Pollen transfer efficiency of Apocynum cannabinum (Apocynaceae): a comparative perspective

Abstract

Pollen transfer efficiency (PTE), the percentage of removed pollen delivered to conspecific stigmas, has been implicated in the morphological evolution, population dynamics, and lineage diversification of flowering plants. Pollinia, the aggregated contents of pollen sacs, present in Apocynaceae subfamilies Asclepiadoideae (milkweeds), Secamonoideae, and Periplocoideae and orchids (Orchidaceae), are the pre-eminent example of a plant trait that elevates PTE (to ca. 25%). However comparison of species with pollinia to “average” flowers (PTE ca. 1%) may over-estimate the gains from pollinia. We hypothesize that elevated PTE evolved in Apocynaceae prior to pollinia. We measured PTE and pollen to ovule ratio, a possible correlate of PTE, in Apocynum cannabinum, a milkweed relative with pollen tetrads (instead of pollinia) and simple bands of style head adhesive (instead of complex pollinium-carrying translators), comparing them to reports of other species collated from the literature. PTE of A. cannabinum is 7.9%, in the 24^th percentile of reports for 35 milkweed species, but more than twice the highest PTE reported for a species with monads (3.4%). The bands of style head adhesive are functionally equivalent to the translators of milkweeds. The pollen to ovule ratio of A. cannabinum, at 19.8, is in the 94^th percentile of ratios reported for milkweeds (mean 9.6). Our results are consistent with the hypothesis that floral novelties of Apocynaceae that evolved prior to pollinia also promote aggregated pollen transport and elevated PTE.