Quantitative developmental analysis of homeotic changes in the inflorescence of Philodendron (Araceae).

BACKGROUND AND AIMS: The inflorescence of Philodendron constitutes an interesting morphological model to analyse the phenomenon of homeosis quantitatively at the floral level. The specific goals of this study were (1) to characterize and quantify the range of homeotic transformation in Philodendron billietiae, and (2) to test the hypothesis that the nature of flowers surrounding atypical bisexual flowers (ABFs) channel the morphological potentialities of atypical bisexual flowers. METHODS: Inflorescences of P. billietiae at different stages of development were observed using SEM. The number of appendices in male, female and sterile flowers were counted on 11 young inflorescences (5-6 flowers per inflorescence). The number of staminodes and carpels on ABFs were counted on 19 inflorescences (n = 143). These data were used for regression and ANOVA analyses. RESULTS: There was an average of 4.1 stamens per male flower, 9.8 carpels per female flower and 6.8 staminodes per sterile male flower. There was an average of 7.3 floral appendices per atypical flower. Staminodes and carpels are inserted on the same whorl in ABFs. A negative exponential relationship was found between the average number of staminodes and the number of carpels in ABFs. If only the ABFs consisting of less than six carpels are considered, there is a linear relationship between the number of carpels and the average number of staminodes. The value of the slope of the regression equation indicates that on average, in P. billietiae, 1.36 carpels are replaced by one staminode. CONCLUSIONS: In P. billietiae, the number of appendages in female flowers imposes a constraint on the maximum total number of appendages (carpels and staminodes) that can develop on ABFs. The quantitative analyses indicate that the average number of different types of floral appendages on an ABF and the number of organs involved in a homeotic transformation are two independent phenomena.

Study of homeosis in the flower of Philodendron (araceae): a qualitative and quantitative approach.

This study deals specifically with floral organogenesis and the development of the inflorescence of Philodendron squamiferum and P. pedatum. Pistillate flowers are initiated on the lower portion of the inflorescence and staminate flowers are initiated on the distal portion. An intermediate zone consisting of sterile male flowers and atypical bisexual flowers with fused or free carpels and staminodes is also present. This zone is located between the sterile male and female floral zones. In general, the portion of bisexual flowers facing the male zone forms staminodes, and the portion facing the female zone develops an incomplete gynoecium with few carpels. The incomplete separation of some staminodes from the gynoecial portion of the whorl shows that they belong to the same whorl as the carpels. There are two levels of aberrant floral structures in Philodendron: The first one is represented by the presence of atypical bisexual flowers, which are intermediates between typical female flowers and typical sterile male flowers. The second one is the presence of intermediate structures between typical carpels and typical staminodes on a single atypical bisexual flower. The atypical bisexual flowers of P. squamiferum and P. pedatum are believed to be a case of homeosis where carpels have been replaced by sterile stamens on the same whorl. A quantitative analysis indicates that in both species, on average, one staminode replaces one carpel.