Monera

Figure 1. Five kingdoms.

In some systems of biological classification, Monera is a kingdom that comprises most living things with a prokaryotic cell organization. For this reason the kingdom was sometimes called Prokaryotae.

Monera traditionally included the bacteria and blue-green algae (cyanobacteria), and sometimes the archaebacteria (Towle 1989).

In the five kingdom system, the other kingdoms are Protista, Fungi, Plantae, and Animalia. In the six kingdom system, the archaebacteria are separated into their own kingdom.

Although the kingdom level traditionally was the highest level, in recent years a higher classification level has been promoted called a "domain," or "superkingdom," and includes three major groups, the Archaea; Bacteria; and Eukaryota. Although sometimes monera can be seen as a kindgom under the domain Bacteria, in Carl Woese's three-domain system, life is divided into 23 main divisions, all incorporated within three domains: Bacteria, Archaea, and Eucarya.

Biological classification

For two centuries, from the mid-18th century until the mid-20th century, organisms were generally considered to belong to one of two kingdoms, Plantae (plants, including bacteria) or Animalia (animals, including protozoa). This system was proposed by Carolus Linnaeus in the mid-18th century. However, this classification had obvious difficulties, including the problem of placing fungi, protists, and prokaryotes. There are single-celled organisms that fall between the two categories, such as Euglena, that can photosynthesize food from sunlight and yet feed by consuming organic matter.

Traditionally objects were classified as animal, vegetable or mineral (see: Systema Naturae). After the discovery of microscopy, attempts were made to fit microscopic organisms into either the plant or animal kingdom. In 1866 Ernst Haeckel proposed a three kingdom system which added Protista as a new kingdom that contained most microscopic organisms. Later, Haeckel proposed the fourth kingdom of which he called Monera. One of his eight major divisions of Protista was called Moneres. Haeckel's Moneres subcategory included known bacterial groups such as Vibrio. Haeckel's Protista kingdom also included eukaryotic organisms now classified as Protist. It was later decided that the Monera had proven to be too diverse to be seriously considered one single kingdom.

In 1969, American ecologist Robert H. Whittaker proposed a system with five kingdoms: Monera (prokaryotes — bacteria and blue-green algae), Protista (unicellular, multicellular, and colonial protists), Fungi, Plantae, and Animalia. This system was widely used for three decades, and remains popular today.

More recently, a classification level higher than a kingdom has been devised, called a "domain." Also called a "Superregnum" or "Superkingdom," a domain is the top-level grouping of organisms in scientific classification. One of the reasons such a classification has been developed is because research has revealed the unique nature of anaerobic bacteria (called Archaeobacteria, or simply Archaea). These "living fossils" are genetically and metabolically very different from oxygen-breathing organisms. Various numbers of Kingdoms are recognized under the domain category.

In the three-domain system, which was introduced by Carl Woese in 1990, the three groupings are:Archaea; Bacteria; and Eukaryota. This scheme emphasizes the separation of prokaryotes into two groups, the Bacteria (originally labelled Eubacteria) and the Archaea (originally labelled Archaebacteria).

His three-domain system, based upon genetic relationships rather than obvious morphological similarities, divided life into 23 main divisions, all incorporated within three domains: Bacteria, Archaea, and Eucarya.

In some classifications, authorities keep the kingdom system as the higher-level classification, but recognize a sixth kingdom, the Archaebacteria.

Coexisting with these schemes is one dividing living organisms into the two main divisions of prokaryote (cells that lack a nucleus: bacteria, etc.) and eukaryote (cells that have a nucleus and membrane-bound organelles: animals, plants, fungi, and protists).

In summary, today there are several competing top classifications of life:

• The three-domain system of Carl Woese, with top-level groupings of Archaea, Eubacteria, and Eukaryota domains;
• The two-empire system, with top-level groupings of Prokaryota (or Monera) and Eukaryota empires.
• The five-kingdom system with top-level groupings of Monera, Protista, Fungi, Plantae, and Animalia.
• The six-kingdom system with top-level groupings of Archaebacteria, Monera, Protista, Fungi, Plantae, and Animalia.

Overall, the majority of biologists accept the domain system, but a large minority uses the five-kingdom method. A small minority of scientists adds Archaea or Archaebacteria as a sixth kingdom but do not accept the domain method.

Prior to its creation these were treated as two separate divisions of plants: the Schizomycetes or bacteria, considered fungi, and the Cyanophyta or blue-green algae. They later are now considered a group of bacteria, typically called the cyanobacteria, and are known not to be closely related to plants, fungi, or animals. Recent DNA and RNA sequence analyses has demonstrated that there are two major groups of prokaryotes, the Bacteria and Archaea, which do not appear to be closer in relationship to each other than they are to the Eukaryotes. All new schemes abandon the Monera and now treat the Bacteria, Archaea, and Eukarya as separate domains or kingdoms.

In 1969 Robert Whittaker published a proposed five kingdom system (Figure 1) for classification of living organisms. Whittaker's system placed most single celled organisms into either the prokaryotic Monera or the eukaryotic Protista. The other three kingdoms in his system were the eukaryotic Fungi, Animals and Plants.

Figure 2. A phylogenetic tree of living things, based on rRNA sequence data, showing the separation of bacteria, archaea, and eukaryotes.

Linnaeus 1735 2 kingdoms	Haeckel 1866 3 kingdoms	Chatton 1937 2 empires	Copeland 1956 4 kingdoms	Whittaker 1969 5 kingdoms	Woese et al. 1977 6 kingdoms	Woese et al. 1990 3 domains
(not treated)	Protista	Prokaryota	Monera	Monera	Eubacteria	Bacteria
					Archaebacteria	Archaea
		Eukaryota	Protista	Protista	Protista	Eukarya
Vegetabilia	Plantae			Fungi	Fungi
			Plantae	Plantae	Plantae
Animalia	Animalia		Animalia	Animalia	Animalia

Based on molecular phylogeny studies, Carl Woese proposed that the prokaryotes (monerans) be divided into two separate groups: Bacteria and Archaea. In Carl Woese's 1990 proposed phylogeny, these three kingdoms are all rooted in a universal common ancestor and this is the most widely accepted categorical phylogeny accepted today (see diagram to the right). However, the most modern interpretation for these three kingdoms is the "Universal and Eukaryote Phylogenetic Tree" based on 16s rDNA as presented on the Tree of Life website.

See also

• Bacterial cell structure
• Kingdom (biology)

Monera also called Prokayotae it is the kingdom of prokaryotic organisms that are unicellular that is a single-celled: consisting of a single cell & lack a true cell nucleus.

External links

• Bacterial evolution by Carl Woese (1987). Woese reviewed the historical steps leading to the use of the term "Monera" and its later abandonment (full text online). Entrez PubMed 2439888

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