Stamen construction, development and evolution in Salvia s.l.

Abstract

Lever-like stamens are among the most important floral traits in Salvia s.l. Their morphology, function and diversity were repeatedly studied to understand the lever mechanism and to reconstruct stamen evolution. The present paper reviews current knowledge on stamen diversity in Salvia species and combines morphogenetic and phylogenetic data. Five stamen types are distinguished based on absence vs. presence of versatile anthers, bithecate vs. monothecate anthers, curved vs. straight connective growth and presence vs. absence of the lower lever arm. Type I (no versatile anthers) is restricted to the four species of the Californian sect. Echinosphace. Versatile anthers evolved at least five times: Type II (bithectae) is the most common one in all old world Salvia s.l. clades, but also appears twice in America. Type III (monothectae, curved connective) is derived from type II (ontogenetic abbreviation). It is not restricted to the Eurasian-African clade I (Salvia s. str.) and the E-Asian clade IV-A, but also underlies some bee-pollinated American species. The new function of the lower lever arm to serve as a sterile barrier goes along with novel growth centres (ontogenetic addition). Type IV (monothecate, straight connective) characterises the majority of the American species (subgen. Calosphace). Type V (no lower lever arm) is derived by ontogenetic abbreviation and most likely homoplastic. It evolved at least four times independently, i.e. twice in Salvia s. str., once in the Californian clade II-B (sect. Audibertia) and once or several times in the East Asian clade. The study shows that stamen diversity results from few developmental changes. The latter appear in different combinations and support subclades rather than clades.