A total evidence approach to understanding phylogenetic relationships and ecological diversity in Selaginella subg. Tetragonostachys.
OBJECTIVE
Several members of Selaginella are renowned for their ability to survive extreme drought and "resurrect" when conditions improve. Many of these belong to subgenus Tetragonostachys, a group of ∼45 species primarily found in North and Central America, with substantial diversity in the Sonoran and Chihuahuan Deserts. We evaluated the monophyly and the age of subgenus Tetragonostachys and assess how drought tolerance contributed to the evolution of this clade.


METHODS
Our study included most Tetragonostachys species, using plastid and nuclear sequences, fossil and herbarium records, and climate variables to describe the species diversity, phylogenetic relationships, divergence times, and climatic niche evolution in the subgenus.


RESULTS
We found that subgenus Tetragonostachys forms a monophyletic group sister to Selaginella lepidophylla and may have diverged from other Selaginella because of a Gondwanan-Laurasian vicariance event ca. 240 mya. The North American radiation of Tetragonostachys appears to be much more recent and to have occurred during the Early Cretaceous-late Paleocene interval. We identified two significant and nested ecological niche shifts during the evolution of Tetragonostachys associated with extreme drought tolerance and a more recent shift to cold climates. Our analyses suggest that drought tolerance evolved in the warm deserts of southwest North America and may have been advantageous for colonization of cold and dry boreal climates.


CONCLUSIONS
Our investigation provides a foundation for future research addressing the genomics of ecological niche evolution and the potential role of reticulate evolution in Selaginella subgenus Tetragonostachys.
Department of Ecology and Evolutionary Biology, University of Arizona, P.O. Box 210088, Tucson, Arizona 85721, USA.
Nils
N
Arrigo
James
J
Therrien
Cajsa Lisa
CL
Anderson
Michael D
MD
Windham
Christopher H
CH
Haufler
Michael S
MS
Barker
D001483:Base Sequence; D005075:Biological Evolution; D002980:Climate; D018744:DNA, Plant / Q000737:chemistry / Q000235:genetics; D021903:DNA, Ribosomal Spacer / Q000737:chemistry / Q000235:genetics; D004463:Ecology; D005580:Fossils; D014644:Genetic Variation; D005843:Geography; D008969:Molecular Sequence Data; D010802:Phylogeny; D012335:RNA, Ribosomal / Q000235:genetics; D032503:Selaginellaceae / Q000235:genetics / Q000502:physiology*; D017422:Sequence Analysis, DNA; D013997:Time Factors
D018744:DNA, Plant; D021903:DNA, Ribosomal Spacer; D012335:RNA, Ribosomal
Ecological niche shift; Ornstein-Uhlenbeck models; Selaginella; Sonoran Desert; lycophyte