Inquiry Question

How does reproduction ensure the continuity of a species?

Content Descriptor

Explain the mechanisms of reproduction that ensure the continuity of a species, by analysing sexual and asexual methods of reproduction in a variety of organisms, including fungi: budding, spores

Budding in yeasts

A scanning electron micrograph of the budding yeast, Saccharomyces cerevisiae. Image credit: By Mogana Das Murtey and Patchamuthu Ramasamy. CC BY-SA 3.0,

Yeasts are unicellular eukaryotes that belong to the kingdom Fungi. Humans have used yeast in fermentation reactions - to produce bread, beer and wine (hence, they are also called Baker's yeast or Brewer's yeast).

Yeasts are unicellular fungi and are classified as budding or fission yeasts based on the manner of cell division. Both budding yeast (Saccharomyces cerevisiae) and fission yeast (Schizosaccharomyces pombe) divide by mitosis. However, budding yeast divide asymmetrically (unequally) to produce one large cell (called the mother cell) and a smaller cell (called the bud). On the other hand, fission yeast produces two similarly-sized daughter cells.

A micrograph of fission yeast (left) and budding yeast (right). The cells have been stained with a dye to show the positions of the nuclei during cell division. In fission yeast (left), the cytoplasm is partitioned in the middle of the cell (after the nuclei's separation) to form equally sized daughter cells. In budding yeast (right, cytoplasmic partitioning is unequal, producing a mother cell and a bud. Image credit: Tang, Z. I. (2010) The Domino and Clock Models of Cell Cycle Regulation. Nature Education 3(9):56

Micrograph showing bud formation in yeast. The shape formed during budding is called a shmoo. Image credit: By Masur - Own work, Public Domain.

Sporulation in fungi: asexual and sexual reproduction

Both unicellular and multicellular fungi can reproduce by producing spores. This process is called sporulation. Some fungi produce spores when conditions are harsh, such as nutrient scarcity. When favourable conditions return, the spores germinate to produce the adult organism.

Sporulation in yeast

When conditions are unfavourable, budding yeast undergoes a type of cell division known as meiosis. During this process, the cell's genome is altered slightly. Furthermore, four daughter cells are formed - each daughter cell has half the number of chromosomes as the parental cell (haploid) from which they originated. Each of the four daughter cells becomes a spore. Spore cells do not grow or undertake normal metabolic activities. When conditions become favourable again, the spores germinate. Two spore cells fuse to form a fertilised cell (this restores the chromosome number). Since sporulation is associated with fertilisation, this type of reproduction is sexual. Mixing the DNA from the different cells during fertilisation enhances the genetic diversity of the progeny.

Left: Sporulation in the budding yeast. The four haploid spores are contained within a structure called the ascus. Right: 1. Budding; 2. Sporulation; 3. Germination and fertilisation. Image credit: modified from 'The yeast cell's life cycle: Budding Conjugation Spore'. By Masur - Own work, Public Domain

Sporulation in multicellular fungi - asexual and sexual cycles

Multicellular fungi often grow by producing hair-like structures called mycelia or hyphae - we see them on mouldy bread.

Fungi growing on bread, showing the mycelium. Image credit: Matt Wharton, licensed under the Creative Commons Attribution-Share Alike 2.0 Generic license.

During asexual reproduction, the fungi produce haploid spores. Spores are dispersed by wind and germinate into new mycelia when they land on suitable surfaces. When conditions are not favourable, the fungi undergo sexual reproduction. The cells of adjacent hyphae will fuse. This brings their nuclei together, fusing to form the diploid state. The diploid cells then undergo meiosis to produce haploid daughter cells. The haploid cells then become spores to perpetuate the organism's life cycle.

Figure showing the asexual and sexual lifecycles of fungi. Image credit: CNX OpexStax, licensed under the Creative Commons Attribution 4.0 International license.

Sporulation in multicellular fungi - sexual reproduction

A group of fungi called basidiomycetes reproduce sexually through sporulation. Basidiomycete is the biological name for mushrooms. Mushrooms are diploid but produce haploid spores. The haploid spores germinate to form new hyphae and mycelia. Diploid cells are formed when two adjacent hyphae fuse.

Figure showing the life cycle of a basidiomycete (mushroom). The fruiting body of the mushroom releases haploid spores (called basidiospores). They germinate to form mycelia. The fusion of mycelia results in the formation of diploid cells. The diploid cells divide by mitosis to form the fruiting body (mushroom). Image credit: OpenStax, licensed under the Creative Commons Attribution 4.0 International license.