ParaHoxozoa (or Parahoxozoa) is a

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...

of animals that consists of

Bilateria The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly and ...

Placozoa The Placozoa are a basal form of marine free-living (non-parasitic) multicellular organism. They are the simplest in structure of all animals. Three genera have been found: the classical ''Trichoplax adhaerens'', '' Hoilungia hongkongensis'', a ...

, and

Cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that ...

. The relationship of this clade relative to the two other animal lineages

Ctenophora Ctenophora (; ctenophore ; ) comprise a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), and ...

and

Porifera Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through t ...

is debated. Some phylogenomic studies have presented evidence supporting Ctenophora as the sister to Parahoxozoa and Porifera as the sister group to the rest of animals (e.g. ). Some studies have presented evidence supporting Porifera as the sister to Parahoxozoa and Ctenophora as the sister group to the rest of animals (e.g. ).

Phylogeny

The tree below, which is congruent with the vast majority of these phylogenomic studies, conveys this uncertainty with a

polytomy An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tr ...

ParaHoxozoa or Parahoxozoa

Though "ParaHox" genes are usually referred to in

CamelCase Camel case (sometimes stylized as camelCase or CamelCase, also known as camel caps or more formally as medial capitals) is the practice of writing phrases without spaces or punctuation. The format indicates the separation of words with a single ...

and the original paper that named the clade used "ParaHoxozoa", the single initial cap format "Parahoxozoa" has become more common in the literature as CamelCase is not standard in zoological nomenclature.

Characteristics

Parahoxozoa was defined by the presence of several gene (sub)classes (HNF, CUT, PROS, ZF, CERS, K50, S50-PRD), as well as Hox/

ParaHox The ParaHox gene cluster is an array of homeobox genes (involved in morphogenesis, the regulation of patterns of anatomical development) from the Gsx, Xlox ( Pdx) and Cdx gene families. Regulatory gene cluster These genes were first shown to b ...

-ANTP from which the name of this clade originated. It was later proposed and contested that a gene of the same class (ANTP) as the Hox/ParaHox, the NK gene and the Cdx Parahox gene, is also present in

, the sponges. Regardless of whether a ParaHox gene is ever definitively identified, Parahoxozoa, as originally defined, is monophyletic and therefore continues to be used as such.

Planula-acoel, triploblasty, and bilaterian similarities

The original Bilateria are hypothesized to be a bottom dwelling worm with a single body opening. A through-gut may already have developed with the Ctenophora however. The through-gut may have developed from the corners of a single opening with lips fusing. E.g. Acoela resemble the

planula A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species and also in some species of Ctenophores. Some groups of Nemerteans also produce larvae that are very similar to the planula, which ...

larvae of some Cnidaria, which exhibit some bilaterian symmetry. They are vermiformes, just as the cnidarian '' Buddenbrockia'' is''.'' Placozoa has been noted to resemble planula. Usually, "Planulozoa" is to the exclusion of Placozoa, but not necessarily. In this case it appears synonymous with Parahoxozoa.

Triploblasty Triploblasty is a condition of the gastrula in which there are three primary germ layers: the ectoderm, mesoderm, and endoderm. Germ cells are set aside in the embryo at the blastula stage, which are incorporated into the gonads during organo ...

developed before the Cnidaria-Bilateria radiation as well.

References

{{Taxonbar, from=Q16976127 Parazoa Superphyla