Abstract
Petals, the inner organs in a differentiated perianth, generally play an important role in pollinator attraction. As such they exhibit an extraordinary diversity of shapes, sizes, and colors. Being involved in pollinator attraction and reward, they are privileged targets of evolution. The corolla of the Ranunculaceae species Nigella damascena consists of elaborate nectariferous petals, made of a stalk, upper, and lower lips forming a nectar pouch, shiny pseudonectaries, and pilose ears. While the main events of petal development are properly described, a few is known about the pattern of organ size and shape covariation and the cellular dynamics during development. In this study, we investigated the relationships between morphogenesis and growth of N. damascena petals using geometric morphometrics coupled with the study of cell characteristics. First, we found that petal shape and size dynamics are allometric during development and that their covariation suggests that petal shape change dynamics are exponentially slower than growth. We then found that cell proliferation is the major driver of shape patterning during development, while petal size dynamics are mostly driven by cell expansion. Our analyses provide a quantitative basis to characterize the relationships between shape, size, and cell characteristics during the development of an elaborate floral structure. Such studies lay the ground for future evo-devo investigations of the large morphological diversity observed in nectariferous structures, in Ranunculaceae and beyond.
Highlights
In the flowers of most angiosperms, sterile structures, collectively called the perianth, surround fertile organs and are organized in two functionally and morphologically differentiated organ types
Focusing on the petal dorsal view, corresponding to the lower lip of the nectar pouch, we found that cell proliferation is prevalent in the first stages, which correspond to the period when most of the mature shapes are achieved
Complex petal shape elaborations have been qualitatively described in many taxa
Summary
In the flowers of most angiosperms, sterile structures, collectively called the perianth, surround fertile organs and are organized in two functionally and morphologically differentiated organ types. It has been shown that petal complexity is progressively acquired during development, as different elaborated features appear sequentially, giving rise to a mature petal that comprises a stalk, upper, and lower lips, a nectar pouch, lobes, pseudonectaries (Liao et al, 2020), trichomes, and color patterns. Some of these features are present in all Nigella species, others are shared by few species only (Yao et al, 2019). The precise quantification of petal shape transformation, as well as the covariation of petal shape with petal growth, during development, is still lacking
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