Evolution of mycoheterotrophy in Polygalaceae: The case of Epirixanthes

The mycoheterotrophic lifestyle has enabled some plant lineages to obtain carbon from their mycorrhizal symbionts. The mycoheterotrophic genus Epirixanthes (Polygalaceae) consists of six species from tropical Asia. Although it is probably closely related to the chlorophyllous genus Salomonia and linked to arbuscular mycorrhizal fungi, lack of DNA sequence data has thus far prevented these hypotheses from being tested. Therefore, the evolutionary history of Epirixanthes remains largely unknown.

We reconstructed the phylogenetic relationships of Epirixanthes based on nuclear ITS and plastid matK data. Divergence times were inferred using a Bayesian relaxed clock approach, and we phylogenetically analyzed its mycorrhizal symbionts. We furthermore assigned these symbionts to operational taxonomic units, compared them with symbionts of other Polygalaceae, and measured their phylogenetic diversity.

We found that Epirixanthes is placed in tribe Polygaleae as sister to Salomonia. Epirixanthes has a Miocene-Oligocene stem age and grows exclusively in symbiosis with fungi of Glomeraceae. Salomonia and some Polygala species are linked to both Glomeraceae and Acaulosporaceae, resulting in higher phylogenetic diversity values. The majority of the symbionts of Epirixanthes are not found in Salomonia or Polygala, although a few shared fungal taxa are found.

Epirixanthes forms a relatively young mycoheterotrophic lineage. The Oligocene-Miocene origin suggests its evolution was influenced by the environmental dynamics in Southeast Asia during this time. Although comparison of fungi from Epirixanthes with those from Salomonia and Polygala suggests some specialization, many other mycoheterotrophic plants are linked to a more narrow set of Glomeraceae.