Floral morphology in Nicotiana:: Architectural and temporal effects on phenotypic integration

Flowers are phenotypically integrated units that may require a precise configuration of floral organs for proper function. Plants, however, are metameric organisms that often exhibit architectural and temporal variation in floral size and shape. Clearly, architectural and temporal effects could interrupt phenotypic integration within flowers, yet such effects have not been explicitly examined. We compared correlations among floral characters for the closely related but morphologically divergent species Nicotiana alata and Nicotiana forgetiana and their artificial F3 hybrids. Principal components analysis of 16 floral traits identified two suites of correlated characters that are maintained in both taxa and in the hybrids, regardless of architectural and temporal variation in floral characters. We interpret the maintenance of these suites as evidence of common developmental regulation of correlated characters. One suite includes measures of corolla tube, gynoecium, and androecium length, while the other includes corolla limb and corolla tube aperture characters. The correlated suites of characters are tightly integrated, independent from characters of the other factor, and they participate in a common function. As such, these character suites represent phenotypic modules that have been maintained despite evolutionary changes in morphology during the divergence of the two species.