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

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 L., 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 a noto-sternotribic 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 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.

Heteromorphic stamens are differentially attractive in Swartzia (Fabaceae)

“…However, this type of dehiscence is not mandatory for buzz pollination ( Buchmann 1985 ), as noted in Swartzia . Modified configurations of longitudinal dehiscence or small basal and apical openings of the dehiscence line, such as a shortening of the anther slit, can be very close to the poricidal anthers ( Endress 1996 ; Saab et al 2021 ). In addition to restricting the release of pollen, other strategies can favour the saving of pollen by the flower in Swartzia .…”

“… Peach and Mazer 2019 ; Telles et al 2020 ; Dellinger et al 2021 ), it is a hypothesis that has wide support in the literature (e.g. Paulino et al 2016 ; Solís-Montero et al 2018 ; Velloso et al 2018 ; Saab et al 2021 ; Valadão-Mendes et al 2022 ). Thus, based on the hypothesis of ‘division of labour’ between the stamens, it could be assumed that there is a difference in the attractiveness between the distinct stamens, in which some stamens would be more attractive ( feeding stamens ) and others would be more cryptic ( pollination stamens ) (e.g.…”

“…Thus, based on the hypothesis of ‘division of labour’ between the stamens, it could be assumed that there is a difference in the attractiveness between the distinct stamens, in which some stamens would be more attractive ( feeding stamens ) and others would be more cryptic ( pollination stamens ) (e.g. Solís-Montero et al 2018 ; Velloso et al 2018 ; Saab et al 2021 ).…”

“…The differential attractiveness of stamen whorls has until now been poorly studied. However, there are differences in visual perceptibility between stamens ( Velloso et al 2018 ; Telles et al 2020 ; Saab et al 2021 ). In the same way, variations in the pattern of volatiles emitted by stamens were detected in some cases ( Solís-Montero et al 2018 ).…”

VISITANTES FLORAIS DE Cassia fistula L. (FABACEAE, CAESALPINIOIDEAE) EM ÁREA URBANA NO MUNICÍPIO DE ALTA FLORESTA-MT

“…As flores, apesar de não ofertarem néctar (SAAB et al, 2021), dispõe de uma grande quantidade de pólen para os seus visitantes (PINTO et al, 2020), principalmente para as abelhas (AGUIAR; SANTOS, 2017). Entretanto, devido a deiscência poricida da antera, não são todos visitantes que conseguem liberar o pólen das flores, pois é necessária a vibração das anteras (buzz pollination) (SILVA et al, 2014).…”