Phloem wedges in Malpighiaceae: origin, structure, diversification, and systematic relevance

Evolution of secondary growth: have pattern, got process? A commentary on 'Molecular phylogeny of Urvillea (Paullinieae, Sapindaceae) and its implications in stem vascular diversity'

This article comments on:

Israel L. Cunha Neto, Yanã C. Rizzieri, Pablo A. Cabanillas, Fabiano M. Martins, Natália F. Marques, Genise V. Somner, Pedro Acevedo-Rodríguez and Joyce G. Onyenedum. Molecular phylogeny of Urvillea (Paullinieae, Sapindaceae) and its implications in stem vascular diversity, Annals of Botany, Volume 132, Issue 5, 10 October 2023, Pages 929–948, https://doi.org/10.1093/aob/mcad093

Molecular phylogeny of Urvillea (Paullinieae, Sapindaceae) and its implications in stem vascular diversity

BACKGROUND AND AIMS

The tribe Paullinieae has the highest diversity of vascular variants among the seed plants. The developmental diversity is better understood in the species-rich genera Paullinia and Serjania; however, the phylogeny and diversity of vascular variants in the smaller genera of Paullinieae remain understudied. Here we investigate the evolution of development of stem vasculatures in the small genus Urvillea.

METHODS

We generate the first molecular phylogeny of Urvillea derived from 11 markers using a maximum likelihood and Bayesian approach. In combination with phylogenetic reconstruction, stochastic character mapping is used to assess evolutionary changes in stem ontogenies, determined from developmental anatomy of stems collected in the field or from herbarium and wood collections.

KEY RESULTS

Urvillea is supported as a monophyletic group and sister to Serjania. There are five stem ontogenies in Urvillea, including typical growth and four different vascular variants. Most stem ontogenies initiate with lobed stems in primary growth. Lobed stems in secondary growth are ancestral in Urvillea, but this ontogeny was lost multiple times. A reversal to typical growth occurred in non-climbing species. Phloem wedges, fissured stems, and ectopic cambia each evolved once independently. Phloem wedges is an intermediate developmental stage in the formation of fissured stems, which is characterized by a continuous fragmentation of vascular tissues. Lobed stems may generate constriction zones and lobes may split or not.

CONCLUSIONS

Urvillea is the third most diverse genus (after Serjania and Paullinia) with respect to the number of vascular variants within Paullinieae. One ontogeny (fissured stems) is exclusive to the genus. Differential cambial activity and ectopic cambia are the main ontogenetic processes generating stem diversity. The evolutionary history of vascular variants demonstrates the large developmental plasticity of the cambium in such a small genus and further demonstrates that complex anatomies have repeatedly evolved within Paullinieae lianas.

Vascular variants in seed plants-a developmental perspective

Over centuries of plant morphological research, biologists have enthusiastically explored how distinct vascular arrangements have diversified. These investigations have focused on the evolution of steles and secondary growth and examined the diversity of vascular tissues (xylem and phloem), including atypical developmental pathways generated through modifications to the typical development of ancestral ontogenies. A shared vernacular has evolved for communicating on the diversity of alternative ontogenies in seed plants. Botanists have traditionally used the term 'anomalous secondary growth' which was later renamed to 'cambial variants' by late Dr. Sherwin Carlquist (1988). However, the term 'cambial variants' can be vague in meaning since it is applied for developmental pathways that do not necessarily originate from cambial activity. Here, we review the 'cambial variants' concept and propose the term 'vascular variants' as a more inclusive overarching framework to interpret alternative vascular ontogenies in plants. In this framework, vascular variants are defined by their developmental origin (instead of anatomical patterns), allowing the classification of alternative vascular ontogenies into three categories: (i) procambial variants, (ii) cambial variants and (iii) ectopic cambia. Each category includes several anatomical patterns. Vascular variants, which represent broader developmental based groups, can be applied to both extant and fossil plants, and thereby offer a more adequate term from an evolutionary perspective. An overview of the developmental diversity and phylogenetic distribution of vascular variants across selected seed plants is provided. Finally, this viewpoint discusses the evolutionary implications of vascular variants.

Stem cambial variants of Taiwan lianas

BACKGROUND

Cambium in lianas, responsible for secondary growth, develop diverse and diagnostic traits during the climbing phase. Studies on the cross-section of Taiwanese liana cambial variants are scarce. We collected multiple stem cross-sections from 287 liana species belonging to 52 families. Each sample was examined on five occasions, and the observations were documented.

RESULTS

The results showed that approximately 22 cambial variants types were displayed in Taiwan lianas. Among these, axial vascular elements in radial segments were the most common, followed by the variants with the irregular conformation and intraxylary phloem. Based on our assessment, we provide the following identification features of a few families: Apocynaceae had intraxylary phloem; Convolvulaceae had intraxylary phloem combined with successive cambia; Lardizabalaceae, Menispermaceae, and Ranunculaceae possessed axial vascular elements in segments; Piperaceae had external primary vascular bundle cylinder combined with axial vascular elements in segments; Vitaceae had axial vascular elements in segments combined with irregular conformation. Axial vascular elements in segments and intraxylary phloem appeared in six or five combination types, showing that these two types combined with many variants are helpful for the identification of lianas. Two species, Momordica charantia var. abbreviata, and Momordica cochinchinensis had a cambium element in the outer cylinder of cortical bicollateral vascular bundles and formed directional layers of successive cambia.

CONCLUSIONS

Our study documented regular secondary growth with a single cambium in 36 species and cambial variants present in 16 species of Taiwanese lianas. Furthermore, we provide crucial baseline data on liana cambial variations, thereby improving our understanding of their morphology and identification.