Evolution of Ac and Dsl elements in select grasses (Poaceae)

Abstract

We present data on evolution of the Ac/Ds family of transposable elements in select grasses (Poaceae). An Ac-like element was cloned from a DNA library of the grass Pennisetum glaucum (pearl millet) and 2387 bp of it have been sequenced. When the pearl millet Ac-like sequence is aligned with the corresponding region of the maize Ac sequence, it is found that all sequences corresponding to intron II in maize Ac are absent in pearl millet Ac. Kimura's evolutionary distance between maize and pearl millet Ac sequences is estimated to be 0.429 +/- 0.020 nucleotide substitutions per site. This value is not significantly different from the average number of synonymous substitutions for coding regions of the Adh1 gene between maize and pearl millet, which is 0.395 +/- 0.051 nucleotide substitutions per site. If we can assume Ac and Adh1 divergence times are equivalent between maize and pearl millet, then the above calculations suggest Ac-like sequences have probably not been strongly constrained by natural selection. The level of DNA sequence divergence between maize and pearl millet Ac sequences, the estimated date when maize and pearl millet diverged (25-40 million years ago), coupled with their reproductive isolation/lack of current genetic exchange, all support the theory that Ac-like sequences have not been recently introduced into pearl millet from maize. Instead, Ac-like sequences were probably present in the progenitor of maize and pearl millet, and have thus existed in the grasses for at least 25 million years. Ac-like sequences may be widely distributed among the grasses. We also present the first 2 Ds1 controlling element sequences from teosinte species: Zea luxurians and Zea perennis. A total of 10 Ds1 elements had previously been sequenced from maize and a distant maize relative, Tripsacum. When a maximum likelihood network of genetic relationships is constructed for all 12 sequenced Ds1 elements, the 2 teosinte Ds1 elements are as distant from most maize Ds1 elements and from each other, as the maize Ds1 elements are from one another. Our new teosinte sequence data support the previous conclusion that Ds1 elements have been accumulating mutations independently since maize and Tripsacum diverged. We present a scenario for the origin of Ds1 elements.

DNA Transposons and the Evolution of Eukaryotic Genomes

“…Two hypotheses can be put forward to explain this paradox. First, some MITEs may arise de novo from the fortuitous juxtaposition of solo TIRs or sequences resembling the TIRs of an autonomous transposon (127,183). A second possibility is that MITE progenitors are the relics of the past invasion of transposons whose autonomous copies have been erased or have not reached fixation within the population (53).…”

Molecular Characterization of the Abp1 5′-Flanking Region in Maize and the Teosintes

“…L08425) as a query also revealed that the 5Ј upstream-most region (870-1240 bp upstream of the ATG) shares similarity with the Ds1 element insertion of the maize Bz1 gene (Schiefelbein et al, 1988; EMBL accession no. X14155) and with a Ds1 insertion in Zea perennis (MacRae and Clegg, 1992; EMBL accession no. X54711).…”

Three novel families of miniature inverted-repeat transposable elements are associated with genes of the yellow fever mosquito,Aedes aegypti

“…The mechanism of transposition of MITE-like elements has not yet been clearly elucidated (6,12). However, a DNA-mediated mechanism seems likely because a few MITE-like elements have been found to have terminal sequences almost identical to those of some DNAmediated elements that have coding potentials (9,10,13). On the other hand, MITE-like elements seem to differ from other nonautonomous deletion derivatives of DNA-mediated elements in being present in high copy numbers and in being relatively homogeneous in size within each family or subfamily (8,12), indicating that they are units of highly successful transposition.…”