Growth & Development

Moraceae inflorescence development highly variable even for similar pollination modes

Transitional stages in fig relatives show increasingly enclosed flowers.

The family Moraceae produces a diverse range of inflorescence types, from simple to highly condensed and complex, as in the fig syconium, in which flowers are contained within an enclosed, urn-shaped receptacle that itself appears fruit-like. This range of inflorescences makes use of several different pollination modes, including wind and various insects such as flies, wasps, and midges.

In a new article published in Annals of Botany, lead author Viviane Gonçalves Leite and colleagues studied the inflorescence development of six different species of Moraceae, each from different genera. Their aim was to determines whether species with similar pollination modes also displayed the same patterns of inflorescence development. The team also attempted to pinpoint, from a phylogenetic perspective, key changes leading to the evolution of the syconium.

Schematic drawings summarizing information about three developmental stages of the
inflorescence, plotted in a recent cladogram of Moraceae. Source: Gonçalves Leite et al. 2021.

Ontogenetic study revealed that the inflorescence development of the six species was highly variable, even for more closely-related species, and that the variation arose quite early in development. Three of the species studied, Maclura tinctoria, Morus nigra and Clarisia ilicifolia, form an elongated structure that develops into a spike, globose head, or oval raceme. The other three, Brosimum gaudichaudii, Castilla elastica and Ficus pertusa, form inflorescences that are initially flat and discoid, but eventually become urceolate (urn-shaped) and envelop the flowers. Even for those with similar modes of pollination, no two species followed the same developmental sequence.

Ficus pertusa, which forms a syconium, is most closely related to Brosimum gaudichaudii, and Castilla elastica. The fact that all three first develop flat, open inflorescences, suggests that selective pressures, perhaps related to predation, favoured the enclosure of the flowers. Castilla elastica forms three different types of inflorescence – a discoid head enclosed by marginal bracts, a fig-like inflorescence with only an apical orifice as an opening, and a bivalvate inflorescence also enclosed by bracts. While these inflorescences aren’t completely enclosed as in figs, they seem to represent a transitional evolutionary stage.

“Such a great diversity of the inflorescence architecture remains a field of research to be explored,” write the authors. “The main gap is still the comprehensive understanding of the structure and function of these inflorescences in the family, which could be filled by studies that relate morphological and ecological aspects to inflorescence development.”

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