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narrow G:C content (1% of the total range), and this narrow fraction corresponds to 17% of the DNA content of the genome. To explain this hybridization pattern, Bernardi and coworkers (Barakat et al., 1997; Carels et al., 1995) reasoned that maize coding genes must be located in “gene-rich” regions and that these gene-rich regions must be flanked by DNA with highly homogeneous G:C contents. They proposed that this flanking DNA could consist of retrotransposons like those flanking the Adh1 gene (San Miguel et al., 1996).

The results from G:C fractionation experiments and studies of the Adh1 region are inconsistent. On the one hand, the study of the Adh1 region, coupled with studies of centromeres and knobs, suggest that retrotransposon distribution is widespread, representing 50% of the genome. On the other hand, Bernardi and coworkers' work implicitly suggests that retrotransposon distributions are heterogeneous, with a higher concentration of retroelements in the 17% of the genome that represents coding DNA. Ultimately, there may be a resolution to differences implied by different studies, but such a resolution will require more sequencing of large chromosomal clones representing diverse genomic regions.

Question 2. What contributes more to the evolution of DNA content: multiplication of repetitive DNA or chromosomal duplication? The evolutionary history of maize suggests that retrotransposon multiplication and chromosomal duplication (by way of polyploidy) each have generated a 2-fold increase in DNA content within the last 16 million years. Hence, the net effect of these two evolutionary processes is similar in maize. In contrast, it seems that the multiplication of repeat sequences is the primary contributor to differences in DNA content between many taxa (Flavell et al., 1974). For example, barley and rice have similar complements of low-copy genes (Saghai-Maroof et al., 1996) but a 12-fold difference in DNA content (Fig. 1). The difference in DNA content is thus probably attributable to differences in the amount of repetitive DNA (Saghai-Maroof et al., 1996).

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. Second, duplication and repeat proliferation are not independent. Duplication plays a role in repeat proliferation, because duplication doubles repetitive DNA as well as low-copy DNA.

Question 3. Are chromosomal duplication events correlated with an increase in the rate of transposition? This question originates from the work



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