The role of genetic and genomic attributes in the success of polyploids

In 1950. G. Ledyard Stebbins devoted two chapters of his book variation and Evolution in Plants (Columbia Univ. Press. New York) to polyploidy, one on occurrence and nature and one on distribution and significance. Fifty year s later, many of the questions Stebbins posed have not been answered, and m any new questions have arisen. In this paper, we review some of the genetic attributes of polyploids that have been suggested to account for the treme ndous success of polyploid plants. Based on a limited number of studies, we conclude: (i) Polyploids. both individuals and populations, generally main tain higher levels of heterozygosity than do their diploid progenitors. (ii ) Polyploids exhibit less inbreeding depression than do their diploid paren ts and can therefore tolerate higher levels of selfing; polyploid ferns ind eed have higher levels of selfing than do their diploid parents, but polypl oid angiosperms do not differ in outcrossing rates from their diploid paren ts. (iii) Most polyploid species are polyphyletic. having formed recurrentl y from genetically different diploid parents. This mode of formation incorp orates genetic diversity from multiple progenitor populations into the poly ploid "species"; thus, genetic diversity in polyploid species is much highe r than expected by models of polyploid formation involving a single origin. (iv) Genome rearrangement may be a common attribute of polyploids. based o n evidence from genome in situ hybridization (GISH). restriction fragment l ength polymorphism (RFLP) analysis, and chromosome mapping. (v) Several gro ups of plants may be ancient polyploids. with large regions of homologous D NA. These duplicated genes and genomes can undergo divergent evolution and evolve new functions. These genetic and genomic attributes of polyploids ma y have both biochemical and ecological benefits that contribute to the succ ess of polyploids in nature.