Apocynaceae display highly complex and diverse floral morphologies. Pollen dispersal units include monads (single pollen grains; e.g. Plumeria), tetrads (Apocynum), and pollen packaged in pollinia, e.g. Asclepias and Cynanchum (Fishbein et al. 2018). All species produce adhesive from the specialized apex of the gynoecium (the style-head). At anthesis, this adhesive may be amorphous or molded into discrete translators; in either case, it functions to attach the pollen grains to each other and to pollinators, effecting aggregated pollen transport (Fallen 1986, Endress and Bruyns 2000, Livshultz et al. 2018). Species of one large lineage, the APSA clade, which includes ca. 3700 of the ca. 4500 species in the family, share the synapomorphic presence of a gynostegium, the structurally integrated style-head and anthers, which functions to place and remove pollen from visitors (Fishbein et al. 2018). It has been hypothesized that these floral modifications are adaptations that increase “pollen transfer efficiency” and reduce loss of pollen in transit between flowers (Harder and Johnson 2008, Livshultz et al. 2018). Selection on male fitness may favor aggregation and high efficiency at the cost of fewer mating opportunities under conditions of low quantity and/or quality of pollination service (Harder and Johnson 2008, Livshultz et al. 2011). Aggregated pollen also makes it more likely that offspring in the same fruit are full siblings, with implications for pollen competition and pollen to ovule ratios (Harder and Johnson 2008).
Many kinds of floral visitors have been documented for the family, and pollination syndromes in the Apocynaceae are diverse (Ollerton et al. 2018). Insects are the main floral visitors (Endress 1994, Ollerton et al. 2018), including bees (Lopes and Machado 1999, Darrault and Schlindwein 2005, Alvina de Araujo et al. 2011, Nogueira de Moura et al. 2011), beetles (Faria-Vieira and Santos- Fonseca 2011), butterflies (Darrault and Schlindwein 2005, Alvina de Araujo et al. 2011, 2014), moths (Haber 1984, Darrault and Schlindwein 2005, Sugiura and Yamazaki 2005, Barman et al. 2018), flies (Ollerton et al. 2009), and wasps (Wiemer et al. 2011). Recent phylogenetically-informed studies have linked evolution of the gynostegium and pollinia with diversification of both corolla morphology and functional groups of pollinators (Fishbein et al. 2018, Ollerton et al. 2018). These recent syntheses highlight the need for continued studies to test the generality of identified trends.
To obtain information on the natural history of pollinator interactions by species of Apocynaceae, and to learn how their flowers function, we studied two related species that are abundant in the flora of the Bahamas. Both are lianas belonging to the APSA clade (made up of subfamily Apocynoideae and exemplars of Periplocoideae, Secamonoideae, and Asclepiadoideae; Livshultz et al., 2007) with gynostegia, pollen in monads, and amorphous style-head adhesive, but contrasting corolla morphology. Here we present our preliminary observations on the floral biology and pollination of Pentalinon luteum (L.) B.F.Hansen and Wunderlin (Odonadenieae) and Echites umbellatus Jacq. (Echiteae).