AUTHOR=Thaowetsuwan Pakkapol , Ritchie Stuart , Riina Ricarda , Ronse De Craene Louis 

TITLE=Divergent Developmental Pathways Among Staminate and Pistillate Flowers of Some Unusual Croton (Euphorbiaceae)

JOURNAL=Frontiers in Ecology and Evolution

VOLUME=8

YEAR=2020

URL=https://www.frontiersin.org/journals/ecology-and-evolution/articles/10.3389/fevo.2020.00253

DOI=10.3389/fevo.2020.00253

ISSN=2296-701X

ABSTRACT=<p><italic>Croton</italic> is a mega-diverse genus of more than 1,200 species with great morphological diversity and highly dimorphic flowers. Staminate flowers generally possess petals and a variable number of stamens and lack of an ovary. Pistillate flowers generally lack petals or have filamentous structures instead; stamens are lacking, and the ovary is generally tricarpellate with divided styles. However, well-developed petals can be found in pistillate flowers of some African species and two New World sections, i.e., sect. <italic>Alabamenses</italic> and sect. <italic>Eluteria</italic> subsect. <italic>Eluteria</italic>. Our objectives are to compare ontogeny in dimorphic flowers of <italic>Croton</italic> which may elucidate the origin of petals, homology of the filamentous structures, nectaries, and diversity of the androecium. The development of staminate and pistillate flowers of <italic>C. alabamensis</italic> (sect. <italic>Alabamenses</italic>) and <italic>C. schiedeanus</italic> (sect. <italic>Eluteria</italic>) was studied under the scanning electron microscope (SEM) and compared with <italic>C. chilensis</italic> (sect. <italic>Adenophylli</italic>) which has filamentous structures in pistillate flowers. In staminate flowers, petals develop in alternation with sepals and later the outermost stamen whorl develops opposite to the petals. In a much later stage, nectar glands emerge in alternation with the petals. In pistillate flowers, filamentous structures and petals share the same early development in alternation with the sepals. However, growth of the filamentous structures of <italic>C. chilensis</italic> becomes arrested, while petals of <italic>C. alabamensis</italic> and <italic>C. schiedeanus</italic> develop similar to those of the staminate flowers. The main conclusions of the study are: (1) Petals and filamentous structures are homologous based on their location and shape in early development. (2) Nectaries with variable morphology develop in antesepalous position and probably represent receptacular outgrowths and are not staminodial. (3) Staminate flower development displays an unexpected diversity, including a floral cup in <italic>C. alabamensis</italic>. All investigated species have an unusual centrifugal initiation of the second stamen whorl. (4) The possible evolutionary loss and potential regain of petals among different species of <italic>Croton</italic> is discussed in relation to heterochrony. We suggest that the filamentous structures in pistillate flowers represent pedomorphic forms of petals of staminate flowers, and that the well-developed petals in <italic>C. alabamensis</italic> and <italic>C. schiedeanus</italic> are derived via a developmental reversion.</p>