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Modulations in seed micromorphology reveal signature of adaptive species-diversification in Dendrobium (Orchidaceae)

During speciation in flowering plants, many 
morphological characters were evolved in response to continuous selection 
pressure. Such characters are largely governed by genetic changes or get 
modulated according to ecological adaptations. To explore the potential of 
latter, a novel comparative analysis of seed micromorphology and ultrastructure 
was performed in twenty species in the genus Dendrobium (Orchidaceae) 
from well-defined altitude based phytogeographical realms i.e. temperate, subtropical and tropical regions. Seed 
morphological characteristics from all collected species were examined using 13 
quantitative traits descriptors. Comparative seed morphology revealed that the 
species from temperate region have larger seed volumes and seed volume/embryo 
volume ratio than species from subtropical or tropical regions irrespective of 
their phylogenetic association. Hence, phylogenetically related species 
emphasized their preference for a climatic region as their prime habitat. 
Similar trends towards gradual increase of percentage free air space were 
observed in the species of tropical to subtropical and subtropical to temperate 
regions. The higher percentage of free air space may have an advantage for the 
species from temperate region with relatively low atmospheric pressure that had 
greater buoyancy for optimal seed dispersal. Also, phylogenetically distant 
species from a particular climatic region, showed characteristic features of 
the testa cell walls sculptured with bead-like orna-mentations or covered 
with cottony white substances. Based on these observations, it may be concluded 
that seed volume, % free air space and seed coat ornamentation in Dendrobium species are signatures of 
adaptation according to climatic preferences rather than their selection during 
speciation.