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

Background and Aims: The inflorescence of Philodendron constitutes an interestingmorphological model to analyse the phenomenon of homeosis quantitativelyat the floral level. The specific goals of this study were (1)to characterize and quantify the range of homeotic transformationin Philodendron billietiae, and (2) to test the hypothesis thatthe nature of flowers surrounding atypical bisexual flowers(ABFs) channel the morphological potentialities of atypicalbisexual flowers. Methods: Inflorescences of P. billietiae at different stages of developmentwere observed using SEM. The number of appendices in male, femaleand sterile flowers were counted on 11 young inflorescences(5–6 flowers per inflorescence). The number of staminodesand 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 staminodesper sterile male flower. There was an average of 7·3floral appendices per atypical flower. Staminodes and carpelsare inserted on the same whorl in ABFs. A negative exponentialrelationship was found between the average number of staminodesand the number of carpels in ABFs. If only the ABFs consistingof less than six carpels are considered, there is a linear relationshipbetween the number of carpels and the average number of staminodes.The value of the slope of the regression equation indicatesthat on average, in P. billietiae, 1·36 carpels are replacedby one staminode. Conclusions: In P. billietiae, the number of appendages in female flowersimposes a constraint on the maximum total number of appendages(carpels and staminodes) that can develop on ABFs. The quantitativeanalyses indicate that the average number of different typesof floral appendages on an ABF and the number of organs involvedin a homeotic transformation are two independent phenomena.