Floral developmental morphology of Amborella trichopoda (Amborellaceae)

We investigated the early floral development of the monotypic genus Amborella, the sister to all other extant angiosperms. Examination of vegetative shoot development revealed that Amborella possesses both decussate and alternate phyllotaxy; one may simply be a special case of the other as a reaction to meristem size and shape. The transition from bracts to tepals is gradual, making it difficult to determine exactly where a flower begins in this species. Although flowers of Amborella are described as having spiral phyllotaxy, the periphery of the flower could be considered unidirectionally whorled. This new observation, together with observations of both spiral and whorled phyllotaxy in other basal angiosperms ( e. g., Nuphar, Drimys, Ceratophyllum), further demonstrates the flexibility of floral development in basal angiosperms; i.e., some basal angiosperms are not fully committed to either spiral or whorled phyllotaxy. The developmental transitions between bracts and perianth, and between stamens and carpels, are continuous. However, the transition between perianth and sporophylls is more distinct, although tepals, stamens, and carpels appear to be structurally homologous. Both stamens and carpels share characteristics of the other sexual organ. We hypothesize that genes correlated with stamen identity (e.g., B-class MADS- box genes) may show a gradual change in expression pattern throughout the floral parts of Amborella and perhaps other basal angiosperms, corresponding to the observed gradual morphological transitions. This fading borders'' model of floral gene expression should be evaluated in Amborella and other basal angiosperms.