Kingdom Protoctista

Kingdom Protoctista

Protoctists can be defined as unicellular eukaryotic organisms other than fungi, plants, and animals. Evolutionarily, members in high kingdoms such as Fungi, Plantae and Animalia, have their ancestors in the Protoctista kingdom. Therefore, protoctists are eukaryotes consisting of unicellular and multicellular members.

The multicellular protoctists consist of an assembly of similar cells such as Spirogyra. The major difference between protists and protoctists is that the former consists of only unicellular microscopic organisms (protozoans) while the latter is the mixture of unicellular and multicellular organisms.

Studies based on the base sequence of mitochondrial and chloroplast DNA indicate that members of kingdoms Fungi, Plantae and Animalia have their ancestors in the kingdom Protoctista.

Characteristics of protoctists

i. Organisms under this kingdom are nucleated; that is, all are eukaryotes.

ii. Some protoctists are unicellular while others are multicellular.

iii. They lack tissue differentiation.

iv. Protoctists are adapted to both, aquatic and terrestrial habitats.

v. They have various types of vesicles that perform different functions. These include their increased surface area to facilitate exchange of materials needed for their survival. For instance, contractile vacuole helps protoctists to discharge excess water taken by osmosis. They also have food vacuole, as in Paramecium, which helps them in digestion of engulfed food particles.

vi. Their cell surfaces are diverse, ranging from just a plasma membrane as in Amoeba, to a stiffer surface as in Euglena to ensure the integrity of the cell.

vii. Many protoctists are involved in endo-symbiotic relationship with other organisms. A good example is a radiolarian which harbors other photosynthetic protoctists. These protoctists, through photosynthesis, synthesise food that is shared by both, while the radiolarian in turn confers protection and provide some metabolites to the other symbiont.

viii. Both asexual and sexual types of reproduction are common in the majority of protoctists, although some members lack sexual reproduction. Asexual reproduction occurs by binary fission (in which one nucleus divides), multiple fission (in which many nuclei divides into multiple daughter cells), or budding (in which a new cell grows on the surface of the mother cell). During all these types of asexual reproduction, an organism replicates its nucleus and divides to form new organisms. Sexual reproduction in protoctists is still primitive, given that it is mainly a recombination of genetic material.

ix. Some protoctists are parasites, while others are free living organisms. The free living members are either autotrophs (such as Spirogyra) or heterotrophs (such as Amoeba proteus)

Classification of protoctists

Classifying protoctists has been a difficult task due to their high diversity. Traditionally, protoctists were subdivided into several groups based on their physical similarities to higher kingdoms of Animals, Plants and Fungi. This text will deal with six selected phyla of Protoctista namely; Rhizopoda, Zoomastigina, Apicomplexa, Euglenophyta, Oomycota, and Chlorophyta.