Reproductive character displacement and potential underlying drivers in a species-rich and florally diverse lineage of tropical angiosperms (Ruellia; Acanthaceae).

Erin A Tripp,Heather B Stone,Kyle G Dexter

doi:10.1002/ece3.7371

Abstract

Reproductive character displacement is a pattern whereby sympatric lineages diverge more in reproductive character morphology than allopatric lineages. This pattern has been observed in many plant species, but comparably few have sought to disentangle underlying mechanisms. Here, in a diverse lineage of Neotropical plants (Ruellia; Acanthaceae), we present evidence of reproductive character displacement in a macroevolutionary framework (i.e., among species) and document mechanistic underpinnings. In a series of interspecific hand pollinations in a controlled glasshouse environment, we found that crosses between species that differed more in overall flower size, particularly in style length, were significantly less likely to produce viable seeds. Further, species pairs that failed to set seed were more likely to have sympatric distributions in nature. Competition for pollinators and reinforcement to avoid costly interspecific mating could both result in these patterns and are not mutually exclusive processes. Our results add to growing evidence that reproductive character displacement contributes to exceptional floral diversity of angiosperms.

Highlights

Floral diversity could have arisen via random drift in floral characters over time (Freckleton et al, 2002; Revell et al, 2008), with floral diversity being higher by chance in older and more diverse clades because they have had greater evolutionary history over which to accumulate floral differences
Competition for pollinators and reinforcement are by no means mutually exclusive, and distinguishing their relative importance as drivers of reproductive character displacement (RCD) remains difficult (Armbruster & Muchhala, 2009; Castillo, 2017; Hopkins, 2013), despite the importance of understanding mechanisms that drive plant species divergence and floral diversification
We examine a species-rich and florally diverse lineage of tropical angiosperms to (a) test for RCD between species pairs, which would indicate a role for community-level processes in driving angiosperm floral diversification, and (b) evaluate evidence in support of two different mechanisms that contribute to RCD: competition for pollinators and reinforcement

Summary

| INTRODUCTION

“Endless forms most beautiful” have motivated biologists for centuries (Carroll, 2005; Darwin, 1859), and the remarkable floral diversity of angiosperms is one prime example. Competition for pollinators and reinforcement are by no means mutually exclusive, and distinguishing their relative importance as drivers of RCD remains difficult (Armbruster & Muchhala, 2009; Castillo, 2017; Hopkins, 2013), despite the importance of understanding mechanisms that drive plant species divergence and floral diversification. We examine a species-rich and florally diverse lineage of tropical angiosperms to (a) test for RCD between species pairs, which would indicate a role for community-level processes in driving angiosperm floral diversification, and (b) evaluate evidence in support of two different mechanisms that contribute to RCD: competition for pollinators and reinforcement. The results from this study have implications for understanding the relative contribution of community-level processes and RCD to floral diversification, especially given few examples are known from the tropics (but see Kay & Schemske, 2008; Muchhala et al, 2014), and serve as steps toward disentangling the underlying drivers of RCD

| MATERIALS AND METHODS

| Molecular methods

Findings

| DISCUSSION