Species delimitation of three species within the Russula subgenus Compacta in Korea: R. eccentrica, R. nigricans, and R. subnigricans.

Distinguishing individual Russula species can be very difficult due to extensive phenotypic plasticity and obscure morphological and anatomical discontinuities. In this study, we use the internal transcribed spacer (ITS) and 28S nuclear ribosomal large subunit (LSU) markers to identify and study the genetic diversity of species in the Russula subgenus Compacta in Korea. We focus on two morphologically similar species that are often misidentified for each other: R. nigricans and R. subnigricans. Based on molecular phylogenetic analyses, we identify three subgroups of R. nigricans, with two from Asia and one from Europe/North America. Surprisingly, we find Korean R. subnigricans are more closely related to R. eccentrica from North America than the type specimen of R. subnigricans from Japan. These molecular data, along with habitat data, reveal that Korean R. subnigricans had previously been misclassified and should now be recognized as R. eccentrica. Both ITS and LSU exhibit high interspecific and low intraspecific variation for R. eccentrica, R. nigricans, and R. subnigricans. These markers provide enough resolutional power to differentiate these species and uncover phylogeographic structure, and will be powerful tools for future ecological studies of Russula.