title: Salt tolerance is evolutionarily labile in a diverse set of angiosperm families.
abstract: BACKGROUND
  Salt tolerance in plants is rare, yet it is found across a diverse set of taxonomic groups. This suggests that, although salt tolerance often involves a set of complex traits, it has evolved many times independently in different angiosperm lineages. However, the pattern of evolution of salt tolerance can vary dramatically between families. A recent phylogenetic study of the Chenopodiaceae (goosefoot family) concluded that salt tolerance has a conserved evolutionary pattern, being gained early in the evolution of the lineage then retained by most species in the family. Conversely, a phylogenetic study of the Poaceae (grass family) suggested over 70 independent gains of salt tolerance, most giving rise to only one or a few salt tolerant species. Here, we use a phylogenetic approach to explore the macroevolutionary patterns of salt tolerance in a sample of angiosperm families, in order to ask whether either of these two patterns - deep and conserved or shallow and labile - represents a common mode of salt tolerance evolution. We analyze the distribution of halophyte species across the angiosperms and identify families with more or less halophytes than expected under a random model. Then, we explore the phylogenetic distribution of halophytes in 22 families using phylogenetic comparative methods.
  
  
  RESULTS
  We find that salt tolerance species have been reported from over one-third of angiosperm families, but that salt tolerant species are not distributed evenly across angiosperm families. We find that salt tolerance has been gained hundreds of times over the history of the angiosperms. In a few families, we find deep and conserved gains of salt tolerance, but in the majority of families analyzed, we find that the pattern of salt tolerant species is best explained by multiple independent gains that occur near the tips of the phylogeny and often give rise to only one or a few halophytes.
  
  
  CONCLUSIONS
  Our results suggest that the pattern of many independent gains of salt tolerance near the tips of the phylogeny is found in many angiosperm families. This suggests that the pattern reported in the grasses of high evolutionary lability may be a common feature of salt tolerance evolution in angiosperms.
authors.affiliation: Division of Ecology, Macroevolution and Macroecology, Evolution and Genetics, Research School of Biology, Australian National University, Brinkin, 0200, Australia. camile.moray@anu.edu.au.
authors.forename: Camile
authors.initials: C
authors.lastname: Moray
authors.affiliation: Division of Ecology, Macroevolution and Macroecology, Evolution and Genetics, Research School of Biology, Australian National University, Brinkin, 0200, Australia. xia.hua@anu.edu.au.
authors.forename: Xia
authors.initials: X
authors.lastname: Hua
authors.affiliation: Division of Ecology, Macroevolution and Macroecology, Evolution and Genetics, Research School of Biology, Australian National University, Brinkin, 0200, Australia. lindell.bromham@anu.edu.au.
authors.forename: Lindell
authors.initials: L
authors.lastname: Bromham
mesh_terms: D000220:Adaptation, Biological; D005075:Biological Evolution; D019684:Magnoliopsida / Q000033:anatomy & histology / Q000145:classification* / Q000235:genetics / Q000502:physiology*; D010802:Phylogeny; D055049:Salt Tolerance