Leaflet Anatomical Diversity in Zamia (Cycadales: Zamiaceae) Shows Little Correlation with Phylogeny and Climate

Cycads (Cycadales) are among the most ancient lineages of extant seed-bearing plants and are the most threatened plant order on Earth, with circa 75% of the 356 accepted species endangered or threatened with extinction. Zamia is the most species-rich (81 spp.) and widely distributed cycad genus in the Americas, notable for its morphological and ecological diversity. Across the genus, there appears to be a high degree of convergence among macromorphological traits, with many characters that are useful for species identification proving uninformative for elucidating relationships among species. However, it remains unknown whether anatomical variation in leaflet structure corresponds with phylogenetic or geographic patterns, as has been investigated in Dioon and Cycas. Here, we present a broad comparative survey of leaflet anatomy across Zamia species with the goals of describing anatomical diversity and uncovering diagnostic characters for resolved clades. Anatomical characters were scored based on the literature and newly prepared sections of leaflets from 20 Zamia species plus the outgroup species Microcycas calocoma. The resulting matrix covers 39 Zamia species representing all five major clades and spanning the geographic distribution of the genus. Anatomical characters scored from leaflet sections were mapped onto a previously published phylogeny and evaluated for their phylogenetic signal. Most anatomical characters examined are not diagnostic for clades, but newly reported mesophyll sclereids may be unique to one large lineage. Given the widespread incongruence between phylogenetic relationships and the distribution of anatomical traits, we tested the relationship between anatomical characters and environmental signals but did not uncover significant correlations between anatomy and ecology. While further work is required to elucidate the evolutionary history of anatomical characters in this genus, this research improves our understanding of micromorphological character evolution, anatomical diversity, and phylogenetic relationships within this highly threatened lineage of plants.

Automated summary

This study provides a comprehensive comparison of leaflet anatomy in Zamia species, describing anatomical diversity and exploring diagnostic characters for resolved clades. The results show widespread incongruence between phylogenetic relationships and anatomical traits, and no significant correlations between anatomy and ecology.