Variation and Evolution in Plants and Microorganisms Toward a New Synthesis 50 Years After Stebbins (2000) / Chapter Skim
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11 Maize as a Model for the Evolution of Plant Nuclear Genomes
11 Maize as a Model for the Evolution of Plant Nuclear Genomes
Pages 187-210
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From page 187... ...
Retrotransposon multiplication has been relatively recentwithin the last 5-6 million years suggesting that the proliferation of retrotransposons has also contributed to differences in DNA content between sorghum and maize. There are still unanswered questions about repetitive DNA, including the distribution of repetitive DNA throughout the genome, the relative impacts of retrotransposons and chromosomal duplication in plant genome evolution, and the hypothesized correlation of duplication events with transposition.
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From page 188... ...
Even after these genomes are sequenced, it will still be a tremendous challenge to understand the evolution of plant nuclear genomes, like the maize genome, for which entire DNA sequences will not be readily available. Maize is a member of the grass family (Poaceae)
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From page 189... ...
Altogether, cytological observations suggested that the maize genome contains extensive regions of homology, probably reflecting chromosomal duplications. Evidence for chromosomal duplication also came from linkage information.
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From page 190... ...
The sequence data were consistent with a segmental allotetraploid model of origin but inconsistent with the other two models of polyploid formation. Hence, the authors concluded that the maize genome was the product of a segmental allotetraploid event.
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From page 191... ...
1 places the segmental allotetraploid event in a phylogenetic context, and this context raises three important points about the comparison of maize to sorghum. First, if the allotetraploid event occurred after maize and sorghum diverged, then the maize genome should be duplicated more extensively than the sorghum genome.
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From page 192... ...
contains errors regarding the timing and phylogenetic context of the allotetraploid event. For example, they suggest that the allotetraploid event occurred after the divergence of maize and Tripsacum, whereas most evidence suggests that the allotetraploid event occurred before the divergence of maize and Tripsacum.
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From page 193... ...
1~. Based on this information, differences in DNA content probably reflect the allopolyploid event and additional evolutionary changes, such as the accumulation of repetitive DNA.
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From page 194... ...
First, different studies use different data, leading to different conclusions. Second, and perhaps more importantly, researchers rarely denote their criteria for defining chromosomal duplications, and thus criteria likely differ among studies.
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From page 195... ...
There is no doubt that polyploidy is common in plants, with up to 70% of angiosperms owing their history to polyploidy (Masterson, 1994; Stebbins,1950~. Furthermore, genetic maps demonstrate that a great number of species contain chromosomal duplications.
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From page 196... ...
First, Southern blot and other analyses suggest that the retrotransposon families in the Adhl region comprise at least 50% of the maize genome; altogether, just three of the retroelement families found in the Adhl region constitute a full 25% of the genome (SanMiguel et al., 1996~. Second, 85% of repetitive DNAs from other regions were also
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From page 197... ...
. The Timing of Retrotransposon Multiplication Maize repetitive DNA seems to be primarily retrotransposons, but the second question remains: when did these retroelements multiply?
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From page 198... ...
If the Adhl region is representative and the retrotransposons in this region constitute 50% of the genome, the maize genome has doubled in size in the last 5-6 million years. Like the polyploid event, retrotransposon proliferation represents a doubling of genome content over a relatively short evolutionary time scale.
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From page 199... ...
This prediction holds in Arabidopsis, where sequence data from chromosomes 2 and 4 indicate an increase in the frequency of transposable elements near centromeres (Copenhaver, 1999~. There are other reasons to suggest that retrotransposon distribution may not be homogeneous throughout the maize genome.
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From page 200... ...
It is premature to make the general statement that repeat proliferation contributes more to the evolution of DNA content than chromosomal duplications for two reasons. First, as mentioned previously, mapping studies are biased against the discovery of duplications, and for this reason, there is as yet no accurate indication of the extent of chromosomal duplication in complex genomes.
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From page 201... ...
In Drosophila, for example, genetic diversity varies along the chromosome as a function of recombination rate (Begun and Aquadro, 1992; Hamblin and Aquadro, 1999~. Loci near centromeres tend to have low recombination rates and also tend to have low levels of genetic diversity, but both recombination rate and genetic diversity increase toward the tip of chromosomes.
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From page 202... ...
To measure genetic diversity in each gene, we will sample DNA sequences from ~70 individuals representing maize, its progenitor, and two other wild Zea taxa. The project has many long-term goals, including (i)
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From page 203... ...
Taxa are abbreviated as follows: maize, domesticated maize; parv, ancestor of domesticated maize (Z. mays subsp.
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From page 204... ...
In crops, artificial selection can cause discordant patterns of genetic diversity among loci. Thus far, levels of nucleotide sequence diversity have been measured in maize and its wild progenitor (Z.
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From page 205... ...
Our ongoing study of 100 genes will help determine whether patterns of evolutionary history among genes are, in fact, correlated with chromosomal location and will also contribute to the overall understanding of the evolutionary forces acting on plant genomes. The authors acknowledge National Science Foundation Grants DBI9872631 and DEB-9815855 and U.S.
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From page 206... ...
(1998) Restriction fragment length polymorphism and divergence in the genomic regions of high and low recombination in self-fertilizing and cross-fertilizing Aegilops species.
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From page 207... ...
(1988) Identification of the genomic locations of duplicate nucleotide sequences in maize by analysis of restriction fragment length polymorphism.
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From page 208... ...
(1998) Vertebrate genome evolution and the zebrafish gene map.
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From page 209... ...
(1996) Nested retrotransposons in the intergenic regions of the maize genome.
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From page 210... ...
(1986) Duplicated chromosomal segments in Zea mays L.: further evidence from Hexokinase isozymes.
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