# Evolutionary Changes in Crassulacean Acid Metabolism (CAM) and Related Traits During the Diversification of Aichryson (Crassulaceae) on the Macaronesian Islands

## TL;DR

This study explores how Crassulacean acid metabolism (CAM) evolved in the Aichryson genus across Macaronesian islands, linking it to climate and life history strategies.

## Contribution

The study provides new insights into the evolutionary dynamics of CAM in Aichryson, revealing shifts in photosynthetic strategies tied to ecological differentiation and island adaptation.

## Key findings

- Annual Aichryson species show high CAM inducibility under stress, while perennials exhibit more constitutive CAM and water storage traits.
- Ancestral state reconstruction suggests a C3 ancestor with low-level CAM capacity, evolving into diverse CAM expressions in response to local climates.
- Ecological differentiation among islands correlates with distinct physiological and life history strategies in Aichryson.

## Abstract

Crassulacean acid metabolism (CAM) is a highly plastic photosynthetic pathway with ecological and evolutionary significance, ranging from weak inducible to strong obligate forms. While most Crassulaceae taxa may be capable of performing CAM, the Macaronesian genus Aichryson has not traditionally been associated with CAM. We integrate phylogenetic, physiological, isotopic, anatomical and bioclimatic data to investigate the distribution, plasticity and evolutionary history of CAM and related traits in Aichryson. Our study includes all 15 accepted species, combining over 1100 occurrence records, carbon isotope (δ13C) data, nocturnal acid titration and a CAM performance experiment under temperature and drought gradients. Multivariate analyses of bioclimatic variables show clear ecological differentiation among the Azores, Madeira and Canary Islands, with life form strongly associated with climatic niche. Annual species are generally restricted to cooler, wetter climates, while the perennial
A. tortuosum
lineage, endemic to the arid eastern Canaries, exhibits increased succulence, lower minimum leaf conductance and higher CAM performance. Ancestral state reconstruction of δ13C data suggests that the ancestor of Aichryson possessed a predominantly C3 physiology with low‐level CAM capacity, from which independent shifts towards stronger CAM expression or reversions to predominant C3 photosynthesis occurred in response to local climatic conditions. Our CAM performance experiment revealed pronounced interspecific differences in nocturnal acid accumulation and plasticity. Some annuals, such as
A. bollei
, exhibited high CAM inducibility under stress, while others, like A. dumosum, maintained low ΔH+ across treatments, likely reflecting relaxed selection in mesic habitats. These physiological traits align with environmental niche and life history, supporting two main strategies: fast‐growing annuals with flexible CAM and slow‐growing perennials with more constitutive CAM and investment in leaf longevity, cuticular properties and water storage. These findings support a ‘CAM continuum’ and highlight the roles of ecological differentiation and climatic filtering in shaping CAM evolution. Aichryson emerges as a model system for understanding CAM plasticity and the interplay between photosynthetic pathways, life history and insular biogeography.

We explore evolutionary changes in Crassulacean acid metabolism (CAM) and associated traits during the diversification of Aichryson (Crassulaceae) across the Macaronesian Islands. By combining field data, physiological measurements and phylogenetic analysis, we identify shifts in photosynthetic strategy linked to habitat transitions. Our findings highlight the role of CAM evolution in island adaptation and lineage diversification.