Androecial evolution in Caryophyllales in light of a paraphyletic Molluginaceae.

PREMISE OF THE STUDY: Caryophyllales are highly diverse in the structure of the perianth and androecium and show a mode of floral development unique in eudicots, reflecting the continuous interplay of gynoecium and perianth and their influence on position, number, and identity of the androecial whorls. The floral development of five species from four genera of a paraphyletic Molluginaceae (Limeum, Hypertelis, Glinus, Corbichonia), representing three distinct evolutionary lineages, was investigated to interpret the evolution of the androecium across Caryophyllales. •
METHODS: Floral buds were dissected, critical-point dried and imaged with SEM. The genera studied are good representatives of the diversity of development of stamens and staminodial petaloids in Caryophyllales. • KEY
RESULTS: Sepals show evidence of petaloid differentiation via marginal hyaline expansion. Corbichonia, Glinus, and Limeum also show perianth differentiation via sterilization of outer stamen tiers. In all four genera, stamens initiate with the carpels and develop centrifugally, but subsequently variation is significant. With the exception of Limeum, the upper whorl is complete and alternisepalous, while a second antesepalous whorl arises more or less sequentially, starting opposite the inner sepals. Loss or sterilization of antesepalous stamens occurs in Glinus and Limeum and is caused by altered carpel merism and inhibition by sepal pressures. •
CONCLUSIONS: Outer stamens of Hypertelis correspond with petaloids of Caryophyllaceae and suggest that staminodial petaloids and outer alternisepalous stamens are interchangeable in the Caryophyllales. We emphasize a switch in the position of first formed stamens from antesepalous to alternisepalous following the divergence of Limeum; thus stamen position is an important synapomorphy for the globular inclusion clade.