Variation and Evolution in Plants and Microorganisms Toward a New Synthesis 50 Years After Stebbins (2000) / Chapter Skim
Currently Skimming:
17 The Role of Genetic and Genomic Attributes in the Success of Polyploids
17 The Role of Genetic and Genomic Attributes in the Success of Polyploids
Pages 310-330
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 310... ...
This mode of formation incorporates genetic diversity from multiple progenitor populations into the polyploid "species"; thus, genetic diversity in polyploid species is much higher than expected School of Biological Sciences, Washington State University, Pullman, WA 991644236 This paper was presented at the National Academy of Sciences colloquium "Variation and Evolution in Plants and Microorganisms: Toward a New Synthesis 50 Years After Stebbins," held January 27-29, 2000, at the Arnold and Mabel Beckman Center in Irvine, CA.
|
From page 311... ...
devoted two chapters to polyploidy and addressed the following issues: the frequency, taxonomic distribution, and geographic distribution of polyploidy; the origins of polyploidy and factors promoting polyploidy; the direct effects of polyploidy; the polyploid complex; the success of polyploids in extreme habitats (including weeds) ; ancient polyploidy; and the role of polyploidy in the evolution and improvement of crops.
|
From page 312... ...
In fact, many aspects of the genetic systems of polyploids may contribute to the success of polyploid plants. These characteristics range from the molecular level to the population level and include increased heterozygosity, reduced inbreeding depression and an associated increase in selfing rates, increased genetic diversity through multiple formations of a polyploid species, genome rearrangements, and ancient polyploidy and gene silencing.
|
From page 313... ...
OUTCROSSING RATES IN POLYPLOIDS AND THEIR DIPLOID PROGENITORS Some aspects of polyploid success have been attributed to improved colonizing ability, which may involve higher selfing rates than those of
|
From page 314... ...
Theoretical models predict reduced inbreeding depression in polyploids relative to their diploid parents, because of the buffering effect of additional genomes: Deleterious alleles are masked by the extra genomes (Stebbins, 1971; Richards, 1986; Barrett and Shore, 1989~. Both allopolyTABLE 2.
|
From page 315... ...
These data were interpreted as evidence for reduced inbreeding depression in the tetraploid, with the lower inbreeding depression allowing for increased selfing rates. There are few estimates of selfing rates in polyploid fern species, largely because polyploid fern populations often lack sufficient levels of segregating allozyme markers; however, selfing rates have been estimated in a few diploid-tetraploid pairs.
|
From page 316... ...
THE GENETIC IMPLICATIONS OF RECURRENT POLYPLOID FORMATION The application of isozyme analysis and DNA techniques to the study of polyploid ancestry dramatically altered our view of polyploid origins. Although morphological or cytological differences among populations of a few polyploid species suggested evidence of repeated polyploid formation (see example in Ownbey, 1950)
|
From page 317... ...
Recurrent formation of a polyploid species has implications for the taxonomy of polyploids, our understanding of the ease with which and rate at which polyploidization can occur, and, most relevant here, the genetic diversity of polyploid "species." In this section, we will address (i) the proportion of polyploid plant species that are known to have formed recurrently, (ii)
|
From page 318... ...
mirus in different locations had the same isozyme multilocus genotype, chloroplast DNA haplotype, and rDNA repeat, and, in many cases, they co-occurred with the diploid progenitor species, T dubius and T
|
From page 319... ...
The concept of recurrent formations forces us to consider polyploid species not as genetically uniform, as previous models of polyploid formation imply, but as genetically variable. In fact, multiple formations may represent a significant source of genetic diversity in polyploid species, as a polyploid species may comprise multiple, genetically different lineages.
|
From page 320... ...
ANCIENT POLYPLOIDY AND GENE SILENCING Basal Angiosperms Estimates of ancient polyploidy generally have relied on chromosome number alone; Stebbins (1950) , for example, viewed those plants with a base chromosome number of n = 12 or higher to be polyploid, and others (Goldblatt, 1980; Grant, 1981, 1982)
|
From page 321... ...
First, if they are indeed polyploids, then these plants should exhibit extra copies of their genes above the level that TABLE 3. Angiosperm families with high chromosome numbers, suggested to be of ancient polyploid origin (Stebbins, 1950)
|
From page 322... ...
Clades with families of putative ancient polyploid origin are indicated in bold. Numbers below branches are jackknife support values.
|
From page 323... ...
At least two possible explanations can explain this paradox of high chromosome numbers and genetic diploidy. First, these plants are ancient polyploids that have undergone extensive gene silencing to produce genetic diploids, and second, they may have achieved high chromosome numbers through another mechanism, such as chromosomal fission.
|
From page 324... ...
If this species is of ancient polyploid origin but has since undergone substantial gene silencing, then pseudogenes should be detectable in the genome. Five clones of the CAB genes were analyzed by Pichersky et al.
|
From page 325... ...
Polyploids also harbor higher levels of genetic and genomic diversity than was anticipated, with recurrent formation from genetically divergent diploid parents and possibly genome rearrangements contributing genetic diversity. This genetic diversity results in greater biochemical diversity, which also may be beneficial to the polyploid (Levin, 1983~.
|
From page 326... ...
How extensive has gene silencing been, and is there evidence for the cooption of duplicated genes for new function? The study of polyploidy is a dynamic and open area of research, ranging from molecular genetic comparisons to population genetics, with important implications for the biology and evolution of the majority of plant species.
|
From page 327... ...
richardii (Parkeriaceae) by restriction fragment length polymorphisms.
|
From page 328... ...
(1989) Chloroplast DNA variation and multiple origins of autopolyploidy in Heuchera micrantha (Saxifragaceae)
|
From page 329... ...
(1989) Tetrasomic inheritance and chromosome pairing behaviour in the naturally occurring autotetraploid Heuchera grossulariifolia (Saxifragaceae)
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.