Evolutionary History of Redwoods Revealed By Transcriptome and Targeted Sequence Capture

Coast redwoods (Sequoia sempervirens) are known for their great height (over 100m) and advanced age (over 2,000 years), but less so for being the only hexaploid conifer (2n=6x=66). Though these colossal ancient trees are limited to foggy coastal forests in California and Oregon, fossils suggest a broader historical range. How, when, and where polyploidization took place remains a mystery, though diverse putative genome donors and polyploidization mechanisms have been proposed. The closest relatives of Sequoia are giant sequoia, Sequoiadendron giganteum, of the Californian Sierra Nevada and dawn redwood, Metasequoia glyptostroboides, endemic to China. These three monotypic genera comprise the living redwood clade. Despite restricted current distributions, redwood-like fossils occur across the Northern Hemisphere. Historical distributions include range overlap of redwood lineages, suggesting hybridization among lineages is plausible. Our project combines redwood transcriptomes with targeted sequence data to shed light on the evolution of this enigmatic lineage.

Foliar transcriptome sequencing of the three redwoods yielded over 70,000 expressed sequences. Preliminary analysis showed that, for a majority of genes, copies from S. sempervirens and S. giganteum are sister. This pattern implies either autoployploidy or allopolyploidy involving only genome donors within the Sequoia/Sequoiadendron clade. To further clarify polyploidization mechanisms, a subset of 900+ genes were chosen for targeted sequencing to isolate homeologous gene copies from S. sempervirens. Subsequent phylogenetic comparison among homeologs in S. sempervirens and its relatives will reveal whether hybridization with an extant lineage contributed to polyploidy in Sequoia.