Is sex necessary?

Abstract

Fungal sexual reproductive modes have markedly high diversity and plasticity, and asexual species have been hypothesized to arise frequently from sexual fungal species. A recent study on the red yeasts provides further support for the notion that sexual ancestors may give rise to shorter-lived asexual species. However, presumed asexual species may also be cryptically sexual, as revealed by other recent studies.

Figure 1

The life cycle of a unicellular yeast. The budding yeast Saccharomyces cerevisiae is an example of a sexual unicellular yeast, with haploid cells of two mating types, a and α, that mate when pheromones produced by each cell is recognized by pheromone receptors expressed on the opposite cell type: this stimulates fusion of the two cells to produce an a/α diploid cell that grows vegetatively when nutrients are plentiful, but upon starvation undergoes meiosis to produce four haploid spores that can remain dormant until conditions improve. Reproduced with permission from Figure 2-5 of Morgan DO, The Cell Cycle, Oxford University Press (2007).

Figure 2

Advantages and disadvantages of a sexual or an asexual life cycle.