COMPOSITION OF FLORAL NECTAR IN LITHOSPERMEAE TRIBE (BORAGINACEAE) WITH FOCUS ON NEUROACTIVE SECONDARY METABOLITES AND THEIR POSSIBLE IMPLICATIONS ON DEFINING THE GUILDS OF POLLINATORS

In recent years, a central role was recognized to the study of nectar chemistry in shaping the interactions between plants and pollinators. Some neuroactive molecules such as non-proteogenic amino acids (NPAAs) and biogenic amines have also been reported in nectar. The presence of these compounds has opened various hypotheses of their ecological role as well as their genesis. One of the most interesting considerations is that biogenic amines are normally associated with the presence of microorganisms (mostly lactic acid bacteria) in food. Over the last twenty years, there are many studies that are making contributions on the description of the microbial community in nectar. This project unfolds in four pillars describing the chemistry of the nectar of the Mediterranean Lithospermeae in relation to the pollinator guild and then investigating the presence and possible ecological role of biogenic amines (BioAs). In the first pillar the chemical profile of the nectar of Mediterranean Lithospermeae was described and in the second pillar it was used to identify possible associations with pollinators building plant-pollinator networks. Once revealed the presence of BioAs in nectar, in the third pillar their effect was tested on Bombus terrestris and in the fourth pillar was investigated the possible role of some nectar inhabiting yeast’ species in the production of such compounds. The hexose-dominant profile, restricted to Aegonychon purpurocaeruleum, is associated with a purely generalist guild of pollinators. The sucrose-rich profile, typical of Echium species, is the most interactive, as well as the most widespread. The sucrose-dominant profile, typical of Onosma spp. is the one that depend the most on specialist pollinators. The Onosma species, with corolla tubes up to 23mm, are visited mostly by species morphologically suited to reaching nectar, such as Osmia apicata (Megachilidae: Osminii). The pollinators that interact most with the studied Lithospermeae species are long-tongued bees (Apidae and Megachilidae families) but also specialized or non-specialized dipterans and moths and some short-tongued bees. A particular adaptation has been identified in some instances linking plant and pollinators in a very specialist relationship (e.g. Andrena solenopalpa and Lithodora fruticosa). The amino acid profile is more diversified and not very constant among the Lithospermeae species of the collection. Among the most ubiquitous amino acids are histidine, valine and tyrosine. The concentrations also vary greatly, with proline being the most concentrated amino acid. In 12 out of 15 species, there is a tendency to increase the non-proteinogenic amino acids component in unvisited flowers respect to visited flowers. Biogenic amines were identified in almost all samples, with qualitative and quantitative differences. Octopamine is the most concentrated for Onosma species nectar, with concentrations exceeding 20mM. Other amines identified are tyramine, histamine and tryptamine (mostly traces). Testing diets with octopamine and tyramine on Bombus terrestris, a phagostimulation effect of octopamine, opposite to tyramine, was recorded. Furthermore, bumblebees fed octopamine are less inclined to take flight. Despite the behavioural impact, neither the levels of biogenic amines in the brain nor in the haemolymph gave significantly different results from the control. Of the twelve strains of yeasts (three strains per species) inoculated in artificial nectar medium emulating the nectar of three Lithospermeae species, none seemed to produce the amines of interest in the growth conditions to which they were subjected. It is not possible to say whether the biogenic amines are due to the presence or absence of microorganisms in the nectar, but these molecules, and the entire spectrum of molecules present in the nectar, must be a key factor in the study of the biology of pollination, as well as the role of microorganisms.