A sophisticated case of division of labour in the trimorphic stamens of the Cassia fistula (Leguminosae) flower

Author(s)

Saab, Gabriella da Silva ^[1] ; Mansano, Vidal de Freitas ^[2] ; Nogueira, Anselmo ^[3] ; Maia, Isabele Carvalho ^[3] ; Bergamo, Pedro Joaquim ^{[2, 4]} ; Paulino, Juliana Villela ^[1]

Affiliation

^[1] Univ Fed Rio de Janeiro UFRJ, Fac Farm, Ctr Ciencias Saude, Dept Prod Nat & Alimentos, BR-21941902 Rio De Janeiro, RJ - Brazil

^[2] DIPEQ, Inst Pesquisas Jardim Bot Rio de Janeiro, Rua Pacheco Leao 915, BR-22460030 Rio De Janeiro, RJ - Brazil

^[3] Univ Fed ABC, Ctr Ciencias Nat & Humanas, BR-09606045 Sao Bernardo Do Campo, SP - Brazil

^[4] Univ Estadual Campinas, Inst Biol, Dept Biol Vegetal, UNICAMP, Programa Posgrad Ecol, BR-13083862 Campinas, SP - Brazil

• Document type: Journal article
• Source: AOB PLANTS; v. 13, n. 5 SEP 6 2021.
• Web of Science Citations: 0

Abstract

Buzz-pollinated pollen flowers have pollen as the primary resource for pollinators and must deal with a conflict between the exploitation of pollen grains by bees and pollination success. It has been hypothesized that heterostemony allows division of labour between stamens as a solution to the pollen dilemma. To test the division of labour hypothesis, we chose Cassia fistula, which has a trimorphic androecium and analysed androecium development, pollen grain release mechanisms and visitor behaviour. We explored the reflectance of floral organs and carried out an exclusion experiment to test the attractiveness of each stamen morph to the bee species. Finally, we explored the structural, ultrastructural and functional variation between the pollen grains, including pollen viability across stamen morphs. The differences among the three stamen morphs, which is developed from two whorls of the stamen, are the first evidence of the division of labour in our study system. Large Bombus and Xylocopa bees actively and exclusively exploited the pollen grains from the central poricidal anthers generating pollen deposition on their bodies. The reflectance pattern of floral organs indicated a targeting of these large bees to the central anthers, corroborated by the anther manipulative experiment where only the exclusion of the anthers positioned in the flower centre, especially the intermediate stamens, reduced bee visits. Both results revealed a division of labour, in which the intermediate stamen morph was responsible for both floral attractiveness and pollen resources. Only the largest stamen morph produced germinable pollen grains, highlighting their role as pollinating stamens. The smallest stamen morph has a less clear function, likely representing an economy in pollen production for feeding function. Our findings suggest that the evolution of the trimorphic androecium is associated with division of labour in large pollen flowers and can represent a strong strategy for circumventing the pollen dilemma, optimizing the feeding function by reducing pollen grain investment from central anthers. (AU)

• FAPESP's process: 19/19544-7 - Synergistic effect of multiple mutualists on plants: how bacteria, ants and bees contribute to the evolution of a hyper-diverse lineage of legumes
• Grantee: Anselmo Nogueira
• Support Opportunities: BIOTA-FAPESP Program - Young Investigators Grants
• FAPESP's process: 16/06434-0 - Plant-pollinator interactions in communities and the assembly of functional traits and of the reproductive success
• Grantee: Pedro Joaquim Bergamo
• Support Opportunities: Scholarships in Brazil - Doctorate