Size matters. Particularly when it comes to finding a mate. Across the animal kingdom, body size plays a crucial role in determining how successful individuals are at surviving and reproducing. Large males are often better at fighting off competing males, while large females generally produce more offspring. However, being big isn’t always best. In some environments, small individuals may have a better chance of survival. Choosing a partner the right size may help to ensure a good start in life for your offspring. Recent research in GEE suggests that even simple organisms such as yeast are able to choose the best size mate to ensure their offspring survive and reproduce.
In single-celled organisms, just like their multicellular counterparts, the ideal body size depends on surrounding conditions in the environment. In nutrient rich environments, larger cells do better, whilst in nutrient poor environments, smaller cells prevail. In many multi-cellular animals, females have been shown to select mates based on body size to ensure their offspring the best possible chances of survival. Are unicellular organisms also able to make these kinds of selections? A recent paper by GEE Researchers Dr Carl Smith, Prof. Andrew Pomiankowski and Dr Duncan Greig investigated this phenomenon in yeast.
The Curious Life-Cycle of Yeast
Yeast, being single-celled, are able to reproduce asexually – budding off a new copy of themselves. However, when times are hard, yeast can also enter a sexual phase of reproduction. When nutrient levels are low, a yeast cell will divide into sexual spores. These spores are haploid, carrying only one copy of each chromosome (rather than the usual two), making them analogous to sperm and egg cells. Spores will remain dormant until they sense an improvement in the environment, at which point they spring into action and try to find a mate. Spores come in two mating types and must always mate with another spore of the opposite type – a system based on mating pheromones released by spores to attract the opposite sex.
Body size is important in both the asexual and sexual phases of yeast reproduction. Yeast cells must reach a certain critical size before they can bud asexual copies of themselves, although this critical size is smaller in poorer environments. Likewise, during sexual reproduction, the cell size of the offspring is determined by the cell size of the two spores that fuse together. GEE researchers first investigated how body size influenced reproductive success in both phases of yeast reproduction. Cell size was an important influence on how quickly spores awoke from dormancy to begin searching for a sexual partner; large spores became active 38% more quickly in a nutrient-rich environment, whereas smaller spores were 63% faster when conditions were poor. Asexual reproduction was similarly affected, with large spores budding off asexual copies sooner in good environments and small spores budding sooner in bad environments. Size matters when it comes to yeast reproduction, but bigger isn’t always better.
Are Sexual Yeast Cells Selecting the Right Sized Partner for their Environment?
To answer this question, GEE Researchers created a mating-choice experiment, in which a haploid yeast cell of one mating type is presented with two cells of the opposite type, one small and one large. To mate, two of these haploid cells will fuse to form a single, diploid cell which can resume asexual reproduction. These experiments were performed with different sizes of focal yeast cell, and in different environments, to see whether the focal cells were able to select the best partner – one that would give the final fused cell the best chance of survival. This is a critical choice – mating in these single-celled organisms is an irreversible decision, the size of the ‘offspring’ cell is directly determined by the size of the two haploid cells that fuse, and cell size will influence how quickly the ‘offspring’ can reproduce.
The results of the experiment indicate that yeast cells are indeed able to make careful calculations about their choice of partner and select the one closest to the optimum size for the environment. In nutrient rich environments, yeast cells tended to select larger mates, whereas in poorer environments they tended to select smaller mates. This was true regardless of their own body size, indicating that cells were not simply mating with similar sized partners, but making a choice based upon the mates available and the environment around them. The authors suggest that such seemingly complex behaviour could be generated quite simply – if cells that are better adapted to their environment begin emitting these pheromones earlier, or produce them in larger amounts, they would be more easily detected by a prospective mate.
Size is a crucial determinant of success, survival and reproduction for most living organisms. However, the optimum size often depends upon environmental conditions, as GEE researchers have shown to be the case for yeast. For most organisms, selecting a mate is an important decision that will influence the success of their offspring. For yeast even more so, this decision is critical, as it is irreversible. A new paper from GEE researchers shows that yeast cells also consider the quality of their partner when mating. During sexual reproduction, yeast cells are able to select a mate of the optimum size for the environment, thereby improving the chances that the resulting offspring cell will reproduce.
This research was made possible by funding from the Natural Environment Research Council (NERC), the Biotechnology and Biological Sciences Research Council (BBSRC), the Engineering and Physical Sciences Research Council (EPSRC), The Royal Society and the Max Planck Society.