The domain Eukarya is broken up into four different kingdoms: Protista, Fungi, Plantae, and Animalia. Each of these kingdoms have organisms that are classified under it based on certain defining characteristics.
Biologists believe that protists evolved from bacteria through mutations that made them more complex. As all of the kingdoms in the domain Eukarya, protists are eukaryotic. (They have membrane-bound organelles) [18]. In general, protists are unicellular, unlike most fungi, plants and animals, although there are some exceptions. However, all multicellular protists do not have specialized cells. Aside from these general characteristics, protists are very diverse; indeed, protists seem to be all eukaryotes that do not fit into the category of plant, animal or fungus [18]. One notable characteristic that all protists share is that they favor moist or wet environments. Protists will usually reproduce asexually using binary fission but there are some that will reproduce sexually with conjugation [19]. They are customarily broken up into two major groups: autotrophs (such as algae), similar to plants, and heterotrophs (such as protozoa), similar to animals and fungi [19]. The autotrophs will obtain their food through photosynthesis and the heterotrophs will obtain their food by ingesting smaller cells or organic material. Protists can have motility, just as animals do; however, this is limited to flagella and ciliary movement, as well as pseudopodia, or false feet, which are cytoplasmic extensions that can help move a protist [20].
The kingdom Fungi is composed of organisms such as mushrooms, molds and yeasts [21]. While they are commonly compared to plants because they are not motile and do not have a nervous system, fungi are much closer genetically to animals [22]. Like most plants and animals, fungi are generally multicellular, although yeasts (which are fungi) are unicellular. One of the key characteristics that makes fungi similar to animals and differentiates them from plants is their mode of nutrition. Fungi are heterotrophic and absorb food from outside their body [21]. First, they will release enzymes outside of their bodies to their surroundings. These enzymes break down organic substances that the fungus can eat into smaller pieces that the fungus can absorb directly into its cells [22]. While like most other eukaryotes, fungi reproduce both sexually and asexually, they do this in a manner that is different from any other kingdom. They can reproduce asexually through mitosis, or sexually with another fungus that is compatible with them. Unlike animals and plants, fungi can have up to 20,000 different mating types. For sexual reproduction, fungi will produce specialized spores that spread out and try to find a “‘mate”. Also, the filaments of two different fungi can fuse themselves, including their DNA, into one organism, something that neither plants nor animals are capable of [22]. Some fungi, however, such as yeasts, will only reproduce asexually.
The kingdom Plantae is made up of (eukaryotic) organisms that are all multicellular, except for some algae. The defining characteristic of plants is that they are photoautotrophs: they convert light energy into chemical energy that can be used for cell respiration through a process called photosynthesis [23]. This is in contrast to most other organisms in the domain Eukarya, which, with the exception of some organisms in Protista, are all heterotrophs that ingest their food to obtain energy. Another characteristic of plants is that, like Fungi and Protists, they do not have a nervous system. However, an interesting thing to note is that biologists have very recently found that some plants can send electrical signals to their cells in order to coordinate their actions [24]. Plants are also nonmotile, a trait they share only with Fungi within the Domain Eukarya [23]. Finally, as with the majority of organisms in Domain Eukarya, Plants have the ability to reproduce both sexually and asexually. Vascular plants will mainly reproduce with seeds whereas non-vascular plants will mainly reproduce with spores [25].
The last kingdom, Animalia, is made up of all multicellular, eukaryotic organisms [26]. While all of the other kingdoms in the Domain Eukarya have only some or many organisms that are multicellular, Animalia is the only kingdom that is comprised solely of multicellular organisms. Animals are also all heterotrophic, ingesting their food to get energy [27]. Fungi and some protists also rely on this mode of nutrition; however, as mentioned earlier, plants and some protists get their energy in other ways. Additionally, while not all animals have nervous systems, the only organi
sms that do have nervous systems are in this kingdom. The most complex animals will have a brain that coordinates the functions of the body and processes senses such as sight and smell. Also, almost all animals are capable of some form of motility, and some animals have the ability to make complex movements [26]. Finally, like most members of the domain Eukarya, animals use both sexual and asexual means for reproduction. However, for pro
ducing new organisms (as opposed to individual cells), most animals use sexual reproduction [27].
The differences and similarities between these kingdoms that were just mentioned can be seen on an individual scale between organisms in addition to the larger scale of kingdoms as a whole. Let’s take a closer look at o
ne example organism from each kingdom to see some of the unity and diversity between the organisms in the different kingdoms. The organisms that we will look at are Naegleria fowleri from Protista, Agaricus bisporus from Fungi, Polytrichum juniperinum from Plantae, and octopus vulgaris from Animalia. One characteristic that all of these organisms have in common is that they are eukaryotic, with definitive organelles within their cells. In fact, all organisms within the domain Eukarya have this characteristic, as it is the defining characteristic of this domain. The organism N. fowleri from Protista is unicellular; however, all of the other representative organisms are multicellular, as can be seen in the pictures. All of these organisms are hete
rotrophic, ingesting food to obtain energy, except for p. Juniperinum, which uses photosynthesis to change light energy into chemical energy. Another characteristic that is shared by all of these organisms except one is the absence of a nervous system. Indeed, the only organism that has a nervous system is O. vulgaris. N. fowleri and O. vulgaris are both motile, with N. fowleri using pseudopods and flagella to move, and O. vulgaris using its eight legs. In contrast, A. bisporus and P. juniperinum are both sessile (immobile). Finally, apart from N. fowleri which only reproduces asexually through binary fission, all of the other organisms reproduce both sexually and asexually.
The following picture traces the ancestry of organisms within the domain Eukarya.
