SUMMARY Background We investigated the determinants of sexual identity in the budding yeast, Saccharomyces cerevisiae. The higher fungi are divided into the Ascomycetes and the Basidiomycetes. Most Ascomycetes have two mating types: one (called α in yeasts and MAT1-1 in filamentous fungi) produces a small, unmodified, peptide pheromone, and the other (a in yeasts and MAT1-2 in filamentous fungi) produces a peptide pheromone conjugated to a C terminal farnesyl group that makes it very hydrophobic. In the Basidiomycetes, all pheromones are lipid-modified, and this difference is a distinguishing feature between the phyla. We asked whether the asymmetry in pheromone modification is required for successful mating in Ascomycetes. Results We cloned receptor and pheromone genes from a filamentous Ascomycete and a Basidiomycete and expressed these in the budding yeast, Saccharomyces cerevisiae, to generate novel, alternative mating pairs. We find that two yeast cells can mate even when both cells secrete a-like or α-like peptides. Importantly, this is true regardless of whether the cells express the a- or α-mating type loci, which control the expression of other, sex-specific genes, in addition to the pheromones and pheromone receptors. Conclusions We demonstrate that the asymmetric pheromone modification is not required for successful mating of ascomycete fungi and confirm that, in budding yeast, the primary determinants of mating are the specificity of the receptors and their corresponding pheromones.

Budding yeast cells exist in two mating types, a and α, which use peptide pheromones to communicate with each other during mating. Mating depends on the ability of cells to polarize up pheromone gradients, but cells also respond to spatially uniform fields of pheromone by polarizing along a single axis. We used quantitative measurements of the response of a cells to α-factor to produce a predictive model of yeast polarization towards a pheromone gradient. We found that cells make a sharp transition between budding cycles and mating induced polarization and that they detect pheromone gradients accurately only over a narrow range of pheromone concentrations corresponding to this transition. We fit all the parameters of the mathematical model by using quantitative data on spontaneous polarization in uniform pheromone concentration. Once these parameters have been computed, and without any further fit, our model quantitatively predicts the yeast cell response to pheromone gradient providing an important step toward understanding how cells communicate with each other.

Human reproduction does not happen uniformly throughout the year and what drives human sexual cycles is a long-standing question. The literature is mixed with respect to whether biological or cultural factors best explain these cycles. The biological hypothesis proposes that human reproductive cycles are an adaptation to the seasonal (hemisphere-dependent) cycles, while the cultural hypothesis proposes that conception dates vary mostly due to cultural factors, such as holidays. However, for many countries, common records used to investigate these hypotheses are incomplete or unavailable, biasing existing analysis towards Northern Hemisphere Christian countries. Here we show that interest in sex peaks sharply online during major cultural and religious celebrations, regardless of hemisphere location. This online interest, when shifted by nine months, corresponds to documented human births, even after adjusting for numerous factors such as language and amount of free time due to holidays. We further show that mood, measured independently on Twitter, contains distinct collective emotions associated with those cultural celebrations. Our results provide converging evidence that the cyclic sexual and reproductive behavior of human populations is mostly driven by culture and that this interest in sex is associated with specific emotions, characteristic of major cultural and religious celebrations.

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