Plastid genome evolution in the mycotrophic orchid genus Hexalectris Raf.

Abstract

The leafless, terrestrial genus Hexalectris Raf. comprises nine species of holomycotrophic orchids native to Mexico and the southern USA. The genus belongs to the tribe Bletiinae, and is closely related to the leafy genera Basiphyllaea Schltr. and Bletia Ruiz & Pav. Vegetative morphology in Bletiinae follows a trend of reduction, with well-developed, conduplicate leaves in Bletia, highly reduced, plicate leaves in Basiphyllaea, and absence of leaf laminae in Hexalectris. Estimates of relationships among these genera vary among studies. Recent analyses place Basiphyllaea as sister to Hexalectris, or as nested within Bletia; one analysis places Basiphyllaea + Hexalectris as nested within Bletia. Studies in orchid clades containing holomycotrophs and in holoparasitic Orobanchaceae suggest that plastid genome degradation occurs as a result of relaxed selective pressures on photosynthesis, which parallels reduction in vegetative morphology. Thus, Bletiinae serves as a powerful model for comparison with such clades. To test hypotheses of plastid genome reduction in Bletiinae, we sequenced representatives of Bletia, Basiphyllaea, and Hexalectris, focusing primarily on the latter. All species of Hexalectris were included, with at least two accessions sampled per species, and more intensive sampling in the widespread Hexalectris spicata (Walter) Barnhart, which ranges from Mexico to the eastern USA. Plastid genomes were assembled de novo and annotated, and phylogenetic analyses were conducted based on whole plastomes. Gene losses and pseudogene content were assessed across Bletiinae, as were patterns of selection on individual genes. These patterns are compared with recent mechanistic models of plastome evolution in parasitic lineages.