Genome evolution

Conserved non-coding sequences (CNS) are islands of non-coding sequence that, like protein coding exons, show less divergence in sequence between related species than functionless DNA. Several CNSs have been demonstrated experimentally to function as cis-regulatory regions. However, the specific functions of most CNSs remain unknown. Previous searches for CNS in plants have either anchored on exons and only identified nearby sequences or required years of painstaking manual annotation. Here we present an open source tool that can accurately identify CNSs between any two related species with sequenced genomes, including both those immediately adjacent to exons and distal sequences separated by >12 kb of non-coding sequence. We have used this tool to characterize new motifs, associate CNSs with additional functions, and identify previously undetected genes encoding RNA and protein in the genomes of five grass species. We provide a list of 15,363 orthologous CNSs conserved across al...

****The contents of Chapter 2, regarding Selfish DNA, is currently in prep for publication. Based on comments from my defense the eventual form of this paper will change a bit with some additional content added and a probable change in tone

Transposable elements (TEs), mobile pieces of self-replicating DNA, are one of the driving forces behind genomic evolution in eukaryotic organisms. Their contribution to genome size variation and as mutagens has led researchers to pursue their study in the hope of better understanding the evolution of genomic properties and organismal phenotypes But TEs can also be thought of in a multi-level evolutionary context, with TEs best understood as evolving populations residing within (and interacting with) the host genome. I argue, with empirical evidence from the literature, that the multi-level approach advocated by the classic “selfish DNA” papers of 1980 has become less commonly invoked over the past 35 years, in a favour of a strictly organism-centric view. I also make the case that an exploration of evolution at the level of TEs within genomes is required, one which articulates the similarities and differences between a TE population and a traditional population of organisms. A comprehensive analysis of sequenced eukaryote genomes outlines the landscape of how TE superfamilies are distributed, but also reveals that how TEs are reported needs to be addressed. A proper exploration of evolution at the TE level will require a dramatic change to how TE information is annotated, curated, and stored, and I make several specific recommendations in this regard.

Over the past few decades, plant genomics research has been studied extensively bringing about a revolution in the field of plant biotechnology. Molecular markers, useful for plant genome analysis, have now become an important tool in crop improvement. The development and use of molecular markers for the detection and exploitation of DNA polymorphism is one of the most significant developments in the field of molecular genetics. The presence of various types of molecular markers, and differences in their principles, methodologies and applications require careful consideration in choosing one or more of such methods. No molecular markers are available yet that fulfill all requirements needed by researchers. In this article we attempt to review most of the available DNA markers that can be routinely employed in various aspects of plant genome analysis such as characterization of genetic variability, genome fingerprinting, genome mapping, gene localization, analysis of genome evolution, population genetics, taxonomy, plant breeding, and diagnostics. The emerging patterns make up a unique feature of the analyzed individual and are currently considered to be the ultimate tool for biological individualization.

The crop species within the genus Brassica have highly replicated genomes. Three base 'diploid' species, Brassica oleracea , B. nigra and B. rapa , are likely ancient polyploids, and three derived allopolyploid species, B. carinata , B. juncea and B. napus , are created from the interspecific hybridization of these base genomes. The base Brassica genome is thought to have hexaploid ancestry, and both recent and ancient polyploidization events have been proposed to generate a large number of genome rearrangements and novel genetic variation for important traits. Here, we revisit and refine these hypotheses. We have examined the B. oleracea linkage map using the Arabidopsis thaliana genome sequence as a template and suggest that there is strong evidence for genome replication and rearrangement within the base Brassicas, but less evidence for genome triplication. We show that novel phenotypic variation within the base Brassicas can be achieved by replication of a single gene, BrFLC , that acts additively to influence flowering time. Within the derived allopolyploids, intergenomic heterozygosity is associated with higher seed yields. Some studies have reported that de novo genomic variation occurs within derived polyploid genomes, whereas other studies have not detected these changes. We discuss reasons for these different findings. Large trans-locations and tetrasomic inheritance can explain some but not all genomic changes within the polyploids. Transpositions and other small-scale sequence changes probably also have contributed to genomic novelty. Our results have shown that the Brassica genomes are remarkably plastic, and that polyploidy generates novel genetic variation through gene duplication, intergenomic heterozygosity and perhaps epigenetic change.

SUMMARY Interspecific hybridization is a significant evolutionary force as well as a powerful method for crop breeding. Partial substitution of the AA subgenome in Brassica napus (A n A n C n C n) with the Brassica rapa (A r A r) genome by two rounds of interspecific hybridization resulted in a new introgressed type of B. napus (A r A r C n C n). In this study, we construct a population of recombinant inbred lines of the new introgressed type of B. napus. Microsatellite, intron-based and retrotransposon markers were used to characterize this experimental population with genetic mapping, genetic map comparison and specific marker cloning analysis. Yield-related traits were also recorded for identification of quantitative trait loci (QTLs). A remarkable range of novel genomic alterations was observed in the population, including simple sequence repeat (SSR) mutations, chromosomal rearrangements and retrotransposon activations. Most of these changes occurred immediately after interspecific hybridization, in the early stages of genome stabilization and derivation of experimental lines. These novel genomic alterations affected yield-related traits in the introgressed B. napus to an even greater extent than the alleles alone that were introgressed from the A r subgenome of B. rapa, suggesting that genomic changes induced by interspecific hybridization are highly significant in both genome evolution and crop improvement.

We have constructed a 1736-locus maize genome map containing1156 loci probed by cDNAs, 545 probed by random genomic clones, 16 by simple sequence repeats (SSRs), 14 by isozymes, and 5 by anonymous clones. Sequence information is available for 56% of the loci with 66% of the sequenced loci assigned functions. A total of 596 new ESTs were mapped from a B73 library of 5-wk-old shoots. The map contains 237 loci probed by barley, oat, wheat, rice, or tripsacum clones, which serve as grass genome reference points in comparisons between maize and other grass maps. Ninety core markers selected for low copy number, high polymorphism, and even spacing along the chromosome delineate the 100 bins on the map. The average bin size is 17 cM. Use of bin assignments enables comparison among different maize mapping populations and experiments including those involving cytogenetic stocks, mutants, or quantitative trait loci. Integration of nonmaize markers in the map extends the resources available fo...

The green lineage (Viridiplantae) comprises the green algae and their descendants the land plants, and is one of the major groups of oxygenic photosynthetic eukaryotes. Current hypotheses posit the early divergence of two discrete clades from an ancestral green flagellate. One clade, the Chlorophyta, comprises the early diverging prasinophytes, which gave rise to the core chlorophytes. The other clade, the

The prominent repair mechanism of DNA double-strand breaks formed upon excision of the maize Ac transposable element is via nonhomologous end joining. In this work we have studied the role of homologous recombination as an additional repair pathway. To this end, we developed an assay whereby beta-Glucuronidase (GUS) activity is restored upon recombination between two homologous ectopic (nonallelic) sequences in transgenic tobacco plants. One of the recombination partners carried a deletion at the 5' end of GUS and an Ac or a Ds element inserted at the deletion site. The other partner carried an intact 5' end of the GUS open reading frame and had a deletion at the 3' end of the gene. Based on GUS reactivation data, we found that the excision of Ac induced recombination between ectopic sequences by at least two orders of magnitude. Recombination events, visualized by blue staining, were detected in seedlings, in pollen and in protoplasts. DNA fragments corresponding to rec...