Environmental control of sepalness and petalness in waterilies

Abstract

Existing models of perianth evolution are inadequate to explain morphological variation in tepals of early-divergent angiosperms as conventional concepts regarding the perianth include an unstated assumption that the characteristics of sepalness (possessing sepal-like characters) and petalness (possessing petal-like characters) must be fixed to entire individual organs. However, initial observations found that in Nuphar Smith, Nymphaea L. (Nymphaeaceae) and Schisandra Michx. (Schisandraceae), sepaloid (green) and petaloid (colourful) patches can both occur on the same individual perianth organs. Furthermore, in Nuphar and Nymphaea the region of tepal that is exposed when the flower is in bud ultimately becomes sepal-like, whereas nearly all of the covered tissue becomes petaloid. The positions of these areas on the tepals suggest that the environment determines these developmental fates. In order to test this hypothesis that I employed three different (interlinked) types of research: (1) Morphological study of normal buds found that in Nuphar lutea and Nymphaea caerulea, green and colourful areas on the perianth differ from each other in features that often distinguish sepals and petals. (2) Field experiments found that the development of sepaloid patches in the perianth of these species is initiated by exposure. The results indicate that although light is an important environmental cue, other factors also appear to be involved, such as absent of surface contact. (3) RNA insitu- hybridisation to ascertain whether the expression zones of B-genes in maturing tepals correspond to the petaloid and not the sepaloid regions, was unsuccessful due to high background staining. However, this study identified possibly causes and solutions to aid future in-situ studies. Preliminary observations on other ANA-grade angiosperms made during this study (Illiciaceae and possible Amborella), suggests sepal-petal differentiation within individual perianth organs was the ancestral condition of the perianth, and supports a novel theory on perianth evolution called the Mosaic theory. In this theory sepalness and petalness evolved in early angiosperms but were not fixed to particular organs and were primarily environmentally controlled. At a later stage in angiosperm evolution, sepaloid and petaloid characteristics became fixed to whole organs in specific whorls, thus reducing or removing the need for environmental control in favour of fixed developmental control.