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Development and Morphology of Flowers in Loranthaceae

Premise of research. Loranthaceae flowers exhibit exceedingly variable ground plan, size, and pollination syndromes. The homology of floral organs (especially the calyculus and the perianth), the evolutionary shifts from bisexual to unisexual flowers, and the trends in ovary and ovule reduction, primarily studied in Old World taxa, have been controversial. We investigate the development and morphoanatomy of early- and late-diverging Neotropical lineages with broad floral diversity to test organ homology and postulate floral plesiomorphies and apomorphies throughout the family. Methodology. We examined eight species from six genera, all native to the Colombian Andes. Standard LM and SEM methods were used. Pivotal results. All species studied exhibit bisexual flowers, but partial or total stamen sterilization was observed in the small-flowered Passovia pyrifolia and Peristethium archeri. All flowers examined possess an irregular calyx developed from a ring primordium, distinct from the four to seven petals initially free but postgenitally fused, and four to seven epipetalous stamens. Additionally, a cupular pedicel is present, either free from the calyx in Aetanthus and Psittacanthus or fused to the calyx in Passovia. Polysporangiate anthers were detected in Aetanthus mutisii; they dehisce prior to anthesis, which suggests cleistogamy. The ovary is six- or seven-chambered in Gaiadendron punctatum, but it is variously reduced or even solid in the remaining taxa. Conclusions. Plesiomorphies in the family include sessile, bisexual flowers lacking a cupular pedicel; an irregularly developed, distinct calyx; free, simultaneous petal primordia; versatile and tetrasporangiate anthers; and an ovary with six or seven locules and mamelons. The cupular pedicel is apomorphic in some Psittacanthinae genera. Other apomorphies include the basifixed, polysporangiate anthers with preanthetic dehiscence in A. mutisii. Unisexual flowers are homoplasious and linked to small-flowered taxa; anther sterilization occurs by either reduction in the number of sporangia or a proliferating endothecium. The modification of the ovary across Loranthaceae likely reflects pseudomonomery.