the environment may have detrimental effects on the individual level and then on A. mellifera ligustica population.