Genome size variation and adaptation in rotifer populations

A genome constitutes an organisms complete set of genetic instructions in the form of DNA, a long molecule consisting of four nucleotide bases (letters). Each cell of an organism contains the genome, initially inherited by the mother and the father, and thus contains all of the information needed to build and maintain that organism and allow it to grow and develop. Earlier it was assumed that the size of a genome determines the complexity of its carrier, such that bigger genomes would allow more complex instructions and thus build a more complex organism. However, in reality, genome size is at best weakly related to organismal complexity. For instance, the genome of the marbled lungfish, the biggest animal genome, is 47-times bigger than the human genome. On the other hand, the actual information in a genome is often remarkably small, with, for instance, only 1.5% of the human genome consisting of protein-coding sequences. In fact, the majority of most animal genomes appear to consist of DNA sequences that are a relict of their evolutionary past, and the question as to why the genomes of so many organisms carry so much junk DNA is still an unsolved scientific problem. In the current FWF project, we use the rotifer Brachionus asplanchnoidis to obtain new insights into this problem. What sets this animal model apart is that genome size is highly variable (up to 40%) in B. asplanchnoidis populations, which is due to individuals containing different amounts of satellite DNA (a type of repetitive DNA). Despite these differences in genome size, individuals can interbreed and produce fertile offspring. Here we use this rotifer species to investigate the consequences of genome size variation on evolutionary adaptation, in particular, focusing on the question of whether genome size differences can contribute to phenotypic adaptation (e.g., via individuals of larger genome size having larger cell and body size). To this end, we use an experimental evolution approach to populations that adapt to different environmental conditions in the laboratory. These experiments are accompanied by whole-genome-sequencing of individuals before and after evolution, to identify the responsible changes in the genome. In contrast to most studies, our genomic analyses encompass not only changes in the coding regions but also changes in the junk DNA parts of the genome and thus allow a more complete assessment of evolutionary adaptation.