Environment & Energy

Coast redwood most frequently reproduces by coppice (by sprouting) like quaking aspen.

Commercial and other planted redwoods are overwhelmingly clones. Redwood seeds are small and hard to handle and have very low viability but natural redwood seedlings do occur in nature. Redwood is one of only 3 conifer species with polyploid chromosomes.

Polyploid plants tend to live longer and get larger. Developing polyploid strains has long been a commercial tool as in breeding high yield varieties of rice and wheat.

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Coast Redwood May be the Descendent of Two

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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.

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https://www.savetheredwoods.org/grant/coast-redwood-may-be-the-descendent-of-two/

Why are triploid quaking aspen (Populus tremuloides) common?

Premise: Quaking aspen is a clonal tree species that has mixed ploidy, often with high relative abundance of both diploids and triploids but no haploids or tetraploids. Triploids typically have low fertility, leaving their occurrence apparently unlikely from an evolutionary perspective, unless they provide a "triploid bridge" to generating higher-fitness tetraploids-which are not observed in this species. This study focused on how triploidy can be maintained in quaking aspen.

Methods: A computational model was used to simulate gamete production, sexual reproduction, asexual reproduction, parent survival, and offspring survival in a population. All parameters were assumed to be cytotype-dependent and environment-independent. Sampling methods were used to identify parameter combinations consistent with observed cytotype frequencies.

Results: Many processes and parameter values were sufficient to yield a moderate frequency of triploids, and very few were necessary. The most plausible route involved higher triploid survival at the parent or offspring stage and limited unreduced gamete production by either diploid or triploid parents. Triploid fertility was helpful but not necessary.

Conclusions: The coexistence of diploids and triploids in quaking aspen is statistically likely and promoted by the existence of commonly observed, long-lived triploid clones. However, other mechanisms not captured by the model related to environmental variation could also occur.

https://pubmed.ncbi.nlm.nih.gov/38704729/