Comparative Ovule Ontogeny in Some Members of theTabebuiaAlliance (Bignoniaceae)

Abstract

Editor: Maria von BalthazarPremise of research. Recent phylogenetic studies placed nearly all Neotropical arboreal and shrubby species of the Bignoniaceae (excluding Jacaranda) in the Tabebuia alliance, a clade that includes all of the woody species in this family with palmately compound leaves. However, the taxa assigned to this clade have appeared in these studies as an unresolved trichotomy with Sparattosperma as its sister group. Considering the diversity and taxonomic significance of ovule morphogenesis in the extant angiosperms, this study aimed to contribute to the systematics of the Bignoniaceae by investigating ovule ontogeny in one representative of each clade of the Tabebuia alliance trichotomy: Crescentia cujete, Cybistax antisyphilitica, and Tabebuia roseoalba.Methodology. The analysis is based on light microscopy observations of microtome semiserial sections of ovaries and ovules at several stages of development.Pivotal results. The ovules are anatropous, unitegmic, and tenuinucellate and originate from trizonate ovule primordia. The single integument shows a concurrent epidermal and subepidermal origin, and a dark staining hypostase develops at the chalaza. Meiosis of the megaspore mother cell results in a linear or T-shaped tetrad of megaspores. The chalazal megaspore generates a monosporic Polygonum-type female gametophyte. In comparison to previous studies, our analysis of ontogenetic events demonstrates that embryological features are highly conserved in the Bignoniaceae. However, some peculiar characteristics are congruent with the systematic consideration of this study, especially the pattern of callose wall deposition during megasporogenesis and the occurrence of a protruding versus nonprotruding nucellus during early ovule development. Conclusions.In the context of other embryological studies of the Bignoniaceae, our results support closer phylogenetic relationships among Crescentia, Handroanthus, and Tabebuia in comparison to Cybistax and indicate that the nonprotruding early nucellus is an additional character state that helps to segregate Tabebuia s.s. from other taxa in the Tabebuia alliance with a eusyncarpous ovary.