Beyond taxonomy: anther skirt is a diagnostic character that provides specialized noctuid pollination in Marsdenia megalantha (Asclepiadoideae–Apocynaceae)

Taxonomic diagnostic floral structures can be very informative about details of the floral mechanism, promoting baselines for inferences about pollination system. In a number of species in the milkweed tribe Marsdenieae, the already complex basic asclepiad flower structure is further elaborated by the evolution of one morphological novelty: the anther skirt. Since the functional significance of this trait is poorly understood, we investigate its role in the pollination mechanism on Marsdenia megalantha. We tested the phenotypic integration of anther skirt; we described its functional anatomy; we identified and recorded the function of its nectaries; we described and tested in laboratory its contribution to the pollination mechanism; and we studied the pollination effectiveness of this system. The characteristics of an anther skirt are essential in the noctuid pollination of M. megalantha. This is due to its particular morphological and anatomical features related to its ability both to secrete at night and to contain nectar independently from the stigmatic chamber. The skirt also functions coordinately with the corona as a funnel that directs the noctuid legs to gynostegium. Its effectiveness in producing pollination was confirmed by field observations and manipulative experiments. We suggest that nectar offers at night and mechanical adjustment may be applicable to other representatives of the tribe as a feature that determines noctuid pollination, assigning a functional value for the anther skirt beyond the taxonomy.