Genetic Diversity and Structure in the Philippine Rafflesia lagascae Complex (Rafflesiaceae) inform its Taxonomic Delimitation and Conservation

Abstract

Rafflesia lagascae (Rafflesiaceae) is a rare endo-holoparasitic plant species with a disjunct distribution on Luzon Island in the Philippines. It is morphologically very similar to R. manillana, which is only known from a single population on nearby Samar Island. This study aims to contribute to the taxonomy and conservation of R. lagascae and R. manillana (i.e., the R. lagascae complex) by resolving their patterns of genetic diversity and genetic structure. The results of population genetic analyses of a microsatellite data set indicate that despite their frequently extremely small sizes and geographic isolation, Rafflesia populations display moderate genetic diversity and do not show evidence of pronounced inbreeding. Most populations appear to be strongly genetically differentiated from each other, suggesting limited gene flow between them. Patterns of genetic diversity of staminate and pistillate Rafflesia flowers growing on the same Tetrastigma host plants indicate that the R. lagascae complex is monoecious and that host plants are regularly infected by more than one Rafflesia plant. Both might enable Rafflesia to maintain viable populations in areas with low host densities. PCoA and Bayesian cluster analyses show that the R. lagascae complex is composed of three genetically isolated taxa. One of these constitutes R. manillana, supporting the morphology-based hypothesis that it is taxonomically distinct from R. lagascae. The second taxon in this complex is composed of the single population of R. lagascae from Mt. Labo. This morphologically cryptic population is genetically distinct from the 11 remaining R. lagascae populations that were included in this study and that form the third taxon. We recommend that R. lagascae s. s., the Mt. Labo R. lagascae, and R. manillana should be managed as different conservation entities.