Protists

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 protists: binary fission, budding

Binary fission in protists

Protists are a diverse group of single-celled, eukaryotic organisms. As eukaryotes, protists possess membrane-bound organelles, including the nucleus, mitochondria and (in some) chloroplasts. Some protists divide by binary fission. The process is similar to that in bacteria, with the following differences:

  • The eukaryotic chromosomes are located in the nucleus (an organelle). Therefore, the chromosome and the entire nucleus must be replicated. After replication, the daughter nuclei move to different ends of the cell.
  • The pinching of the cell membrane to form the daughter cells (fission) can occur in different planes (directions). In irregularly shaped cells (for example, the amoeba), fission can occur in any plane. In other protists, the fission may be transverse (across) or longitudinal (length).

Binary fission in protists bestows the same benefits as binary fission in prokaryotes – rapid reproduction rates, genetically similar populations, and the rapid spread of genes that confer advantageous traits through the population).

Budding in protists

Photomicrograph of the protist, Paramecium claudata. The pair of cells in the middle of the image is almost completing cell division (binary fission). Image credit: Arturo Agostino.

Protists also undergo another type of asexual reproduction, known as budding. While binary fission produces two daughter cells similar in their sizes, budding produces daughter cells that are unequal in size (but genetically identical). After nuclear replication, the daughter nuclei are located at opposite ends of the cell. However, the partitioning of the cytoplasm occurs unevenly so that a small amount of cytoplasm surrounds one daughter compared to the other. Eventually, the smaller daughter cell pinches away from the larger cell – it will enter a growth phase and become the same size as the other mature cells of the species. One disadvantage of binary fission is that both daughter cells must enter a growth phase before reproducing. However, in budding systems, the larger daughter cell can enter another reproduction cycle without a long growth phase (the smaller budded cell must grow before reproducing).

Sporulation in a protist – an unusual example of alternation between unicellularity and multicellularity

The protist Dictyostelium discoideum is an amoeboid protist. It is also called a slime mould. It feeds on soil bacteria and normally divides by mitosis. However, when food becomes scarce, this organism undergoes a transformation during which it becomes a multicellular organism! The multicellular stage, called a slug, crawls to new locations, where it forms a fruiting body. Each fruiting body contains numerous spores. When favourable conditions return, the spores disperse and germinate to form new amoeba. Therefore, Dictyostelium alternates between unicellular and multicellular stages.

Figure showing the lifecycle of Dictyostelium discoideum. During starvation, individual cells (amoebae) aggregate to form the multicellular slug. The slug transforms into a fruiting body containing spores. The germination of the spores produces unicellular amoebae. Image credit: Jung Choi, Developmental Biology.
The rich lives of cellular slime moulds.