We addressed this with a series of experiments that revealed how female whiptail lizards ovulate unfertilized eggs carrying the full chromosome complement without loss of heterozygosity. We are now using a combination of ovarian tissue culture, confocal microscopy and electron microscopy to gain insights into the cellular and molecular mechanisms through which barriers to unisexuality are overcome. Interspecific hybridization endows clonal lineages with high levels of heterozygosity and plays a key role in the transition from sexual to parthenogenetic reproduction in vertebrates.
Ploidy elevation further increases heterozygosity as the additional genome expands the genetic repertoire that may permit adaptation to a changing environment or the colonization of new ecological niches. Indeed, two-thirds of parthenogenetic species of whiptail lizards are triploids and many of the diploid intermediates are thought to be extinct. Taking advantage of our laboratory colony, we have produced over hybrids between sexual males and parthenogenetic females.
These experiments resulted in the first tetraploid lizards that have founded new genetically isolated lineages. The availability of sterile hybrids and clonally reproducing lineages has opened a new angle for investigating the molecular basis of parthenogenetic reproduction and the causes for hybrid sterility. We will compare sterile and fertile animals using cytological and molecular approaches to decipher the elements that block parthenogenesis in some hybrids but not others.
We have already sequenced and assembled the genomes of three species of whiptail lizards and are in the process of developing a comprehensive genomic toolkit to shed light on the following topics:. These hypotheses form the framework for explaining why a lack of genetic diversity and limited ability to evolve would condemn obligate unisexuals to extinction.
We have conducted a small-scale study of microsatellites variability and found that some loci varied as much within a parthenogenetic species as they did in sexual species. We will derive a far more comprehensive understanding of the degree of diversity, mutation accumulation and epigenetic variation from the analysis of next-generation sequencing data.
Telomere biology in aging and cancer: early history and perspectives
But combining genomes from two or more species in one organism can also result in incompatibilities. The side-by-side comparison of first-generation hybrids with parthenogenetic individuals, that originated from the same parental species in nature many generations ago, provides a unique opportunity to examine adaptation to the presence of divergent genomes in the same cell and organism.
We will compare allele-specific gene expression in laboratory-synthesized hybrids with individuals of parthenogenetic species known to harbour the same two genomes for thousands of generations. These studies will provide insights into how allele-specific gene expression is established, fixed and maintained over time. Our lizard colonies together with genomic resources generated by our group provide a unique opportunity to elucidate the molecular basis of unisexual reproduction and to understand the barriers that limit parthenogenesis in most vertebrates.
Indeed, in analyzing the origin of telomerase, de Lange proposes a theoretical scheme for type II introns, coupled with the formation of primitive t-loops, to evolve into telomerase, independent of non-LTR retro-transpositions Lambowitz and Belfort, Nevertheless, the review by Servant and Deininger focuses on the use in extant organisms of non-LTR retro-transposition in telomerase-positive cells, providing an example of a mechanism that persists and even co-exists with telomerase through evolution.
The bottom line of these studies is the diversity of telomeric processes.
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This variety could be put into a broader context by a more extensive study of diverse organisms. A major future goal, at least for microbes, is to test hypotheses regarding telomere evolution.
These experiments use techniques for growth of cells at a constant density. One of these instruments used for these experiments is the turbidostat Gresham and Dunham, ; Matteau et al.
Telomeres and telomerase: three decades of progress
Another exciting aspect of this work is that these experiments represent real-time albeit manipulated evolution. The artificial evolutionary approach is having signs of success in yeast and microbes under different conditions, such as oxidative stress Raso et al. KWR was responsible for background and analysis of contributions. AJL was responsible for the structure and comments in the editorial. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
These excellent articles have been written by some of the most talented telomere investigators. We are grateful for their ideas and viewpoints. But those viewpoints could not have reached such a stage of refinement without the dedicated reviewers from the same telomere community who put considerable time into this effort. We hope this two-year effort will catalyze some new approaches and ideas within the telomere and evolution communities.
Chaconas, G. Hairpin telomeres and genome plasticity in Borrelia: all mixed up in the end. Chang, M. Telomerase repeat addition processivity is increased at critically short telomeres in a Tel1-dependent manner in Saccharomyces cerevisiae. Genes Dev. How telomeres solve the end-protection problem. Science , — Gresham, D. The enduring utility of continuous culturing in experimental evolution. Genomics , — Hector, R. Tel1p preferentially associates with short telomeres to stimulate their elongation. Cell 27, — Hirano, Y. Rif1 and rif2 inhibit localization of tel1 to DNA ends.
Cell 33, — Karlseder, J.
Telomeres in evolution and evolution of telomeres
Targeted deletion reveals an essential function for the telomere length regulator Trf1. Cell Biol. Lambowitz, A. Mobile bacterial Group II introns at the crux of eukaryotic evolution. Lopes, A. Detection of novel recombinases in bacteriophage genomes unveils Rad52, Rad51 and Gp2. Nucleic Acids Res. Martina, M.
A balance between Tel1 and Rif2 activities regulates nucleolytic processing and elongation at telomeres. Matteau, D. A small-volume, low-cost, and versatile continuous culture device. Mozgova, I. Functional characterization of domains in AtTRB1, a putative telomere-binding protein in Arabidopsis thaliana. Phytochemistry 69, — Raso, S. Effect of oxygen concentration on the growth of Nannochloropsis sp. Ribeyre, C. Anticheckpoint pathways at telomeres in yeast. Sabourin, M.
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Telomerase and Tel1p preferentially associate with short telomeres in S. Mol Cell.
II. Hybridization, ploidy and parthenogenesis
Shabalina, S. Origins and evolution of eukaryotic RNA interference. Trends Ecol.