Growth & Development

Morpho-anatomical affinities of three paleoendemic podocarp genera

Despite the probable extinction of closely related sister species and genera, and the evolution of morphological structures as adaptations to changing environmental conditions, they still share basic reproductive traits with other podocarps.
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The three relict genera PherosphaeraMicrocachrys and Saxegothaea in Podocarpaceae produce quite distinct seed cone types in comparison with other genera and do not form a clade along with Acmopyle. The detailed seed cone morpho-anatomy of these three relict genera and affinities with other podocarps are poorly known. Khan and Hill aimed to understand the seed cone morpho-anatomy and affinities among these three disjunct relict genera and with other podocarps.

Seed cone with epimatium (Em), seed (Sd), fertile bract (Fbr) and micropyle (Mi). Source: Khan and Hill 2021.

The authors describe the seed cone morpho-anatomical structures of the three relict genera in detail. The three genera produce aggregated multiovulate cones. Of particular interest is the epimatium, a single scale that forms a protective covering around the ovule. Both Microcachrys and Saxegothaea have an asymmetrical free cup-like epimatium. Both species of Pherosphaera lack an epimatium. The ancestral state reconstruction implies that the presence of an epimatium is an ancestral trait in podocarps and is independently lost in Pherosphaera and Phyllocladus. The seed cones are fleshy in Microcachrys and non-fleshy in Saxegothaea and Pherosphaera. The seed cone macrofossils of both extinct and living podocarps also show the presence of an epimatium and fleshiness in podocarps.

Altogether, the morpho-anatomy suggests that Pherosphaera, Microcachrys and Saxegothaea present affinities with each other and other podocarps, but the reconstruction of the ancestral seed cone in Podcarpaceae is quite complex due to multiple convergent evolutions of several structures. These structures (e.g. epimatium, aril and receptaculum) are of low taxonomic value but of great evolutionary and ecological significance, and are responsive adaptations to ever-changing environmental conditions.

RESEARCH ARTICLE

Khan, R., Hill, R.S., 2021. Morpho-anatomical affinities and evolutionary relationships of three paleoendemic podocarp genera based on seed cone traits. Annals of Botany. https://doi.org/10.1093/aob/mcab113

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