A developmental model for the corolla in Rubiaceae. Cryptic character states in corollas of the Spermacoceae alliance

Abstract

Background and aims – In Rubiaceae, most flowers have a tubular corolla. Earlier studies showed that a tubular corolla may develop from an annular primordium or result from postgenital fusion of petals. Often stamen-corolla tubes are formed, concurring with epipetaly. Moreover, flowers with a fenestrated tubular corolla occur. The scattered distribution of species with corolla splits in Rubiaceae suggests a multiple origin. We intended to answer the following questions: (1) should the tubular corollas studied be assigned to a single or to several character states?; and (2) are the corolla splits studied homologous? We investigated the corolla development in five species of the Spermacoceae alliance with corolla splits in comparison with a Spermacoce species without. Methods – Floral developmental studies using scanning electron microscopy (SEM) and light microscopy (LM). Results – In Spermacoce, the tubular corolla with epipetalous stamens originates from an annular primordium. In Sacosperma and Pentas, the tubular corolla develops similarly with the stamens fixed basally. In Paederia and Pentodon, individual petal primordia originate on a ring wall. Next, stamen primordia appear adaxially on the ring wall. Then, the bases of the petals fuse partially, resulting in fenestrated tubular corollas. In Sacosperma and Pentas, corolla splits are actively formed in an initially closed tubular corolla. Conclusions – The tubular corollas of the species studied in Spermacoce, Sacosperma and Pentas are early sympetalous. Those in Paederia and Pentodon are late sympetalous. The tubular corolla results from a combination of three developmental processes: the formation of (1) a stamen-corolla tube, (2) a corolla tube sensu stricto, and (3) postgenital fusion of petals. The final form of the tubular corolla and the stamen insertion depend on the relative activity of these processes. Consequently, similar looking tubular corollas may constitute different character states. The corolla splits studied constitute two different cryptic character states in respectively Sacosperma / Pentas and Paederia/Pentodon .