Morpho-anatomy of the flower-to-fruit transition in selected Andean Loranthaceae

Abstract

ABSTRACT: Loranthaceae (Santalales) is a family of hemiparasitic plants that exhibits unique structural modifications, such as: (1) structural reduction of the carpels, compensated by the formation of a massive hypanthium and (2) ategmic ovules, these reduced to the embryo sac; (3) migration of the embryo sac nuclei from the ovary to the style and ectopic fertilization in the style, followed by the repositioning of the proembryo(s) to the ovarian region; and (4) fruits developing from inferior ovaries fused with the hypanthium, with abundant viscin. Other atypical features recorded include the presence of dimorphic embryos (i.e. embryos have foliose or prismatic cotyledons) in the genus Psittacanthus and the absence of endosperm described in Psittacanthus and Aetanthus.

Conventional serial sectioning of flowers and fruits at different developmental stages of Aetanthus colombianus A.C. Sm., Gaiadendron punctatum (Ruiz & Pav.) G. Don., and Tristerix secundus (Benth.) Kuijt, allowed us to conclude that the flowers of these species go through: (1) migration of the nuclei of the megagametophyte, which reach the distal third portion of the style; (2) formation of a secretory parenchyma associated with the vascular bundles of petals and stamens; and (3) growth of a sclerenchymatic structure that delimits the megagametophyte in the flower and the proembryo in the fruit. Conversely, the fruit formation follows: (1) development of the pendular embryo, facing the schlerenchymatic tissue, with a conspicuous hypocotyl, and formation and expansion of the viscin in the epicarp; (2) development of two foliose cotyledons surrounded by massive endosperm (ruminate in Gaiadendron), and lack of seed coat; and 3) the fruit proper is delimited by the vascular bundles of the gynoecium, and surrounded by a thick epicarp with extra-carpellary viscin. This work lays the foundation for comparative evo-devo studies on the genetic bases for ovule reduction in hemiparasitic neotropical Santalales.