Genome-wide transposon analyses: annotation, movement and impact on plant function and evolution

Abstract

The diversity of life forms around us is astounding: a walk in the woods, or even down the street, shows us organisms of different morphologies: two legs, four legs, wings; different capacity of interaction with our environment: plants photosynthesizing while bacteria break down our garbage. How can life take on so many forms? While there is some increase of genes when comparing the most simple eukaryotes to the most complex ones it is clear that organism complexity is not the result of the number of genes. Therefore is has been postulated that the complexity of an organism arises from the complexity of its gene regulation, rather than the number of genes. This regulation must come then from the non-gene part of the genome.

We now know that genes constitute but a small portion of genomes, (about 5% of the human genome). The advent of whole-genome sequencing has enabled us to get a more complete picture of what is in a genome, and with that has come the surprise that a significant part of all genomes characterized is constituted of transposable elements (TEs).

Transposable elements are mobile genetic sequences, meaning that they have the capacity to change their position within the genome of a single cell. The goal of my dissertation has been to investigate the role of TEs in plants and their impact on gene and genome evolution. For this I have taken two approaches. The first is a study in the newly sequenced genome of Cucumis melo, an important crop plant in Spain. In the context of this project I have characterized the transposon landscape in the genome, and identified TE related polymorphisms between seven different varieties. This project has of interest the fact that this is an important plant for agriculture and domestication is a particularly relevant evolutionary context in which to study the impact of transposons, as the lines analyzed come from different geographic and selection backgrounds. In the context of this project I have developed a pipeline for genome annotation, and a software for detection of polymorphisms using next-generation paired-end sequencing data. This yielded insight into the dynamics of transposon evolution in this genome, and the selective forces that have shaped the transposon landscape. To our knowledge this is the first analysis that uses polymorphic TEs to investigate differential chromosomal distribution of recent and old transposons, and thus revealing at an intra-species scale the timeline of selection. The results obtained are promising for studying the contribution of mobile elements to the evolution of two genetically similar yet phenotypically different cultivated varieties.

In order to study the impact of transposition on gene regulation, I investigated MITE families which have amplified a transciption factor binding site (TF BS) in the model plant Arabidopsis thaliana. This project focuses on the potential impact on gene regulation networks of the redistribution of this TFBS, a phenomenon that has been described for various master TF in animals but not yet to my knowledge in plants. This study combines in silico analysis with molecular data such as microarrays and ChIP, and is a striking example of one of the many manners in which TEs can impact gene regulation.

The work in this dissertation highlights the contradictory nature of transposable elements: on one hand, they are invasive, and on the other, are the source of essential innovations. Here we provide insight as to the functions they play in plant genome evolution.