FLORAL PAEDOMORPHY LEADS TO SECONDARY SPECIALIZATION IN POLLINATION OF MADAGASCARDALECHAMPIA(EUPHORBIACEAE)

Abstract

The traditional evolutionary interpretation of Von Baer's "laws" of embryology is that retention of early developmental forms into adulthood (paedomorphosis) leads to the evolution of simpler or more generalized morphology and ecology. Here we show that paedomorphosis can also be involved in an increase in ecological specialization, in this case of plant-pollinator relationships. A paedomorphic transition from generalized pollination (by several functional types of pollinators) to specialized pollination (by one or a few species in one functional type) occurred in a clade of endemic Madagascar vines (Dalechampia spp., Euphorbiaceae). This evolutionary transition involved staminate flowers that fail to develop "normally," instead holding mature pollen inside virtually unopened, bud-like flowers. This paedomorphic morphology restricts reward access to "buzz-pollinating" bees, including Xylocopa species (carpenter bees), which can remove pollen by sonication. This is one of very few reports of paedomorphic specialization, and, as far as we are aware, the first documented case of a rapid reversal to specialized pollination in a lineage of plants that had previously switched from specialized to generalized pollination in conjunction with dispersing to a new region. K E Y W O R D S :Buzz-pollination, flower development, heterochrony, neoteny.

Molecular Phylogeny and Pollen Evolution of Euphorbiaceae Tribe Plukenetieae

“…Although our taxon sampling of Dalechampiinae is sparse (4 of ca. 130 species) and geographically limited to Madagascar, prior studies with much greater taxon sampling indicate its monophyly (Armbruster et al 2009(Armbruster et al , 2013, as does its unique pseudanthial inflorescence. The strongly supported relationship of Dalechampia (Dalechampiinae) + Astrococcus (Plukenetiinae) embedded within the tribe ( Fig.…”

“…In our study, sampling of Dalechampiinae was limited to four species of Madagascan Dalechampia, based on accessible material. We justify using only a few specimens given that the broader phylogeny of Dalechampia is already well known (Armbruster et al 2009(Armbruster et al , 2013 and because the monophyly of the genus is strongly supported by its pseudanthial inflorescence synapomorphy. Five species of Bernardia Houst.…”

“…ex L., known for its unique pseudanthial inflorescence and specialized pollination biology (Webster and Webster 1972;Webster and Armbruster 1991). Broader phylogenetic relationships in the tribe are poorly known outside of Dalechampia (Armbruster et al 2009(Armbruster et al , 2013, and morphological, palynological, and molecular evidence suggests some genera are paraphyletic (Gillespie 1994a;Wurdack et al 2005). Our paper presents the first comprehensively sampled molecular phylogeny of Plukenetieae and elucidates the generic relationships of its taxa and patterns of pollen morphology evolution.…”

“…Molecular phylogenetic analyses have also been conducted on Dalechampia, with a focus on evolutionary and ecological questions (Armbruster and Baldwin 1998;Armbruster et al 2009Armbruster et al , 2013. The most comprehensive phylogeny (Armbruster et al 2009(Armbruster et al , 2013 recovered strong support for an early division in Dalechampia, resulting in two major lineages defined by the number of cymule branches in the male subinflorescence (four-vs. five-armed).…”

“…The most comprehensive phylogeny (Armbruster et al 2009(Armbruster et al , 2013 recovered strong support for an early division in Dalechampia, resulting in two major lineages defined by the number of cymule branches in the male subinflorescence (four-vs. five-armed). Species relationships were mostly well resolved within each lineage, although their taxonomic significance or concordance with the sectional classification were not discussed.…”