Showing papers by "Elizabeth A. Zimmer published in 1990"
TL;DR: The findings suggest that not only is introgressive hybridization presently occurring in parapatry between I. fulva and I. hexagona, but that past hybridization between these species has resulted in introgression into areas of allopatry.
Abstract: Genetic variation associated with the natural hybridization of Iris fulva and I. hexagona was investigated to test for the occurrence of introgression. These species have been viewed as a classic example of the process of introgressive hybridization (Anderson, 1949). However, more recent studies have concluded that there has not been an exchange of genetic material between I. fulva and I. hexagona (Randolph et al., 1967). Our analysis has involved the examination of both allopatric and parapatric populations of I. fulva and I. hexagona with reference to diagnostic ribosomal DNA markers. The pattern of variation in the parapatric population indicates the presence of the repeated backcrossing necessary to the process of introgressive hybridization. Indeed, in the region of parapatry, we suggest that localized introgression of ribosomal sequences has occurred into both I. fulva and I. hexagona. Significantly, we have also detected the presence of the diagnostic ribosomal markers from each species in allopatric populations of the alternate species. Our findings suggest that not only is introgressive hybridization presently occurring in parapatry between I. fulva and I. hexagona, but that past hybridization between these species has resulted in introgression into areas of allopatry.
101 citations
TL;DR: Advantages of sequence data for reconstructing evolutionary trees include their wide scope, the large number of characters, the easier use of objective methods for building and testing trees, the use of information from mechanisms of nucleotide changes, the lower cost of obtaining information, and the predictability of finding useful characters.
Abstract: Advantages of sequence data for reconstructing evolutionary trees include their wide scope, the large number of characters, the easier use of objective methods for building and testing trees, the use of information from mechanisms of nucleotide changes, the lower cost of obtaining information, and the predictability of finding useful characters. There are however still many problems estimating the reliability of the results of tree reconstruction. These are discussed, with examples, under the three headings of sampling error, methodological problems, and human errors. The methodological problems are the hardest to solve. They include the large number of trees, incomplete use information, inconsistency (converging to an incorrect tree), problems derived from unknown selection pressures on sequences, and trees being an inappropriate model. To overcome these problems, a good method for reconstructing trees should have the properties of being fast, efficient, consistent, robust and falsifiable. Considerable progress has been made but present methods are still best considered as 'Exploratory Data Analysis' (EDA) techniques.
48 citations
TL;DR: Results suggest that unmethylated and Eco RI polymorphic sites occur in the same repeat units in the rDNA array.
Abstract: The restriction endonucleases Hpa II and Msp I were used to examine cytosine methylation in the ribosomal RNA genes (rDNA) of inbred lines of maize and species of teosinte. In all of the rDNAs examined, Msp I (not sensitive to mCpG) digestion yielded a distribution of lower molecular weight fragments indicative of multiple recognition sites. The majority of the rDNA arrays in an individual were inaccessible to Hpa II (sensitive to mCpG) cleavage, but a significant fraction (10–25%) was cleaved at least once by Hpa II into repeat unit length fragments (9.1 kbp). In some maize inbred lines, one or two additional fragment populations (less than 9.1 kbp in length) were also produced by Hpa II digestion. All of the unmethylated Hpa II sites mapped to the intergenic spacer (IGS), and the major unmethylated site was located approximately 800 bp 5′ to the start of the 18S RNA coding sequence. An Eco RI polymorphism, present in the 26S gene of certain inbred lines and hybrids, was utilized to investigate the organization of unmethylated repeat units in the rDNA array. In double digest experiments with Hpa II/Eco RI, the fragments from repeat units with two Eco RI sites were sensitive to Hpa II digestion, whereas, the fragments from repeat units with a single Eco RI site were almost completely resistant to Hpa II digestion. Similar digestion patterns were also observed in Eco RII (sensitive to mCNG)/Eco RI digests. These results suggest that unmethylated and Eco RI polymorphic sites occur in the same repeat units.
22 citations
13 citations