Abstract

Vitamins are essential components of all cells, as they provide cofactors for enzymes of central metabolism. Animals must obtain vitamins from their diet, but in fact a widespread and complex distribution of vitamin auxotrophy exists across the whole tree of life. I will present recent work investigating the evolutionary origins of vitamin dependence, using an experimental evolution approach with the fast-growing vitamin B12 -independent alga Chlamydomonas reinhardtii. We observe, after fewer than 500 generations of growth in medium supplemented with vitamin B12 (1000 ng/L), the evolution of a B12 -dependent clone that rapidly displaces its ancestor. Genetic characterisation of this line reveals that a type-II Gulliver-related transposable element (GR-TE) has integrated into the B12 -independent methionine synthase gene (METE). We have captured this transposition event in action, and witness the origin of a new unitary pseudogene, and its subsequent rise in frequency within the population. This selective sweep is reproducible with as little as 200 ng/L vitamin B12 . Our study exhibits how loss of a superfluous gene can occur rapidly and in response to subtle environmental cues, with significant consequences for eco-physiological flexibility.