Convergent evolution in calyptrate flowers of Syzygieae (Myrtaceae)

Shedding a fused perianth as a calyptra at anthesis is a trait that has evolved independently multiple times in angiosperm evolutionary history. However, calyptras do not correspond to homologous structures in all cases. Here, we describe calyptra development in the evolutionary context of Myrtaceae tribe Syzygieae. We use scanning electron and light microscopy to contrast calyptra development in calyptrate and non-calyptrate species in the group. Results show that calyptras in Syzygieae are not all homologous, but correspond to two ontogenetically distinct structures involving different perianth whorls that resemble each other by convergence: a calycine structure, in which the sepals are fused; and a pseudocalyptra, in which petals fall as a single unit but are not fused. Presence of non-calyptrate flowers is the ancestral state in the tribe, and both calyptra types appeared multiple times in the evolution of the group, with infrequent reversals from the calyptrate to the non-calyptrate state. Results highlight the fact that similar terminology in non-homologous structures can affect even relatively restricted lineages. The recurrent evolution of the calyptra in Syzygieae, with little evidence for reversal, shows that these structures may be advantageous in certain conditions but also correspond to an evolutionary dead-end in the group.