Vascular variants in seed plants—a developmental perspective

Abstract

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.

Abstract

‘Vascular variant’ is the developmental process by which atypical vascular growth emerges in a lineage in relation to their putative ancestor. These atypical ontogenies emerge from atypical procambium, cambium and de novo cambium patterning, outlining three developmental based categories: procambial variants, cambial variants and ectopic cambia. In turn, each category of vascular variants includes several anatomical patterns that are predictably conserved within a given taxon and, therefore, useful in the species description. These atypical—and frequently complex ontogenies—develop in stems and roots of all major lineages of seed plants, outlining a striking case of convergent evolution. The vascular variants' framework illuminates the evolution of the development of the massive diversity of vascular ontogenies in distantly related plants and promises to facilitate comparative studies that may be informative at various biological scales with meaningful evolutionary implications.