Balducci, Marco Giuseppe (2024) Quantifying the influence of mutualists on the distribution of Mediterranean orchids. [Tesi di dottorato]

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Collection description

The distribution and abundance of plant populations are often linked to the availability of mutualists. Orchids exhibit interactions of varying strength and specificity with two types of mutualists; (1) animal pollinators to ensure fruit set and (2) particular fungi, the orchid mycorrhizal (OrM) fungi, to germinate and survive. The reproductive success of pollinator-dependent orchids is frequently pollen limited and factors that attract and maintain pollinators, such as flowering population size may be important in determining individual fecundity. However, the relationship between flowering population size and reproductive success is highly variable and may depend on their reward strategy. We performed an extensive literature survey to test the extent of pollen limitation from experimental hand-pollination studies in a phylogenetic context. We quantified how both flowering population size and fruit set varied according to latitude, longitude, and reward strategy. We found that pollen limitation was high and widespread in orchids, with no phylogenetic signal. While natural fruit set was phylogenetically constrained, and that per capita fecundity decreased with increasing latitude and longitude. This supports the hypothesis that the outcome of biotic interactions varies geographically and may play a key role in explaining orchid distributions. However, the influence of population size on genetic diversity and OrM fungal communities is still largely unknown. We therefore investigated the population genetic structure and diversity of Orchis italica, a widespread Mediterranean orchid. We found a low genetic differentiation within and among populations of O. italica in southern Italy and Greece. Lastly, interactions with mycorrhizal fungi are increasingly recognized as an important factor underlying orchid distribution and abundance. However, the geographical distribution of OrM and how their communities vary over large geographical areas is less well understood. We therefore investigated the abundance and diversity of OrM communities associated with O. italica using both fungal isolation and high-throughput sequencing. Isolation from adult roots and protocorms coupled with manipulative in vitro germination experiments revealed that O. italica has a relatively narrow specialisation on members of Tulasnella for germination and early development. Metabarcoding, however, revealed a more taxonomically diverse community of OrM fungi, with 79 OrM fungal operational taxonomic units (OTUs) found associating with O. italica, 35 of which were members of the Tulasnellaceae. Five Tulasnellaceae OTUs and one Ceratobasidiaceae OTU were found in all 23 populations in southern Italy and Greece. Abundance of these OTUs, as measured by number of reads, increased from west to east and decreased from south to north. Taken together, these results highlight the importance of quantifying and identifying the distribution of belowground mutualists, such as mycorrhizas, in understanding and predicting the current and future distribution of this species.

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