Bryophytes’ remarkable capacity to tolerate extreme abiotic conditions allows us to enhance our understanding of the diversity of molecular mechanisms involved in plant stress response. Here, we used next generation sequencing to study DNA methylation and gene expression changes in plants from four populations of the metallophyte moss Scopelophila cataractae experimentally exposed to either Cd or Cu. These populations previously showed differences in tolerance to both metals, so here, we aimed at uncovering the molecular basis of this phenotypic differentiation. We found no evidence of genetic differentiation among the populations studied. The epigenetic data, however, showed limited but significant population-specific changes in DNA methylation in response to both metals. Exposure to acute Cu stress in the laboratory led to the downregulation of genes involved in heavy metal tolerance in both populations regardless of their tolerance level, but this response was quantitatively higher in the most tolerant. We propose that chronic exposure to varying levels of heavy metals in the field led to potentially non-genetically-based intraspecific differentiation for heavy metal tolerance in S. cataractae. The most tolerant plants invested more in constitutive protection and were more efficient in entering a conservative state when faced with acute Cu stress.

Phenotypic variation in natural populations is crucial for rapid adjustment to challenging environmental conditions such as exposure to heavy metal pollution. Hence, the study of variation in traits related to plant response to heavy metal stress provides valuable insight into the likelihood of a population’s survival. We investigate the patterns and mechanisms of intraspecific phenotypic variation for heavy metal accumulation and tolerance in bryophytes, one of the most resilient yet relatively understudied taxa. Two terrestrial mosses exposed to Cd and Cu in the laboratory, the heavy metal specialist Scopelophila cataractae and the facultative metallophyte Ceratodon purpureus, showed intraspecific differences in tolerance to these metals. Only the heavy metal specialist showed differences in accumulation which in the case of Cu could be associated to preferential relocation towards the stem to protect their main photosynthetic organs. We also report the first evidence for sexual dimorphism for Cd tolerance in C. purpureus (females being more tolerant than males). Our results provide support for high variation in the capacity of bryophytes to respond to environmental challenge despite potentially low levels of genetic variation and lack of previous exposure to stress, as well as evidence for metal-dependent, sex-specific differences in heavy metal tolerance in bryophytes.