Unravelling evolution one nucleotide at a time.

Throughout her journey of becoming a microbiology researcher, Siv Andersson would change course every four or five years. ‘I usually just turn around, see what’s available and run off into the direction that looks most exciting, interesting, and challenging’. Every few years, important life decisions impacted her career and somehow paved her journey into academic research. After postdocs at the Laboratory of Molecular Biology in Cambridge and Columbia Medical School in New York, she became an Associate Professor at Uppsala University in 1997, where she had finished her Ph.D. Dissertation. In 2000, she became full Professor for Molecular Evolution and was Head of the Department of Evolution, Genomics, and Systematics at the Evolutionary Biology Centre from 2003 to 2009. Now being more open for long-term goals, Siv investigates how bacteria evolved throughout time; she even looked at time ranges of several million years. Siv and her group explored how two lineages of the bacterium Buchnera aphidicola adapted to their specific hosts, the pea aphid and the wheat aphid (Tamas et al. 2002). This endosymbiosis was established ∼150 million years ago, and the two lineages diverged ∼50–70 million years ago. Interestingly— and completely unexpectedly—they found that even though both lineages were living as endosymbionts with their respective hosts for such a long time, their gene contents barely differ. ‘When we looked at the gene maps and saw they were identical; we were just silent. And then the Ph.D. student started panicking because he thought the samples had been mixed up and the same bacterium had been sequenced twice.’ Later, they found that indeed a high degree of divergence was apparent at the nucleotide sequence level. Yet, no inversions, translocations, duplications, or gene acquisitions seemed to have happened throughout this extensive time period. With both endosymbionts having lost the genetic elements for a recombination machinery, their genome size and flexibility were reduced, which instead increased genome stability and left them with the same genomic architecture. As the next step, Siv aimed to understand the mechanisms of how B. aphidicola adapted to its aphid host (Tamas et al. 2008). This endosymbiont has one of the smallest and most A-T-rich bacterial genomes and some of its transcripts with poly(A) sequences contain frameshift mutations resulting in nonfunctional gene products. Yet, as Siv and her group found, transcriptional slippage of the polymerase can rescue these mutations and—against the odds—lead to functional gene products. Even though a seemingly inefficient mode of information processing, regulation mechanisms like these could be helpful in designing synthetic genomes.