Seed Diversity in Menispermaceae: Developmental Anatomy and Insights into the Origin of the Condyle

Abstract

Most members of the Menispermaceae, a family of predominantly dioecious climbers, are characterized by having seeds that are diverse in form, many of which have an adaxial intrusion of the endocarp known as the condyle. The condyle is a distinctive feature of variable shape that has historically played a prominent role in the taxonomy and classification of the family. To understand the morphological basis of the condyle, I undertook a developmental study of carpels and fruits in selected lineages in Menispermaceae and in related families in the Ranunculales. The results indicated that the condyle is the outcome of differential development of the adaxial portion of the ovary wall, which corresponds to the placentary region. Condyles can be grouped into two general types. In the Calycocarpum condyle type, a broad region of the middle zone of the adaxial ovary wall proliferates, resulting in a convex condyle, and the anatropous ovules develop into concave-convex seeds. In the Menispermum condyle type, proliferation is accentuated in the inner zone of the adaxial ovary wall. Unequal growth in the middle and inner zones of the lateral ovary wall results in bilaterally compressed, laminiform condyles, and the hemianatropous ovules develop into curved seeds. Further variation in condyle shapes in the Menispermum condyle type originates by differential development of areas above and below the funicle. Additionally, endocarp ornamentation, a common feature in many Menispermaceae, results from differential lignification patterns of the inner zone of the fruit wall. Within Ranunculales, the condyle represents a shift in ovary wall development that took place on the branch leading to the Menispermaceae. The two types of condyles described here are potential synapomorphies for the two subfamilies recognized in the Menispermaceae.