Living organisms have evolved a variety of reproductive strategies, both of a sexual and asexual nature, to make sure that their genes live on in the next and future generations. Asexual reproductive strategies have the most use in prokaryotic and bacterial species but have also been seen in some eukaryotic species as well. Two well-known asexual reproduction strategies include binary fission and budding, which both create daughter organisms identical to their ‘parent’ cell. These strategies have the same function of reproduction, but go about it in different ways, and this is where the important distinction comes in. Fission is, in general terms, the splitting of one single cell into two daughter cells, and the original parent cell is lost, while in budding, the daughter cell grows off of the parent cell before popping off, leaving the parent cell completely intact (Binary Fission, 2021; Budding Cells). Interestingly, both styles of reproduction can occur in certain species of fungi, meaning they are not kingdom specific, and have been seen in fascinating locations, such as the fission-based reproduction of mitochondria and the budding-based reproduction of some plant species (Binary Fission,2021; Budding Cells).
However, the greatest distinction between the two relates to how they undergo their reproduction. It has been found that organisms that use fission-based reproduction, such as Schizosaccharomyces pombe, do not undergo regular mitosis, which a budding style of reproduction does (Schizosaccharomyces pombe, 2021; Budding Cells). Another interesting distinction between the reproduction styles is that budding, like in Saccharomyces cerevisiae yeast, can have multiple rounds of reproduction can occur near simultaneously (Budding Cells; Saccharomyces cerevisiae, 2021). Multiple daughter cells can grow off of the parent cell, and in some cases new daughter cells can begin to grow on older daughter cells that are still attached to the original parent cell. In contrast, fission-based reproduction must complete one cycle before beginning another.
There are a few reasons why knowing how a cell reproduces is important, and once such reason is identifying their genus, which can help when trying to understand their relations. Marine yeasts are a commonly identified genus, and it is interesting to point out that they all use variations of budding, including creating fruiting bodies to reproduce. The first genus is Rhodotorula, which is a part of the Basidiomycota phylum, tends to look pink or red, and seems to like plastic – especially that in medical equipment like catheters, which is where a majority of infections caused by this yeast seem to occur (Wirth, 2012; Britannica, 2017). Cryptococcus is another genus found under the Basidiomycota phylum, though it is far more pathogenic to the human species than its pink, plastic-loving cousin (Cryptococcus, 2010). Also, Cryptococcus yeasts come in circular shape, their cells look like balls, while Rhodotorula look quite oblong.
Debaryomyces is another common genus that reproduces by budding but has also been seen undergoing sexual reproduction in certain species, which is interesting for a yeast (Wrent et al, 2012). Candida is the final broad marine yeast genus and gave rise to one of the most common human fungal pathogens, Candida albicans (Alby, 2010). Candida is another coccus-shaped yeast that undergoes budding as its reproductive method, however recent research has shown that it can also undergo sexual reproduction, including stages of meiosis (Alby, 2010).
It is interesting, however, to point out how cell reproduction varies even among a common type of yeast. Mitchison-Field et al. conducted a study of different species of marine yeasts, and found that four species of black yeast, gathered from different locations, had widely different styles of reproduction (Mitchison-Field, 2019). The four species used in the study were H. werneckii, K. petricola, A. pullulans, and P. salicorniae, and the examination of these black yeasts showcased the wide variety of replication speeds, patterns, and shapes that yeast species can share.
| H. werneckii | K. petricola | A. pullulans | P. salicorniae | |
| Cell Cycle Duration | 750 minutes | 500 minutes | 150 minutes | N/A |
| Time to First Bud | 200-250 minutes | 150 minutes | 175 minutes | N/A |
| Growth Pattern | A mix of splitting in half and then budding a new cell off the end of one of the others | One cell buds off of another, growing out in a chain in a single direction | One cell can have multiples growing off one end before those are pushed away to make room for others | The central ‘body’ cells replicate normally while the hyphal cells stretch out with each replication |
| # Of Nuclei | 1 per cell | 1 per cell | Multiple per cell, daughter cells do not have nuclei until they reach a sufficient size | 1-2 per cell, dependent on cell type |
| Cell Shape | Ovular rectangles, long and slightly narrow | Circular, colonies look like beads on a string | Football shaped, oblong, with daughter cells the cells look like a carrot | Spider-like, with arms that stretch out |
| Colony Color | Blue-gray | Blue-black | Yellow | Red-black |
From previous studies on common types of yeast, it has been seen that budding yeasts replicate themselves rapidly, though the amount of time varies widely between species (Duina, 2014). This variety of cell replication time is seen even in the black yeasts, with H. werneckii taking the longest to finish a single cell cycle, while P. salicorniae has a cell cycle duration that is difficult to distinguish. This could be, perhaps, because H. werneckii has a different replication pattern than the other species, undergoing a mix of septation and budding, rather than just budding. P. salicorniae on the other had has a very specific type of replication that includes hyphae creation along with regular replication of body cells. The combination of these two types of replications make it difficult to time the cycle for one specific cell.
However, because of the very specific replication of P. salicorniae, its shape is distinguishable. Rather than the normal cell shapes of H. werneckii and K. petricola, P. salicorniae shares a feature with A. pullulans:both have very odd replication shapes. P. salicorniae, with its hyphae stretching out like arms, looks similar to a spider or octopus that stretches even father as it grows, and A. pullulans looks like a lemon with smaller lemons growing out of its top. These growth patterns, when stained, also give a good look at how the DNA within the mother cells gets into the daughter cells (Mitchison-Field, 2019). The cells of H. werneckii, K. petricola, and P. salicorniae tend to show one nucleus per cell, and their nuclei move to their daughter cells during the cell replication process. A. pullulans is the odd one out here, as its cells tend to show multiple nuclei hiding within, and its daughter cells don’t always show nuclei until they are a larger size.
The experimental study completed by Mitchison-Field and her team gave a wonderfully in depth look at how a single type of replication can vary by cell type. All of the yeasts studied by Mitchison-Field et al. replicated by budding, which matches what is known about the four main yeast species: Cryptococcus, Rhodotorula, Candida, and Debaryomyces. This makes sense, as all are types of yeasts, and tend to share certain characteristics. However, unlike the main species, the black yeasts used in this study show a much more interesting variety of replication patterns used during their cell cycles. The yeasts in this study also show a much more variable colony-shape, showing the incredible uniqueness of different species.
References
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Britannica, T. E. o. E. 2017. Basidiomycota: phylum of fungi. Encyclopaedia Britannica.
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Wrent, P., Rivas, E. M., Gil de Prado, E., Peinado, J. M. and de Silóniz, M. I. (2014) ‘Debaryomyces’, in Batt, C.A. and Tortorello, M.L. (eds.) Encyclopedia of Food Microbiology (Second Edition). Oxford: Academic Press, pp. 563-570.