Pancrustacea is the clade that comprises all and all Hexapoda ( and relatives). This grouping is contrary to the Atelocerata hypothesis, in which Hexapoda and Myriapoda are sister taxon, and Crustacea are only more distantly related. As of 2010, the Pancrustacea taxon was considered well accepted, with most studies recovering Hexapoda within Crustacea. The clade has also been called Tetraconata, referring to having a four-part cone in the ommatidium. The term "Tetraconata" is preferred by some scientists in order to avoid confusion with the use of "pan-" to indicate a clade that includes a crown group and all of its stem group representatives.
Molecular studies
A
monophyletic Pancrustacea has been supported by several molecular studies, in most of which the subphylum Crustacea is
paraphyletic with regard to hexapods (that is, that hexapods, including insects, are derived from crustacean ancestors). This means that within Pancrustacea, only some members are actually crustaceans, hexapods being the main exception.
The evidence for this clade derives from molecular data and morphological characteristics. The molecular data consists of comparisons of Cell nucleus ribosomal RNA , ribosomal RNA genes, and protein coding genes. The morphological data consists of structures (see arthropod eye), the presence of neuroblasts, and the form and style of axonogenesis by pioneer .
Regier et al. (2005)
In a 2005 study of nuclear genomes Regier
et al. suggest that
Hexapoda is most closely related to
Branchiopoda and
Cephalocarida +
Remipedia, thereby hexapods are "terrestrial crustaceans", thus supporting the Pancrustacea hypothesis that
maxillopoda are not monophyletic (in the following cladograms Maxillopoda subclasses are
highlighted). In addition, there appeared some evidence against the
monophyly: that Ostracoda subclass
Podocopa may form a clade with
Branchiura.
Regier et al. (2010)
A 2010 study of nuclear genomes (Regier
et al.) strongly supports Pancrustacea and strongly favour
Mandibulata (
Myriapoda + Pancrustacea) over
Paradoxopoda (Myriapoda +
Chelicerata). According to this study, Pancrustacea is divided into four lineages: Oligostraca (
,
Mystacocarida,
Branchiura,
Pentastomida), Vericrustacea (
Malacostraca,
Thecostraca,
Copepod,
Branchiopoda),
Xenocarida (
Cephalocarida,
Remipedia) and
Hexapoda, with Xenocarida as a sister group to the Hexapoda (comprising "Miracrustacea").
New clades proposed by Regier et al. are:
Of these proposed clades, only Multicrustacea was confirmed in later molecular studies.
von Reumont et al. (2012)
In a 2012 molecular study, von Reumont
et al. challenge the monophyly of Vericrustacea: they present four versions of Pancrustacea cladogram (figures 1–4), and in all four figures
Remipedia is a sister group to
Hexapoda, and
Branchiopoda is a sister group to (Remipedia + Hexapoda). Thus, their data strongly suggest that Branchiopoda is more closely related to Hexapoda and Remipedia than to Multicrustacea. Based on these data, they propose the following scenario of evolution of Branchiopoda, Remipedia and Hexapoda: under the impact of predatory fishes their common ancestors go to the
littoral zone, then ancestors of Branchiopoda go to the
vernal pool, whereas ancestors of Remipedia go to the
anchialine cave, and ancestors of Hexapoda go to the
land.
Jondeung et al. (2012)
Another molecular study (of mitochondrial genomes), conducted in 2012 by Jondeung
et al., strongly support monophyletic Pancrustacea and places
Malacostraca +
Entomostraca and
Branchiopoda as the sister clade to Hexapoda and places
Cirripedia +
Remipedia as a basal lineage of Pancrustacea.
Oakley et al. (2013)
In 2013 combined study of morphology, including fossils, and molecular data, including expressed sequence tag, mitochondrial genome, nuclear genome, and ribosomal DNA data Oakley
et al. obtained support for three pancrustacean clades: Oligostraca (Ostracoda, Mystacocarida, Branchiura, Pentastomida), Multicrustacea (Copepoda, Thecostraca, Malacostraca) and a clade they refer to as Allotriocarida (Branchiopoda, Cephalocarida, Remipedia, Hexapoda), as well as for monophyly of Ostracoda. Within Multicrustacea they obtained support for a clade they suggest the name
Hexanauplia: Thecostraca + Copepoda. Relations within Allotriocarida remain uncertain: sister taxon to Hexapoda is either Remipedia, or the clade Branchiopoda + Cephalocarida, however, authors are inclined to the first version (see "Conclusion", 4), which is also consistent with von Reumont
et al. (2012) results.
New proposed by Oakley et al. clades are:
Note: the Allotriocarida clade was also recovered in 2005 by Regier et al. as Clade #33, but relations within it were different, and they did not choose a name for it.
Rota-Stabelli et al. (2013)
In 2013 Rota-Stabelli
et al. used the signal in the 62 protein-coding genes assembled by Regier
et al. in 2010 to improve the knowledge of the internal relationship in the Pancrustacea group. This data set infers a highly supported
nucleotide tree that is substantially different from the corresponding, but poorly supported,
amino acid one. The discrepancy between the nucleotide-based and the amino acids-based trees is caused by substitutions within synonymous
codon families (especially those of
serine-TCN and AGY): different arthropod lineages are differentially biased in their usage of
serine,
arginine, and
leucine synonymous codons, and the serine bias is correlated with the topology derived from the nucleotides, but not the amino acids. The authors suggest that a parallel, partially compositionally driven, synonymous codon-usage bias affects the nucleotide topology. As substitutions between serine codon families can proceed through
threonine or
cysteine intermediates, amino acid data sets might also be affected by the serine codon-usage bias. The analyses suggests that a
Dayhoff recoding strategy would partially ameliorate the effects of such bias. Although amino acids provide an alternative hypothesis of pancrustacean relationships, neither the nucleotides nor the amino acids version of this data set bring enough genuine phylogenetic information to robustly resolve the relationships within group, which should still be considered unresolved. However the amino acid tree seems to be more likely since it appears to be free from the synonymous codon-family bias affecting the nucleotide one. Most of the inferences based on amino acids sequences support a clade which includes
Branchiopoda,
Remipedia,
Copepoda and
Hexapoda (
group A). Using the best amino acids substitution model,
CATGTR, also
Cephalocarida falls inside this group. In all the analyses group A (with or without Cephalocarida) is sister-group of a clade composed by
Malacostraca, Oligostraca and
Thecostraca (
group B).
The following diagram shows the tree resulting from the Dayhoff recoding.
Lozano-Fernandez et al. (2019)
The relationship of Hexapoda and the crustacean classes is shown in the following phylogenetic tree, which shows Allotriocarida, along with Oligostraca and Multicrustacea, as the three main divisions of subphylum Pancrustacea, embracing the traditional crustaceans and the hexapods (including insects).
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Position of Tantulocarida
According to Petrunina A.S. and Kolbasov G.A., the sixth subclass of Maxillopoda Tantulocarida may lie within Thecostraca, forming a clade with thecostracan infraclass Cirripedia (if so, Thecostraca excluding Tantulocarida is paraphyletic):
See also
