Heterochrony and repurposing in the evolution of gymnosperm seed dispersal units

Abstract

Background Plant dispersal units, or diaspores, allow the colonization of new environments expanding geographic range and promoting gene flow. Two broad categories of diaspores found in seed plants are dry and fleshy, associated with abiotic and biotic dispersal agents, respectively. Anatomy and developmental genetics of fleshy angiosperm fruits is advanced in contrast to the knowledge gap for analogous fleshy structures in gymnosperm diaspores. Improved understanding of the structural basis of modified accessory organs that aid in seed dispersal will enable future work on the underlying genetics, contributing to hypotheses on the origin of angiosperm fruits. To generate a structural framework for the development and evolution of gymnosperm fleshy diaspores, we studied the anatomy and histochemistry of Ephedra (Gnetales) seed cone bracts, the modified leaves surrounding the reproductive organs. We took an ontogenetic approach, comparing and contrasting the anatomy and histology of fleshy and papery-winged seed cone bracts, and their respective pollen cone bracts and leaves in four species from the South American clade. Results Seed bract fleshiness in Ephedra derives from mucilage accumulated in chlorenchyma cells, also found in the reduced young leaves before they reach their mature, dry stage. Cellulosic fibers, an infrequent cell type in gymnosperms, were found in Ephedra, where they presumably function as a source of supplementary apoplastic water in fleshy seed cone bracts. Papery-winged bract development more closely resembles that of leaves, with chlorenchyma mucilage cells turning into tanniniferous cells early on, and hyaline margins further extending into “wings”. Conclusions We propose an evolutionary developmental model whereby fleshy and papery-winged bracts develop from an early-stage anatomy shared with leaves that differs at the pollination stage. The ancestral fleshy bract state may represent a novel differentiation program built upon young leaf anatomy, while the derived dry, papery-winged state is likely built upon an existing differentiation pattern found in mature vegetative leaves. This model for the evolution of cone bract morphology in South American Ephedra hence involves a novel differentiation program repurposed from leaves combined with changes in the timing of leaf differentiation, or heterochrony, that can further be tested in other gymnosperms with fleshy diaspores.

All together now: Cellular and molecular aspects of leaf development in lycophytes, ferns, and seed plants

“…Leaves of Ephedra species are reduced to scale-like structures lacking differentiation between palisade and spongy mesophyll (Dörken, 2013(Dörken, , 2014San Martin et al, 2022;Figures 3D, I). Two YABBY homologs (EdiYABD and EdiYABD) were found in Ephedra distachya, both expressed in the outer layer of the ovule integument in a pattern similar to angiosperm "reproductive" YABBY genes, and in the cone bracts where one of them, EdiYABD, showed a higher expression level on the abaxial side, similar to "vegetative YABBYs" in leaves of angiosperms (Finet et al, 2016) (Du et al, 2020).…”

The ontogeny of disparity in Cupressaceae seed cones

“…Morphological disparity is ultimately generated by variation in patterns of growth and development among species and clades. Changes in development, especially heterochrony in a broad sense (Smith, 2001;McNamara & McKinney, 2005;Webster & Zelditch, 2005), have been widely studied for their potential role in the evolution of morphological diversity in animals (Gould, 1977;Klingenberg, 1998;Smith, 2003;McNamara & McKinney, 2005) and plants (DiMichele et al, 1989;Friedman & Carmichael, 1998;Olson, 2012;Buend ıa-Monreal & Gillmor, 2018;San Martin et al, 2022;Petrone-Mendoza et al, 2023). But, plant reproductive structures, especially those of seed plants (which have been the most common focus for studies of disparity e.g.…”