Evolutionary and functional potential of ploidy increase within individual plants: somatic ploidy mapping of the complex labellum of sexually deceptive bee orchids.

Background and Aims: Recent tissue-level observations made indirectly via flow cytometry suggest that endoreplication (duplication of the nuclear genome within the nuclear envelope in the absence of subsequent cell division) is widespread within the plant kingdom. Here, we also directly observe ploidy variation among cells within individual petals, relating size of nucleus to cell micromorphology and (more speculatively) to function.

Methods: We compared the labella (specialized pollinator-attracting petals) of two European orchid genera: Dactylorhiza has a known predisposition to organismal polyploidy, whereas Ophrys exhibits exceptionally complex epidermal patterning that aids pseudocopulatory pollination. Confocal microscopy using multiple staining techniques allowed us to observe directly both the sizes and the internal structures of individual nuclei across each labellum, while flow cytometry was used to test for progressively partial endoreplication.

Key Results: In Dactylorhiza, endoreplication was comparatively infrequent, reached only low levels, and appeared randomly located across the labellum, whereas in Ophrys endoreplication was commonplace, being most frequent in large peripheral trichomes. Endoreplicated nuclei reflected both endomitosis and endocycling, the latter reaching the third round of genome doubling (16C) to generate polytene nuclei. All Ophrys individuals studied exhibited progressively partial endoreplication.

Conclusions: Comparison of the two genera failed to demonstrate the hypothesized pattern of frequent polyploid speciation in genera showing extensive endoreplication. Endoreplication in Ophrys appears more strongly positively correlated with cell size/complexity than with cell location or secretory role. Epigenetic control of gene overexpression by localized induction of endoreplication within individual plant organs may represent a significant component of a plant's developmental programme, contributing substantially to organ plasticity.