The following points highlight the top six concepts of the kingdom system of organisms classification. The concepts are: 1. Two Kingdom Systems 2. Three Kingdom System 3. Four Kingdom Systems 4. Five Kingdom Systems 5. Six Kingdom System 6. Eight Kingdom System.
1. Two Kingdom Systems (Lennaeus’ Concept):
When we look long back in the prehistoric times, we find that man used to consider two different kinds of populations, plants and animals, existing on the planet Earth. Even in our ancient Indian Holy literature we find mention of ACHAR (non-motile) for plants and SACHAR (motile) for animals; JAR (non-motile) for plants and CHETAN (motile) for animals.
Early in the history, however, this prescientific opinion became formalized in scientific terms: Lennaeus (1758) recognised two primary kingdoms, the Plantae and the Animalia (Fig. 2.1), on the basis of some structural and functional characters such as locomotion, response to external stimuli, mode of nutrition, conductile and contractile system, and cell wall.
Organisms that freely moved from one Place to another, well responded to external stimuli, ate holophaged food, possessed conductile and contractile system in their body and that lacked cell-wall were considered to be animals and put under the kingdom Animalia.
Contrary to it, the organisms that did not freely moved from one place to another, did not respond to external stimuli, did not eat food, lacked conductile and contractile system in their body, and that possessed cell wall were considered to be plants and put under the kingdom Plantae.
This bipartite division of organisms was, in fact, a satisfactory one till the man was only well familiar with highly differentiated advanced organisms like cats and cabbages, buffallo and mango.
When exploration of the microbial world got underway in the 18th and 19th centuries, new forms of tiny creatures like microalgae, micro fungi, multicellular invertebrates (e.g., the rotifers) and unicellular protozoa and bacteria came into knowledge.
These minute forms of life were allocated to ‘plant’ and ‘animal’ kingdoms in the early attempts (around 1800), Multicellular microalgae and micro-fungi, which were mostly immotile and in some cases “plant-like” in form, found natural place in the plant kingdom.
Motile microscopic forms like multicellular invertebrates and unicellular protozoa and bacteria were lumped together as one group of animals, the infusoria, in animal kingdom.
Subsequently, however, it was perceived by the biologists that the group ‘infusoria’ was a heterogenous group and they transferred bacteria from it to the plant kingdom. In this way, finally, the kingdom Plantae came to represent angiosperms, gymnosperms, bryophytes, pteridophytes, fungi (micro and macro), algae (micro and macro) and bacteria; and, the kingdom Animalia to all animals including protozoa.
Therefore, some microorganisms (microalgae, micro fungi, and bacteria) were considered to be plants and others (protozoa) to be animals and were accommodated in both the kingdoms of the two kingdom system.
2. Three Kingdom System (Haeckel’s Concept):
As the knowledge of the properties of various groups of microbial life exploded around the middle of the 19th century, it became apparent that at this level of biological knowledge a division of the living world into two kingdoms cannot really be maintained on a logical and consistent ground.
Although the two kingdom system suited at the level of well-developed advance organisms, it failed to satisfy biologists at the level of microbial forms of life.
Many of the microbes possessed both “plant-like” and “animal-like” characteristics simultaneously, and many others enjoyed such characteristics which were unique to them and not found in either plants or animals.
Slime moulds, which were considered to be protozoan and grouped under the kingdom Animalia, were found phagotrophic and amoeboid (animal-like) in their vegetative state but they resembled true fungi in their reproductive state (plant-like).
Motility by means of flagella was found the only animal-like character in flagellate protozoa: many of them possessed cell wall and were phototrophs (plant-like). Bacteria were found having very little in common with either plants or animals.
Many algae and fungi, which were earlier thought to be immotile thus classified as plants, were found either motile or producing motile structures (zoospores, gametes etc.) during their life.
In view of the foregoing, attempts were made to find a solution and the same was proposed in 1866 by E. Haeckel, a German zoologist and Darwin’s disciple.
Haeckel suggested that the inconsistencies of the two kingdom system could be avoided by the recognition of a third kingdom, and he proposed Protista as a new kingdom to accommodate organisms exhibiting characters either common to both plants and animals, or unique to their own.
Thus, a three kingdom system consisting of kingdoms Protista, Plantae, and Animalia (Fig. 2.2) came into being. This arrangement by Haeckel was done on the basis of morphological complexities and tissue system, division of labour, and mode of nutrition.
Organisms lacking morphological complexities, tissue system, division of labour, and enjoying diversified type of modes of nutrition were segregated and put under the kingdom Protista (algae, fungi, protozoa and bacteria).
Organisms having diverse tissue-system with well- defined division of labour and maximum morphological complexities in their body remained segregated from protists and were bifurcated into two categories: those enjoying autotrophic mode of nutrition were considered to be plants and put under kingdom Plantae, and those that have entirely holophagic (phagotrophic) mode of nutrition were considered to be animals and put under kingdom Animalia. According to this system, all known microorganisms came to be recognised as protists; neither plants nor animals.
3. Four Kingdom Systems (Copeland’s Concept):
Although the three kingdom system of Haeckel provided logical solution for the inconsistencies of two kingdom system, it could not remain unchallenged. With the discovery of electron microscope (Knoll and Ruska, 1932) and further improvements in it with developments in associated preparative techniques for biological materials, it became possible to examine the structure of cell with very high degree of resolution.
This led to the recognition of a profoundly important dichotomy in the internal architecture of the cells of the existing protists. It was found that the cells of less complex protists do not have a membrane-bound nucleus and also lack membrane-bound organelles; contrary to it, the more complex protists possessed true nucleus and membrane-bound organelles like those of plants and animals.
The former category was called prokaryotes (Pro = without + karyon = nucleus) and the latter as eukaryotes (eu = true + karyon = nucleus) (Table 2.1).
Difference between Prokaryotes and Eukaryotes:
Eukaryotes:
1. Size:
Large (104-105 µm)
2. Cell wall:
Present in plants only. Mostly cellulose, hemicellulose and pectate (green plants); chitinous (fungi)
3. Nucleus:
Membrane-bound
4. Nucleolus:
Present
5. DNA:
(a) Mostly in linear chromosomes in nucleus
(b) In mitochondria, chloroplasts
6. DNA packaging:
Histones package DNA into nucleosomes
7. Membrane-bound systems:
Chloroplasts, mitochondria, ER, lysosomes, golgi bodies, etc.
8. Plasma membrane:
Phospholipids, sterols, proteins. May form vesicles or microvilli (animals only)
9. Ribosomes:
80S type
10. Cilia; flagella:
9 + 2 system; powered by ATP
11. Organization:
Extensive developed tissue system (except some algae and fungi)
Prokaryotes:
1. Size:
Small (1-10 µm)
2. Cell wall:
Present in all prokaryotes (except mycoplasmas).
3. Nucleus:
Nuclear membrane absent
4. Nucleolus:
Absent
5. DNA:
(a) Mostly in the form of a single loop attached to plasma membrane
(b) In plasmids
6. DNA packaging:
No histones
7. Membrane-bound systems:
None. Plasma membrane may infold to trap photosynthetic pigments (if present).
8. Plasma membrane:
Phospholipids and proteins; sterols absent. No vesicles, no microvilli.
9. Ribosomes:
70S type
10. Cilia; flagella:
Absent in cyanobacteria; Bacterial flagella superficially resemble monobular, made up of unique protein (flagellin); powered by H+ pumps
11. Organization:
Unicells, or short chain of similar cells
This newly recognized line of demarcation run through kingdom Protista which was subjected to a division into two groups: the lower protists including all prokaryotic protists (bacteria and blue green algae), and the higher protists including all eukaryotic protists (algae other than blue green, fungi and protozoa).
On account of this clear cut division of protists, Copeland (1959) came forward with a four kingdom system to classify the living being.
He created a new kingdom Monera to accommodate lower protists i.e. the prokaryotic protists (bacteria and blue green algae), and retained the higher protists i.e. the eukaryotic protists (algae other than blue green, fungi and protozoa) under the kingdom protists but he called this kingdom as Protoctista.
The kingdoms ‘Plantae’ and ‘Animalia’ remained as such but the terms ‘Plantae’ and ‘Animalia’ were replaced by Metaphyta and Metazoa respectively. Thus, the four kingdoms of Copeland were: Monera, Protoctista, Metaphyta, and Metazoa. (Fig. 2.3).
According to this system the microbes spread into two kingdoms: the bacteria and blue green algae (cyanobacteria) in Kingdom Monera, and microalgae, microfungi and protozoa in kingdom Protoctista. Thus, the microorganisms came to be recognized as monerans and protoctistans, neither plants nor animals.
4. Five Kingdom Systems (Whittakar’s Concept):
Scheme:
Although most of the inconsistencies of protists came to an end with the proposal of two separate kingdoms (Monera and Protoctista) in four kingdom system, the heterogeneity among protoctists were still discernible. All fungi were non-photosynthetic and enjoyed very distinct mode of nutrition (absorptive; osmotrophic).
Similarly, some of the algae have lost their photosynthetic ability (e.g., some euglenophycophytes), lacked cell wall, moved freely and thus were considered closely related with protozoa; other algal forms exhibiting more extensive organizational development and totally photosynthetic nature were considered more closely related with plants.
Considering such inconsistencies of protoctistans. R.H. Whittakar (1969) proposed a comprehensive five kingdom system comprising of kingdom — Monera, Protista, Fungi (Mycophyta), Plantae, and Animalia (Fig. 2.4).
He retained bacteria and cyanobacteria (blue green algae) under kingdom Monera, retained microalgae (the pliytoplanktonic algal forms considered closely related with protozoa), protozoa, and slime moulds under Kingdom Protista (he adopted term Protista rather than Protoctista of Copeland); separated, fungi completely form protoctists and accommodated them under a new kingdom Fungi (Mycophyta); segregated extensively developed completely photosynthetic algal forms (the macroalgae considered closely related with plants) from protoctists and accommodated them along with plants under kingdom Plantae; and, retained invertebrate and vertebrate animals as such under kingdom Animalia. In this way, the microorganisms spread into three kingdoms (Monera, Protista, Fungi) and came to be recognised as monerans, protistans and fungi (mycophytans); neither plants nor animals.
Whittaker’s system of organisms classification is based on evolutionary relationship of phenotypic (observable) characteristics in which three levels of cellular organization are thought to have evolved along three different lines of nutritional strategies: photosynthesis, absorption, and ingestion. (Fig. 2.5).
The phenotypic characteristics taken into account to raise five kingdoms by Whittaker are:
(1) Cell type-prokaryotic or eukaryotic,
(2) Level of organization—solitary and colonial unicellular organization or multicellular, and
(3) Nutritional type.
Summary of the General Characteristics:
The summary of the general characteristics of Whittacker’s five kingdoms is the following:
1. Kingdom Monera (Archaebacteria, Bacteria, and Cyanobacteria):
(i) The monerans consist of all the prokaryotes, and majority of them are represented by the smallest organisms on earth.
(ii) The nuclei of the monerans are not organized with nuclear membrane, nucleoplasm, chromatin fibres and nucleoli, and are referred to as ‘incipient nuclei’.
(iii) The moneran cells typically lack in cell organelles such as chloroplasts, mitochondria and other membrane-bound organelles like endoplasmic reticulum, Golgi bodies, etc.
(iv) The photosynthetic pigments are present in the form of chromatophores, which can be compared with a single lamella of a granum in the plastids (chloroplasts) of algae and higher plants.
(v) The respiratory enzymes, which are mainly the concern of mitochondria in other organisms, however, are present along the infolding’s of plasma membrane called ‘mesosomes’.
(vi) Nutrition absorptive, chemosynthetic, photohetetrophic, or photoautotrophic. Since many of the monerans are photosynthetic, they possess photosynthetic pigments but the same are different from those of other photosynthetic organisms.
(vii) The monerans possess cell walls (except mycoplasmas and some archaebacteria) beyond any doubt but the cell wall composition is unique. Unlike other organisms, the chief constituent of the monerans cell wall is ‘peptidoglycan’ except archaebacteria in which the main constituent is thought to be usually proteinaceous).
(viii) Flagella, if present, are 8 stranded lacking 9 + 2 arrangement; each strand is made up of a protein named flagellin.
(ix) The monerans are also unique in having specific type of ribosomes distributed in cytoplasm having 70 sedimentation co efficient (70S) as against 80 sedimentation coefficient (80S) ribosomes arrayed on membranes in other organisms.
(x) Monerans reproduce asexually, they lack true sexual reproduction.
(xi) The mode of recombination of hereditary characters in monerans can be attributed to alternative pathways of sexuality (parasexuality), namely, transformation, conjugation, transduction and mutation.
(xii) The nuclear genetic material in monerans is represented by a single molecule of DNA per cell and the cell division does not involve any precision in the distribution of genetic material among the daughter cells. This is accomplished by a process known as amitosis.
2. Kingdom Protista (Phytoplanktonic Algae, Protozoa, and Slime Moulds):
(i) Eukaryotic with solitary or colonial unicellular organization without any differentiation into tissues and organs.
(ii) Mostly aquatic forms called planktons; the planktons may be photosynthetic and cell-walled (phytoplanktons) or may be non-photosynthetic and wall-less (zooplanktons).
(iii) Nutrition absorptive, photosynthetic, or ingestive. Photosynthetic pigments are chlorophylls present in plastids (chloroplasts).
(iv) The protistan cells possess cell organelles such as mitochondria, Iysosomes, centrioles and other membrane-bound organelles like endoplasmic reticulum, Golgi bodies, etc.
(v) The cellular organization is of two envelop type, i.e., besides plasma membrane, internal membranes occur around certain organelles.
(vi) Genetic material is organized in the form of a true nucleus. DNA is associated with histone proteins.
(vii) Flagella, if present, are 11 stranded with 9 + 2 arrangement and are made up of a protein named tubulin.
(viii) All forms reproduce asexually; many have true sexual reproduction with plasmogamy, karyogamy, and meiosis. However, an embryo stage is absent.
3. Kingdom Fungi (The Fungi):
(i) Fungi are ubiquitous found in any conceivable habitat.
(ii) They are versatile by virtue of their high degree of adaptability. Anything that can be decomposed to yield energy invites fungi to colonize it.
(iii) They are eukaryotes but their eukaryotism is specific in some aspects. The fungal nuclei are minute to the extent that they cannot be easily observed in compound microscope. They are so plastic that they pass easily through minute septal pores. The nuclear division of fungi is intra-nuclear, i.e., the nuclear membrane is persistant and stages of division complete within it by the process called karyochorisis.
(iv) One-to-all known fungi are heterotrophic and absorb food from the environment employing extracellular digestion.
(v) The vegetative body of fungi is usually filamentous; the filament is called a hypha, which is threadlike, extensively branched, and surrounded by cell wall.
(vi) The vegetative body, even when it forms tissues, is never differentiated into root and shoot and, most important of all, has no specialized vessels for internal transport of nutrients.
(vii) The cell wall is characteristically mainly constituted of chitin (commonly called fungal cellulose); recently, cellulose has been found as chief constituent of cell wall (members of Oomycetes).
(viii) Fungi reproduce usually asexually and sexually by means of asexual and sexual spores, respectively. The main asexual spores formed are either sporangiospores formed inside sporangia or they are conidiospores (conidia) formed exogenously on hyphae or specialized hyphal branches called conidiophores. The sexual spores are oospores, zygospores, ascospores and basidiospores.
(ix) The reserve food material is glycogen (animal starch).
4. Kingdom Plantae (Macroalgae and Plants):
(i) Primarily autotrophic; some heterotrophic, a few saprophytic.
(ii) Organisms multicellular (except some algae) with walled and frequently vacuolate eukaryotic cells.
(iii) Simple multicellular to advanced tissue organization.
(iv) Autotrophism by means of photosynthesis; photosynthetic pigments are chlorophylls present in plastids (chloroplasts).
(v) Reproduction primarily sexual, with haploid and diploid stages alternating with each other (alternation of generations); haploid stage reduced in higher members of the kingdom.
(vi) Development of individuals, as a result of sexual reproduction, proceeds from solid embryos except in the algal groups.
(vii) Food reserve is usually starch and fat.
(viii) Growth is usually indefinite; growing points well-defined.
5. Kingdom Animalia (Invertebrate and Vertebrate Animals):
(i) Individuals multicellular with wall-less eukaryotic cells.
(ii) Multi-cellularity accompanied with cellular tissue and organ-system levels of organization with complex cell junctions.
(iii) Nutrition primarily ingestive with digestion in an internal cavity, but some forms are absorptive and some lack digestive cavity.
(iv) Reproduction mainly sexual with meiosis forming gametes; haploid stages other than gametes almost lacking above lowest phyla.
(v) Zygote develops into an embryo.
(vi) Animals are motile or mobile (except sponges).
(vii) Muscle cells present for mobility and nerve cells for conduction of impulses.
(viii) Centrioles occur in cells.
Merits of Five Kingdom System:
The five kingdom system bears following merits:
(i) Placement of prokaryotes separately in the kingdom monera is well appreciable as the prokaryotes differ from all other organisms (eukaryotes) in their cellular, physiological, and reproductive organization.
(ii) Many intermediate or transitional forms occur in unicellular eukaryotes, which had been included both amongst plants and animals. Placement of these unicellular eukaryotes into kingdom protista has removed this anomaly.
(iii) Fungi have their own unique physiological, biochemical, and structural characteristics and have never been related to plants. Their placement in the form of a separate kingdom was long overdue.
(iv) Creation of five kingdoms is based on the levels of organization and nutritional strategy which evolved very early and became established in later groups that are existing even today.
(v) As a result of this system, the animal and plant kingdoms appear more homogenous than they were in two-kingdom system.
(vi) Five kingdom system has tried to bring out phytogenetic relationships even amongst the primitive forms hence appears more natural.
Demerits of Five Kingdom System:
The five kingdom system is not accepted by all because of the following demerits:
(i) It lacks distinction between archaebacteria and eubacteria.
(ii) The kingdom protista still appears too diverse to be taxonomically useful.
(iii) The boundry-lines between the kingdoms monera, protista, fungi, and plantae are ill-defined. For convenience, the brown algae are probably not closely related to the plants even though this system places them in kingdom plantae, and monera and protista contain both walled and wall-less organisms, photosynthetic and non-photosynthetic organisms, and unicellular and filamentous or mycelial organisms.
(iv) A system based on phytogenetic relationships amongst organisms cannot be established in real terms until all the distinct evolutionary tendencies are taken into account. Five kingdom system does not satisfy in this respect.
For example, photosynthetic bacteria put under kingdom monera resemble certain green algae in the fact that both obtain hydrogen from sources other than water. Similarly, Euglena can be photosynthetic as well as heterotrophic and further it can have absorptive as well as ingestive mode of heterotrophic nutrition.
5. Six Kingdom System (Grey and Doolittle’s Concept):
Though Whittaker’s five kingdom system solved many problems, lack of distinction between archaebacteria (archaea) and bacteria remained untouched. Grey and Doolittle (1982) proposed to classify organisms into two superkingdoms the Prokaryota and the Eukaryota.
The superkingdom prokaryota further divided into two kingdoms Archaeobacteria and Eubacteria, whereas the super-kingdom Eukaryota divided into four kingdoms, Protista, Fungi, Plantae and Animalia. Therefore, they suggest adopting six distinct kingdoms to classify all the organisms (Fig. 2.6).
According to this proposal, the microorganisms spread in four kingdoms (Archaeobacteria, Eubacteria, Protista, Fungi) out of six all.
6. Eight Kingdom System (Cavalier-Smith’s Concept):
The Kingdom Protista was still too diverse to be taxonomically useful. Many attempts have been made to divide protists into better-defined kingdoms and in this regard, an eight-kingdom system (Fig. 2.7) was given by Cavalier-Smith (1987). Cavalier-Smith, using ultra-structural characteristics as well as rRNA sequences, divides all organisms into two Empires and eight kingdoms.
The Empire Bacteria consists of two kingdoms, the Eubacteria and the Archaeobacteria. The Empire Eukaryota contains six kingdoms of eukaryotes Archezoa, Protozoa, Chromista, Fungi, Plantae and Animalia; Archezoa and Chromista are the two new kingdoms of eukaryotes.
The kingdom Archezoa consists of primitive eukaryotic unicellular microorganisms (e.g. Giardia) that possess 70S ribosomes and lack cell organelles like golgi apparatus, mitochondria, chloroplasts and peroxisomes.
The kingdom Chromista is represented by diatoms, brown algae, crypto-monads and oomycetes; mainly the photosynthetic microorganisms that have their chloroplasts within the lumen of the rough ER rather than in the cytoplasmic matrix (as is found in the members of kingdom Plantae).
The four kingdoms Plantae, Fungi, Animalia and Protozoa are retained but their boundaries have been adjusted to better define each kingdom and differentiate it from the others. 