Origin of polyploidy in coast redwood (Sequoia sempervirens)

The majestic coast redwood (Sequoia sempervirens; Cupressaceae) is one of the few polyploid conifers, and the only hexaploid (2n=6x=66). A California endemic of economic, ecological, and cultural value, coast redwoods are long-lived trees (some over 2,000 years) and are among the very tallest tree species in the world (up to 115 meters). Their geographical range is limited to the coast of central and northern California, dependent on fog in that region, but the fossil record suggests a broader historical distribution. 

Though Sequoia sempervirens was identified as a hexaploid long ago, how it obtained six of each chromosome (three pairs) is not well understood. The closest living relatives of S. sempervirens are the dawn redwood (Metasequoia glyptostroboides,) native to China, and the giant redwood (Sequoiadendron giganteum) endemic to interior central California. The South American alerce (Fitzroya cupressoides) is morphologically similar to the redwoods (tall, long-lived, reddish wood) and is a tetraploid (2n=4x=44) in the same conifer family. To resolve the origin of hexaploidy in coast redwood, we are using transcriptome sequencing to identify low-copy nuclear genes in S. sempervirens, followed by targeted multiplex sequencing of these genes in S. sempervirens and its close relatives.