‘False resupination’ in the flower-pairs of Thalia (Marantaceae)

• Flower-pairs in Thalia geniculata conduct a lateral rotation of 90° before anthesis.
• Those of Thalia dealbata bend backwards by 90° during development.
• Manipulation and gravity experiments show that movements are genetically fixed.
• Spatial constraints play a decisive role in floral development and movements.
• Resupination processes in angiosperms are shortly reviewed.

Resupination, i.e., the turn by 180°, arranges zygomorphic flowers in a suitable position for pollination. It is well-known from orchids, but also occurs in further angiosperm families. Within the genus Thalia L. (Marantaceae), T. dealbata has erect and T. geniculata pendulous inflorescences. Nevertheless, in both species the zygomorphic flower-pairs show the right position for pollination. Thus, resupination is expected in Marantaceae. To reconstruct spatial arrangement, flower development was investigated from meristem formation to anthesis. Early morphogenetic stages were analysed under the scanning electron microscope, late stages were documented by time-lapse records. Gravity experiments were added to test for plastic or genetically fixed flower-pair position. Flowers in both Thalia species develop almost identically. They originate from a common primordium and show the unusual floral organ sequence already described for the family. In T. dealbata, the flower-pair gets its final position by bending 90° backwards, while in T. geniculata a lateral rotation of 90° takes place shortly before anthesis. In both species, position is genetically fixed. Results clearly indicate that the flower-pairs in Thalia do not show resupination according to the given definition. Instead of the expected 180° turn in one species, flower-pairs in both species each dislocate for 90°.