A macro‐ecological perspective on crassulacean acid metabolism (CAM) photosynthesis evolution in Afro‐Madagascan drylands: Eulophiinae orchids as a case study

Summary

• Crassulacean acid metabolism (

  CAM
  ) photosynthesis is an adaptation to water and atmospheric
  CO
  ₂ deficits that has been linked to diversification in dry‐adapted plants. We investigated whether
  CAM
  evolution can be associated with the availability of new or alternative niches, using Eulophiinae orchids as a case study.

• Carbon isotope ratios, geographical and climate data, fossil records and

  DNA
  sequences were used to: assess the prevalence of
  CAM
  in Eulophiinae orchids; characterize the ecological niche of extant taxa; infer divergence times; and estimate whether
  CAM
  is associated with niche shifts.

• CAM
  evolved in four terrestrial lineages during the late Miocene/Pliocene, which have uneven diversification patterns. These lineages originated in humid habitats and colonized dry/seasonally dry environments in Africa and Madagascar. Additional key features (variegation, heterophylly) evolved in the most species‐rich
  CAM
  lineages. Dry habitats were also colonized by a lineage that includes putative mycoheterotrophic taxa.

• These findings indicate that the switch to

  CAM
  is associated with environmental change. With its suite of adaptive traits, this group of orchids represents a unique opportunity to study the adaptations to dry environments, especially in the face of projected global aridification.