Coast Redwood May be the Descendent of Two

Origins of Polyploidy in Coast Redwood (Sequoia sempervirens (D.DON) ENDL.) and Relationship of Coast Redwood to other Genera of Taxodiaceae

Photo by Miguel Vieira, Flickr Creative Commons
Photo by Miguel Vieira, Flickr Creative Commons

“The history of any taxon, community, or ecosystem provides a context for interpreting its current distribution and status”
— Reed F. Noss, The Redwood Forest

Dr. Raj Ahuja and Dr. David Neale have taken a big stride in coming closer to knowing the origin of polyploidy in coast redwood.

Polyploid organisms have more than two sets of each of their chromosomes, packages of DNA that hold recipes for making proteins. The human cell contains pairs of chromosomes. We have two copies of 23 chromosomes in every cell. The coast redwood, on the other hand, has a rare makeup of six copies of each chromosome, making it a hexaploid. Polyploidy is relatively common among flowering trees, but rare among gymnosperms, or cone bearing trees. The coast redwood is one of only three polyploid conifers in the world.

But ancestors of the coast redwood all have paired chromosomes, so how did it evolve into a hexaploid? Ahuja and Neale used a variety of methods to compare the giant tree to other members of its family. These include analyzing fossil patterns, physical characteristics, embryonic development patterns, pictures of chromosomes, the structure of molecules within cells, and the structure of pollen and stomata (cells in the epidermis that function much like human sweat glands by releasing water). Their work reveals that the coast redwood is most closely related to the dawn-redwood (Metasequoia) and the giant sequoia (Sequoiadendron) and that these three are more closely related to each other than they are to other members of their family.

Their results suggest that the coast redwood most likely became a hexaploid in the Cretaceous period (~144 – 65 million years ago) in one of three ways: the number of chromosomes per set multiplied in one ancestor, the genomes of two ancestors hybridized to yield additional sets, or the genomes of three ancestors hybridized.

“The current working hypothesis is the two-ancestor option,” says Dr. Neale.

In addition to the hexaploid nature of the redwood genome, a mosaic of microscopic fungi living in the cells of redwood leaves contribute their own set of genes to the tree’s biological complexity. Organisms attempting to live on or attack redwoods encounter the defensive resiliency built into such complexity. More genes translate to more options for defense against intruders. This means that a large redwood tree or a clonal clump of redwood sprouts may even present as much chemical and physical diversity to its potential pests and pathogens as does a whole field of genetically variable wildflowers.

Further research, especially sequencing genomes of the Taxodiaceae species and comparing them to coast redwood’s genome, will get us closer to understanding this genetic anomaly.

Dr. Ahuja and Dr. Neale’s report was published in the journal Silvae Genetica under the title “Origins of polyploidy in coast redwood (Sequoia sempervirens (D.Don) Endl.) and relationship of coast redwood to other genera of Taxodiaceae”

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