Pancrustacea
Encyclopedia
Pancrustacea is a clade
, comprising all crustacean
s and hexapods
. This grouping is contrary to the Atelocerata
hypothesis, in which Myriapoda
and Hexapoda
are sister taxa, and Crustacea are only more distantly related. As of 2010, the Pancrustacea taxon is considered well accepted. The clade has also been called Tetraconata, referring to the square ommatidia of many of its members. That name is preferred by some scientists as a means of avoiding confusion with the use of "pan-" to indicate a clade that includes a crown group
and all of its stem group representatives.
A monophyletic Pancrustacea has been supported by several molecular studies, in most of which the subphylum Crustacea is paraphyletic with regard to insects (that is, that insects are derived from crustacean ancestors)
The evidence for this clade derives from molecular data and morphological characteristics. The molecular data consists of comparisons of nuclear
ribosomal RNA
gene
s, mitochondrial ribosomal RNA genes, and protein
coding genes. The morphological data consists of ommatidial structures (see arthropod eye
), the presence of neuroblasts, and the form and style of axonogenesis by pioneer neuron
s.
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
, comprising all crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s and hexapods
Hexapoda
The subphylum Hexapoda constitutes the largest grouping of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura . The Collembola are very abundant in terrestrial environments...
. This grouping is contrary to the Atelocerata
Atelocerata
Atelocerata Heymons, 1901 is a proposed clade of arthropods that includes Hexapoda and Myriapoda , but excludes Crustacea and Chelicerata...
hypothesis, in which Myriapoda
Myriapoda
Myriapoda is a subphylum of arthropods containing millipedes, centipedes, and others. The group contains 13,000 species, all of which are terrestrial...
and Hexapoda
Hexapoda
The subphylum Hexapoda constitutes the largest grouping of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura . The Collembola are very abundant in terrestrial environments...
are sister taxa, and Crustacea are only more distantly related. As of 2010, the Pancrustacea taxon is considered well accepted. The clade has also been called Tetraconata, referring to the square ommatidia of many of its members. That name is preferred by some scientists as a means of avoiding confusion with the use of "pan-" to indicate a clade that includes a crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
and all of its stem group representatives.
A monophyletic Pancrustacea has been supported by several molecular studies, in most of which the subphylum Crustacea is paraphyletic with regard to insects (that is, that insects are derived from crustacean ancestors)
The evidence for this clade derives from molecular data and morphological characteristics. The molecular data consists of comparisons of nuclear
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
ribosomal RNA
Ribosomal RNA
Ribosomal ribonucleic acid is the RNA component of the ribosome, the enzyme that is the site of protein synthesis in all living cells. Ribosomal RNA provides a mechanism for decoding mRNA into amino acids and interacts with tRNAs during translation by providing peptidyl transferase activity...
gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
s, mitochondrial ribosomal RNA genes, and protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
coding genes. The morphological data consists of ommatidial structures (see arthropod eye
Arthropod eye
The arthropods ancestrally possessed compound eyes, but the type and origin of this eye varies between groups, and some taxa have secondarily developed simple eyes...
), the presence of neuroblasts, and the form and style of axonogenesis by pioneer neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
s.