Does floral phenotyping reflect species integrity and natural hybridization of endangered Calanthe species on an oceanic island?

INTRODUCTION: Floral phenotyping provides invaluable information for conserving genetic diversity of endangered plants. Endangered orchid, Calanthe aristulifera was selected to assess the applicability of phenotyping for identifying genetic variability and species integrity, given its morphological variations and evidences of natural hybridization on an oceanic island.

METHODS: Phenotyping using flower color and size (two phenotypes for each species) were compared with molecular data from genotyping-by-sequencing across C. aristulifera and coexisting allied (Calanthe sieboldii) and hybrid (Calanthe × kibanakirishima) species to track their species integrity and patterns in hybridization.

RESULTS AND DISCUSSION: Principal coordinates analysis and phylogenetic clustering showed no genetic difference between the two phenotypes of C. aristulifera, by which C. aristulifera populations featured higher species integrity and lower genetic diversity than the coexisting allied species despite the divergence in floral morphology. Inversely, floral morphological divergence corresponded to interspecific genetic variability and the level of hybridization within C. × kibanakirishima. Hybrid index analysis particularly reflected the asymmetrical backcrossing and introgression toward C. sieboldi instead of C. aristulifera as a result of the similarities in floral morphology between the hybrids and C. sieboldii. Variations in flower lip and spur sizes were related to such genetic variations. In terms of biodiversity conservation, overall findings exhibit that divergence in floral morphologies may not ensure well-preserved genetic diversity or reduced species integrity of the endangered orchid. Some species may require the exigent protection of genetic diversity from the risk of genetic bottleneck, regardless of their marked phenotypic divergence.