Floral hosts of leaf-cutter bees (Megachilidae) in a biodiversity hotspot revealed by pollen DNA metabarcoding of historic specimens

choice differences of six species of Megachilid bees collected from different regions in South Africa. By analysing the pollen carried by these closely related species that differ in habitat preference, this study aimed to test if regions with high levels of floral endemism and species richness will also show high levels of pollinator specificity as pollinators have a known role in driving floral diversification. If bees from the more plant diverse Succulent Karoo exhibited a narrower range of floral choices than bees from the Savanna, we could expect that more plant family and species OTUs would be identified in the pollen loads of bees from the Savanna than in those from the Succulent Karoo. In contrast, the data presented here showed no significant difference in the number of plant families, or species OTUs visited by bees endemic to the Suc- culent Karoo and Savanna biomes. This suggests that bees in the highly plant-diverse Western region of South Africa are no more specialised than those to the less- diverse east. Our data, however, show a difference between bees endemic to just one biome with narrow habitat pref- erence and those that forage across multiple biomes. A distinct difference was shown in the mean number of both plant family and species OTUs visited between widespread bees and those of bees endemic to either the Succulent Karoo or the Savanna. NMDS analysis supports this finding, with the widespread bee group characterised by a different pollen assemblage than that of the bee species collected in the Succulent Karoo or Savannah.

Fig 4. Sample-based rarefaction curve to estimate plant species richness for each bee species studied. Megachile venusta (e) was shown to have the highest estimated species richness. Sample labels represent bee species and are labelled as follows: (a) M. felina (b) M. karooensis (c) M. maxillosa (d) M. murina (e) M. venusta (f) M. niveofasciata. https://doi.org/10.1371/journal.pone.0244973.g004 PLOS ONE Floral hosts of Megachilidae revealed by metabarcoding PLOS ONE | https://doi.org/10.1371/journal.pone.0244973 January 21, 2021 10 / 16