How yeast cells find their mates

Accurate detection of extracellular chemical gradients is essential for many cellular behaviors. Gradient sensing is challenging for small cells, which experience little difference in ligand concentrations on the up-gradient and down-gradient sides of the cell. Nevertheless, the tiny cells of the yeast Saccharomyces cerevisiae reliably decode gradients of extracellular pheromones to find their mates. By imaging the behavior of polarity factors and pheromone receptors during mating encounters, we found that gradient decoding involves two steps. First, cells bias orientation of initial polarity up-gradient, even though they have unevenly distributed receptors. To achieve this, they measure the local fraction of occupied receptors, rather than absolute number. However, this process is error-prone, and subsequent exploratory behavior of the polarity factors corrects initial errors via communication between mating partners. The mobile polarity sites convert the difficult problem of spatial gradient decoding into the easier one of sensing temporal changes in local pheromone levels.