Developmental analysis of merosity and sexual morphs in Rubiaceae: A case study in Rubia and Cruciata

• Overlapping initiation of petal and stamen whorls in Cruciata is remarkable.
• The results support the role of space on the evolution of merosity.
• Meristem size and relative organ size play important role in genesis of merosity.
• The gynoecium gets relatively late inhibited in male flowers of Cruciata.

The Rubiaceae is characterized by considerable diversity in floral features. To shed light on the variation of floral developmental patterns in Rubiaceae, a comparative study was carried out on pentamerous flowers of Rubia tinctorum L. and tetramerous flowers of Cruciata (Cruciata laevipes Opiz.; Cruciata glabra (L.) Ehrend.), two herbaceous genera of the tribe Rubieae. We aim to study the mechanism responsible for meristic variation and to discuss the expression of andromonoecy in C. laevipes. Developmental peculiarities include the time overlap in the initiation of petal and stamen whorls in Cruciata, the lack of calyx, and a very weak sympetaly in all species examined. Our results show that changes in initial meristem size can play a significant role in meristic variation. The late expression of gender differentiation in C. laevipes may be an explanation for the plasticity of andromonoecy in response to the environmental changes.