Origin of Polyploidy in Sequoia: Tous pour un, pas de deux, or ménage à trois?

Coast redwoods (Sequoia sempervirens) are well-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 the foggy coastal forests of central and northern California and southwestern Oregon, the redwood fossil record suggests a broader historical range across the Northern hemisphere.  How, when, and where polyploidization took place remains a mystery, though diverse putative genome donors and polyploidization mechanisms have been proposed. We estimated phylogenetic trees to compare relationships among gene copies in S. sempervirens and its close relatives, in order to reveal whether hybridization with an extant lineage contributed to polyploidy in the Sequoia lineage. Our results indicate Sequoia is an autopolyploid, or that allopolyploidy occurred within the California redwood clade. Our results also suggest duplicate gene pairs in Sequoia are much younger than expected based on fossil estimates of whole genome duplication, which may be attributed to continued multisomic inheritance in Sequoia. To address this, we consider the role of diploidization in polyploid evolution, and whether slow diplodization helps explain why polyploid gymnosperms are so rare.