Paxgene diversity in the basal cnidarianAcropora millepora(Cnidaria, Anthozoa): Implications for the evolution of thePaxgene family

Abstract

Pax genes encode a family of transcription factors, many of which play key roles in animal embryonic development but whose evolutionary relationships and ancestral functions are unclear. To address these issues, we are characterizing the Pax gene complement of the coral Acropora millepora, an anthozoan cnidarian. As the simplest animals at the tissue level of organization, cnidarians occupy a key position in animal evolution, and the Anthozoa are the basal class within this diverse phylum. We have identified four Pax genes in Acropora: two (

Cubozoan jellyfish: an Evo/Devo model for eyes and other sensory systems

“…Perhaps the examples given above of photoreception leading to coordinated behavior by directly controlling ciliary motion in the unicellular alga, as well as the larval sponge and cubozoan jellyfish, represent primordial eye/mechanoreceptors before each sensory system diverged to elaborate either vision (eyes) or mechanoreception (ears). The hypothesis of an eye/ear evolutionary relationship is reinforced by the fact that PaxB is a hybrid Pax protein, containing a Pax2-like paired domain and octapeptide, but has a Pax6-like homeodomain (Groger et al, 2000, Kozmik et al, 2003, Miller et al, 2000, Sun et al, 2001, Sun et al, 1997) (see Fig. 6).…”

“…Despite the importance of Pax6 in eye development and the presence of eyes in Tripedalia, we have been able to clone only PaxB, not Pax6, from this species (Kozmik et al, 2003). Cnidarian PaxB genes cluster with the Pax2/5/8 subfamily in a phylogenetic tree analysis (Miller et al, 2000). This cluster most likely represents an ancient group of genes within the Pax family since its members were found in sponges (Hoshiyama et al, 1998).…”

“…Even if Pax6 does exist in Tripedalia but has escaped our efforts to detect its gene or cDNA, our co-transfection tests suggest that the jellyfish Pax6 protein would neither bind nor activate the J3-crystallin promoter, as does PaxB (Kozmik et al, 2003). Four Pax genes (PaxA, PaxB, PaxC and PaxD) have been found in corals (also cnidarians) (Miller et al, 2000). However, none represents a true Pax6 ancestor and none has the three characteristic amino acids (IQN) at positions 42, 44 and 47 of the Pax6 paired domain (Kozmik et al, 2003).…”

Perplexing Pax: From puzzle to paradigm

“…Indeed, the DNA binding paired domain that Pax proteins utilize for target gene selection remains strikingly highly conserved, and with conservation of this domain comes conservation of function. While higher vertebrates have nine Pax genes and Drosophila have ten (Bopp et al, 1989;Walther et al, 1991;Balczarek et al, 1997;Sun et al, 1997;Jun et al, 1998;Miller et al, 2000;Dominguez et al, 2004), the simplest of multicellular animals, the placazoans (specifically, Trichoplax adhaerens) who lack nerve or muscle cells and any kind of body symmetry, have a single Pax gene (Hadrys et al, 2005). Given that conserved functional domains of Pax genes predate the origin of nervous and muscular systems, Pax proteins must play a functional role more fundamental than the specification of heterogeneous cell types.…”

Historical perspective on the development and evolution of eyes and photoreceptors

“…However, RNA interference and gene transfer experiments provide functional genetic evidence that Pax 6 and sine oculis are involved in eye development in planarians. In order to study the universality of Pax 6 control in eye development we have adopted a novel strategy; if Pax 6 expression is conserved in evolution, the gene have fewer Pax genes than Bilaterians, and so far no bona fide Pax 6 homolog has been found in corals (Miller et al, 2000;Plaza et al, 2003), in hydrozoans (Gröger et al, 2000;Sun et al, 2001) and in cubozoans (Sun et al, 1997;Kozmik et al, 2003). Until the complete genomic sequences are available, the possibility that a bona fide Pax 6 gene exists in cnidarians cannot be excluded.…”