Is LEAFY a useful marker gene for the flower-inflorescence boundary in the Euphorbia cyathium?

Abstract

The flower-like reproductive structure of Euphorbia s.l. (Euphorbiaceae) is widely believed to have evolved from an inflorescence, and is therefore interpreted as a special type of pseudanthium, termed a cyathium. However, fuzzy morphological boundaries between the inflorescence, individual flowers, and organs have fuelled the suggestion that the cyathium does not merely superficially resemble a flower but could actually share developmental genetic pathways with a conventional flower. To test this hypothesis, immunolocalizations of FLORICAULA/LEAFY (LFY), a protein associated with floral identity in many angiosperm species, were performed in developing cyathia of different species of Euphorbia. Expression of the LFY protein was found not only in individual floral primordia (as predicted from results in the model organisms Arabidopsis and Anthirrhinum), but also in the cyathium primordium and in the primordia of partial male inflorescences. These results provide further evidence that the evolution of floral traits in pseudanthial inflorescences often involves expression of floral development genes in the inflorescence apex. This finding blurs the conventional rigid distinction between flowers and inflorescences.

Fig. 1.

Immunolocalization of LFY protein in different ontogenetic stages of the cyathium. (A, D, H) SEM images; all others longitudinal sections (LS). (A) Euphorbia myrsinites, SEM image of a young cyathium. Cyathium primordia in red, bracteoles and male inflorescence subtending bracts in green, male partial inflorescences in yellow. The dotted line shows the approximate position of the sections in (B) and (C). (B) Euphorbia milli. The cyathium primordium is subtended by a bract and shows strong expression of LFY protein. (C) Euphorbia tithymaloides, young cyathium showing expression in the male inflorescence primordia (asterisks). (D) Euphorbia myrsinites, SEM images of a somewhat older cyathium. Colours are the same as in (A). (E) Euphorbia milli, young cyathium showing expression of LFY protein in male flower primordia (arrowheads) and in a cyathial primordium (encircled). (F) Euphorbia myrsinites, young cyathium showing expression of LFY protein in young male flowers and in the gynoecium. (G) Euphorbia myrsinites, older cyathium with LFY expression in young male flowers and young gynoecium. (H) Euphorbia pteroneura, older cyathium, bracteoles removed, cyathium dissected to show the central gynoecium (i.e. naked and stalked female flower) flanked by two bundles of stamens (i.e. male flowers). The gynoecium is dissected to show two young ovules (note that the integuments are just starting to develop). At the rim of the involucre one nectary is visible. Colours are the same as in (A). (I) Euphorbia nicaeensis, similar developmental stage to that in (H), LFY expression in male flowers, ovules, nectary, and cyathial primordium (arrowhead). Bl, bracteoles; i, involucre; G, gynoecium/female flower; n, nectary; o, ovule; p, cyathial primordium; s, male flower/stamen. Scale bars, 100 μm in A, B; 200 μm in C–F; 500 μm in G–I.