Patterns and consequences of floral formula variation in Phlox (Polemoniaceae)

Abstract

The numbers of organs produced in each of the floral whorls often follow highly consistent and predictable patterns within species, genera, and often higher taxonomic groups. The floral formula is assumed to exhibit low variability due to historically strong selection pressures on suites of integrated reproductive traits. This dissertation examines natural levels and patterns of floral formula variation, the reproductive consequences of formula variation, and the selection potential for a new floral formula. When analyzed for floral organ number variation, most wild Phlox longifolia and greenhouse-grown Phlox drummondii plants were abnormal for at least one flower, with abnormal organ numbers occurring least often in P. longifolia gynoecia and most often in P. drummondii corollas. The gynoecium was the most independent whorl, whereas organ numbers in the other whorls were correlated. Direction of abnormality was largely subnumerary in all whorls of P. longifolia and supernumerary in P. drummondii. Abnormal whorls were often coordinated for direction of variation and for equal organ numbers. Abnormalities increased in frequency over time in P. longifolia and decreased in P. drummondii. Phlox drummondii abnormalities were more frequent among low order branch positions than higher orders. Early floral development in Phlox may be unstable, becoming more stable over time in the absence of environmental stress. Organ number variation within the reproductive whorls significantly affected reproductive output. Stamen and carpel number were positively correlated with pollen and seed number, respectively. Gynoecia with more carpels yielded more seeds per fruit and greater total seed mass per fruit than gynoecia with fewer carpels, suggesting plants producing more carpels per flower would yield greater lifetime seed volume. One round of upward and stabilizing selection on mean carpel number in Phlox drummondii did not result in carpel number divergence but did indirectly affect the frequency of abnormality in the corolla and the whole floral formula, suggesting that selection on organ number within a specific floral whorl may act to alter the sensitivity of stable organ number expression, thus increasing overall instability. Overall, this dissertation represents (1) the first formal documentation of floral formula variation in Phlox longifolia, (2) the first extensive analysis of patterns of variation with respect to degree and direction in all four whorls simultaneously, (3) the first evidence of effect of organ number variation on reproductive output in Phlox, and (4) the first attempt at carpel number selection in Phlox.