Origin and genetic nature of polyploidy in paleoendemic coast redwood (Sequoia sempervirens (D. Don) Endl.)

It is not known when the polyploid coast redwood (Sequoia sempervirens) evolved from its diploid ancestors, and what is its type of polyploidy. Whether close relatives of Sequoia, giant sequoia (Sequoiadendron giganteum) and dawn redwood (Metasequoia glyptostroboides), have possibly contributed to the ancestry of hexaploid of Sequoia remains an open question. The nature of hexaploidy in Sequoia has baffled biologists for more than a century. Based on the chromosome configurations in Sequoia, G. Ledyard Stebbins was the first geneticists who postulated in 1948 that Sequoia is an autoallohexaploid (AAAABB), and an ancient species of Metasequoia might have been one of the putative ancestors of Sequoia. After its chromosome number (2n=6x=66) was confirmed in hexaploid Sequoia, the type of polyploidy in Sequoia has been further investigated for the past 70 years by a number of investigators, using cytogenetic and genetic data. Although an autoallohexaploid (AAAABB) origin of Sequoia has remained one of the dominant hypotheses until recently, an alternative hypothesis, amongst other possible origins, was also put forth by Ahuja and Neale (2002), that Sequoia may be partially diploidized autohexaploid (AAAAAA), derived from some ancestral species of Sequoia, thus carrying a single ancestral genome. Cytogenetic, molecular genetics, and genome sequence data now support the hypothesis that Sequoia originated as an autohexaploid.