Comparative morphological analysis of two parallel mycoheterotrophic transitions reveals divergent and convergent traits in the genus Pyrola (Pyroleae, Ericaceae)

The genus Pyrola includes species with different degree of mycoheterotrophy; some species possess individuals that rely on all carbon through their associations with fungi (full mycoheterotrophy, FM), whereas some species obtain carbon through both fungi and photosynthesis by itself (partial mycoheterotrophy, PM). To investigate how plant functional traits of photosynthesis and reproduction are related to the degree of mycoheterotrophy in the initial stage of the transition from PM to FM, we determined morphological traits in FM (or nearly FM) and PM species in two independent lineages, P. picta and P. japonica complexes. We used herbarium specimens and examined leaf number, leaf area, flower number, and scape length in FM or nearly FM species (P. aphylla and P. subaphylla) and PM species (P. picta s.l. and P. japonica). We found a leaf area reduction in FM (or nearly FM) species in both lineages, suggesting that this is a convergent trait. The number of flowers was not significantly different between FM (or nearly FM) and PM species in both lineages. On the other hand, differences in the variation between FM (or nearly FM) and PM species were found in some traits between the two lineages. The FM (or nearly FM) species in one lineage only possessed rudimentary leaves, whereas that in the other linage possessed a few small, ordinary leaves in addition to those with only rudimentary leaves. The scape length of the FM (or nearly FM) species was significantly longer than that of PM species in one lineage, whereas it was shorter in the other lineage. The different and common variations are divergent and convergent traits, respectively, that could be associated with the transition to FM in Pylora. In addition, shoots of both PM species occasionally lacked ordinary leaves, possibly indicating possession of these shoots is preadaptation for the transition to FM in Pyrola.