Biomedical subjects

L E Talbert

Publications and source records attributed to L E Talbert.

5 recordsLinked to original sources

PubMed1995

Molecular analysis of evolutionary patterns in U genome wild wheats.

The theory of pivotal-differential evolution states that one genome of polyploid wheats remains stable (i.e., pivotal) during evolution, while the other genome or genomes may become modified (i.e., differential). A proposed mechanism for apparent modification of the differential genome is that different polyploid species with only one genome in common may exchange genetic material. In this study, we analyzed a set of sympatric and allopatric accessions of tetraploid wheats with the genomic constitutions UM and UC. The U genome of these species is from Triticum umbellulatum and is considered to be the pivotal genome. The M and C genomes, from T. comosum and T. dichasians, respectively, are considered to be the differential genomes. Low copy DNA was analyzed using "sequence tagged site" primer sets in the polymerase chain reaction, followed by digestion with restriction enzymes. Genetic similarity matrices based on shared restriction fragments showed that sympatric accessions of different U genome tetraploid species did not tend to share more restriction fragments than did allopatric accessions. Thus, no evidence for introgression was found. Analysis of the diploid progenitor species showed that the U genome was less variable than the M and C genomes. Additionally, comparison of diploid and polyploid species using genome-specific primer sets suggests a possible polyphyletic origin for T. triunciale and T. machrochaetum. Thus, our results suggest that the differential nature of the M and C genomes may be the result of variability introduced by the diploid progenitors and not the result of frequent introgression events after formation of the polyploid.

Base Sequence↗

PubMed1995

Variability in wheat based on low-copy DNA sequence comparisons.

The chromosomes of the B genome of hexaploid wheat (AABBDD) do not pair completely with those of any of the diploid species with genomes similar to B. Various biochemical and molecular analyses have suggested that each of the five diploid species in section Sitopsis of Triticum are ancestral to B. These observations have led to the hypothesis that the B genome may be polyphyletic, descending from more than one diploid ancestor. This hypothesis may account for differences between the wheat B genome and the diploids and also for variability that currently exists among different wheat accessions. In this study, we cloned and compared nucleotide sequences for three low-copy DNA fragments from the B and D genomes of several wheat accessions and from diploid relatives of the B and D genomes. Our results suggested that the amount of DNA sequence variability in wheat is low, although somewhat more variability existed in the B genome than in the D genome. The B genome of wheat was significantly diverged from all the Sitopsis diploid species, and Triticum speltoides was closer to B than to other members of this section. The D genome of wheat was very similar to that of its progenitor, Triticum tauschii. No evidence for a polyphyletic origin of the B genome was found. A more parsimonious hypothesis is that the wheat B genome diverged from its diploid ancestor after the original hybridization event occurred.

Cloning, Molecular↗

PubMed1989

Mu transposable elements are structurally diverse and distributed throughout the genus Zea.

The Robertson's Mutator stock of maize exhibits a high mutation rate due to the transposition of the Mu family of transposable elements. All characterized Mu elements contain similar approximately 200-bp terminal inverted repeats, yet the internal sequences of the elements may be completely unrelated. Non-Mutator stocks of maize have a 20-100-fold lower mutation rate relative to Mutator stocks, yet they contain multiple sequences that hybridize to the Mu terminal inverted repeats. Most of these sequences do not cohybridize to internal regions of previously cloned Mu elements. We have cloned two such sequences from the maize line B37, a non-Mutator inbred line. These sequences, termed Mu4 and Mu5, have an organization characteristic of transposable elements and possess approximately 200-bp Mu terminal inverted repeats that flank internal DNA, which is unrelated to other cloned Mu elements. Mu4 and Mu5 are both flanked by 9-bp direct repeats as has been observed for other Mu elements. However, we have no direct evidence that they have recently transposed because they have not been found in known genes. Although the internal regions of Mu4 and Mu5 are not related by sequence similarity, both elements share an unusual structural feature: the terminal inverted repeats extend more than 100 bp internally from Mu-similar termini. The distribution of these elements in maize lines and related species suggests that Mu elements are an ancient component of the maize genome. Moreover, the structure of the Mu termini and the fact that Mu termini are found flanking different internal sequences leads us to speculate that Mu termini once may have been capable of transposing as independent entities.

Base Sequence↗

PubMed1988

Sequence, genomic distribution and DNA modification of a Mu1 element from non-mutator maize stocks.

The increased mutation rate of Mutator stocks of maize has been shown to be the result of transposition of Mu elements. One element, Mu1, is present in 10-60 copies in Mutator stocks and approximately 0-3 copies in non-Mutator stocks. The sequence, structure and genomic distribution of an intact Mu1 element cloned from the non-Mutator inbred line B37 has been determined. The sequence of this element, termed Mu1.4-B37, is identical to Mu1 and it is flanked by 9-bp direct repeats indicative of a target site duplication. Mu1.4-B37 is not in the same genomic location in all stocks, which further suggests that it transposed into its genomic location in B37. We previously reported that in genomic DNA this element is modified such that certain methylation-sensitive restriction enzymes will not cut sites within the element. This is similar to that observed for Mu elements in Mutator stocks that have lost activity. We report herein that the Mu1.4-B37 element loses its modification and becomes accessible to digestion when placed in an active Mutator stock by genetic crosses. This suggests that factors conditioning unmodified elements are dominant in the initial cross between Mutator and non-Mutator stocks. In F2 individuals that have subsequently lost Mutator activity the Mu1.4-B37 element again becomes modified as do most of the Mu elements in the stock. Thus, the modification state of the Mu1.4-B37 element and the other Mu1-like elements correlates with Mutator activity. We hypothesize that factor(s) within an active Mutator stock may inhibit the modification of Mu elements, and that this activity is missing in non-Mutator stocks and may become limiting in certain Mutator stocks resulting in DNA modification.

Base Sequence↗

PubMed1988

Characterization of a highly conserved sequence related to mutator transposable elements in maize.

Mutator stocks of maize exhibit a high mutation rate correlated with the activity of a family of transposable elements. Mu1 and, to a lesser extent, the closely related Mu1.7 elements are responsible for most mutator-induced mutations that have been characterized. These elements are found in 10-60 copies in mutator stocks, and zero to a few intact elements exist in nonmutator maize stocks. Additionally, the component parts of Mu elements exist separately in the maize genome. The Mu terminal inverted repeats are found in multiple copies in all maize lines and related Zea species tested, and Mu internal sequences exist unassociated with Mu termini. In the present paper, we describe the structure and genomic distribution of one Mu-homologous sequence termed MRS-A (for Mu-related sequence). DNA sequencing shows that MRS-A is closely related to the internal region of Mu1 and Mu1.7 elements. However, it has no Mu termini and does not have the structure of a transposable element. This sequence is present in one or two copies in all maize lines and is highly conserved in the genus Zea. A similar sequence exists in a species within the genus most closely related to Zea, Tripsacum dactyloides, although the T. dactyloides genome does not contain any Mu termini or intact Mu elements. Furthermore, an RNA transcript homologous to MRS-A and its flanking DNA is found in both mutator and nonmutator maize plants. These results suggest that MRS-A represents a stable, functional region of the maize genome, and we speculate that a similar sequence was encompassed by Mu termini to generate a Mu transposable element.

Base Sequence↗