Abstract Detail

Development and Structure

Smith, Annika [1], Soltis, Douglas E. [1], Soltis, Pamela S. [1].

Receptacular or appendicular floral growth: a false dichotomy? A histo-ontogenetic analysis of garden nasturtiums (Tropaeolum) as a case study for an alternative interpretation.

There is a long history of disagreement about the ontogenetic identity of the nectar spur of Tropaeolum: is it derived from the floral axis (receptacle), or is it an appendage formed from the fusion of the perianth organs?  This disagreement is not merely semantic: it is rooted in fundamentally different interpretations of the growth of the floral meristem, and the relationship between morphology and vasculature. In addition, the same underlying disagreement extends beyond Tropaeolum to many other angiosperms with contested floral structures, such as many hypanthia and inferior ovaries. After briefly overviewing the history of the two competing interpretations of receptacular versus appendicular structures, we suggest that appendicular/receptacular is often a false dichotomy that does not accurately describe floral diversity. As an alternative, we demonstrate an approach that distinguishes how localized intercalary growth in distinct zones of the receptacle underlies diverse floral morphologies and anatomies, summarized in a schematic diagram of the floral axis, modified from Leins & Erbar (2010). We analyze the developmental morphology and growth of Tropaeolum flowers using contrast-enhanced nano-CT scans of flowers across major developmental stages, tracing the development of the nectar spur in the context of other floral organs. 3D models and 2D sections reveal both external developmental morphology and internal anatomy (e.g. vascular architecture, and heterogeneous tissues in the receptacle). Our results demonstrate: 1) no evidence of fusion between the perianth organs during the growth of the floral cup and subsequent development of the spur; 2) the formation of the floral cup and spur via cell divisions in receptacle cortex tissue below the insertion of perianth organs; and 3) a pattern of vascular architecture showing that the spur develops in a zoning of the receptacle cortex that is spatially separated from the perianth. Finally, for multiple species of Tropaeolum, we also quantify the zonal growth of the intercalary meristem within the cortex of the developing flowers in comparison to growth of both the receptacle pith and the other floral organs.  Given these results, we suggest that the spur of Tropaeolum is derived from a narrow zone of growth in the cortex of the receptacle. Furthermore, this interpretation is consistent with 1) comparative interpretations of vegetative cortex expansion, 2) what is known about the characteristics of the floral apical meristem, 3) existing evo-devo genetic data, and 4) the placement of Tropaeolum in the rosid clade, in which modifications in the size and shape of the receptacle are key components of floral diversity.

1 - University Of Florida, Florida Museum Of Natural History, 1659 Museum Rd., PO Box 117800, Gainesville, FL, 32611, United States

none specified

Presentation Type: Oral Paper
Number: DS1004
Abstract ID:978
Candidate for Awards:Katherine Esau Award,Maynard F. Moseley Award

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