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.
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All data necessary to evaluate this manuscript are included. Slide images are housed at the Cornell University Plant Anatomy Collection (CUPAC), Ithaca, New York and living specimens are maintained at Montgomery Botanical Center and Fairchild Tropical Garden in Coral Gables, Florida.
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We thank Patrick Griffith and the entire Montgomery Botanical Center staff for maintaining crucial living collections and making them freely available to scientists; P. Barry Tomlinson for generously providing his expertise and profound anatomical insight; Chad Husby and Yisu Santamarina for providing access to specimens at Fairchild Tropical Garden; Jennifer Svitko at the Cornell University Plant Anatomy Collection for assisting with imaging; Dennis Stevenson for assisting with identification of anatomical structures; Melissa Whitaker for field support; Rory Maher for making data collection possible; and members of the Specht Lab at Cornell University for providing helpful feedback throughout the project.
RAEG. was supported by a Cornell College of Agriculture and Life Sciences Alumni Association academic enrichment grant and an undergraduate research grant from the Dextra Undergraduate Research Endowment. SS was supported by National Science Foundation Postdoctoral Research Fellowship in Biology (1906333). Consejo Nacional de Ciencia y Tecnología (CB 2011–169468-B) awarded to APV provided partial funding for anatomy research on Mexican cycads.
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Glos, R.A.E., Salzman, S., Calonje, M. et al. Leaflet Anatomical Diversity in Zamia (Cycadales: Zamiaceae) Shows Little Correlation with Phylogeny and Climate. Bot. Rev. 88, 437–452 (2022). https://doi.org/10.1007/s12229-021-09272-0