Characterization of an amphioxus paired box gene,AmphiPax2/5/8: developmental expression patterns in optic support cells, nephridium, thyroid-like structures and pharyngeal gill slits, but not in the midbrain-hindbrain boundary region

Abstract

On the basis of developmental gene expression, the vertebrate central nervous system comprises: a forebrain plus anterior midbrain, a midbrain-hindbrain boundary region (MHB) having organizer properties, and a rhombospinal domain. The vertebrate MHB is characterized by position, by organizer properties and by being the early site of action of Wnt1 and engrailed genes, and of genes of the Pax2/5/8 subfamily. Wada and others (Wada, H., Saiga, H., Satoh, N. and Holland, P. W. H. (1998) Development 125, 1113–1122) suggested that ascidian tunicates have a vertebrate-like MHB on the basis of ascidian Pax258 expression there. In another invertebrate chordate, amphioxus, comparable gene expression evidence for a vertebrate-like MHB is lacking. We, therefore, isolated and characterized AmphiPax2/5/8, the sole member of this subfamily in amphioxus. AmphiPax2/5/8 is initially expressed well back in the rhombospinal domain and not where a MHB would be expected. In contrast, most of the other expression domains of AmphiPax2/5/8 correspond to expression domains of vertebrate Pax2, Pax5 and Pax8 in structures that are probably homologous - support cells of the eye, nephridium, thyroid-like structures and pharyngeal gill slits; although AmphiPax2/5/8 is not transcribed in any structures that could be interpreted as homologues of vertebrate otic placodes or otic vesicles. In sum, the developmental expression of AmphiPax2/5/8 indicates that the amphioxus central nervous system lacks a MHB resembling the vertebrate isthmic region. Additional gene expression data for the developing ascidian and amphioxus nervous systems would help determine whether a MHB is a basal chordate character secondarily lost in amphioxus. The alternative is that the MHB is a vertebrate innovation.

Evolutionary Transition in the Regulation of Vertebrate Pronephros Development: A New Role for Retinoic Acid

“…Following the treatments, embryos and larvae exhibited significant malformations, particularly in the pharyngeal region (Figure 5A-F), confirming the activity of the pharmacological compounds [53,54]. WMISH for Pax2/5/8 and Lim1/5 during normal development revealed that, at the L0 stage, both genes were conspicuously expressed in Hatschek's nephridium (Figure 5A,B) [32,33], located in the anterior mesoderm on the left side of the body in a dorsomedial position close to the mouth (Figure 5A',B') [34]. When comparing the signals of Pax2/5/8 and Lim1/5 between control and treated specimens, we found that exogenous RA led to a complete loss of expression of both genes in Hatschek's nephridium and other anterior mesodermal tissues (Figure 5C,D), while treatments with BMS493 had no significant effect on their expression in this organ (Figure 5E,F).…”

“…Due to the position of this organ and its particular histological development, its homology with the vertebrate pronephros has been the subject of a long-lasting debate [34,63,64]. The expression of Pax2/5/8 and Lim1/5 [32,33] in Hatschek's nephridium provides further evidence for this homology. However, the manifestation of Pax2/5/8 and Lim1/5 in the most anterior somite of amphioxus, on only one side of the body, raises the question of whether the excretory system of the last common ancestor of amphioxus and vertebrates was like that of extant amphioxus.…”

“…Amphioxus, thus, provides an outgroup essential to gain insights into the evolutionary origin of the vertebrate kidney. Previous examinations of the development of the nephric system in amphioxus have revealed that homologs of the vertebrate pronephric markers Pax2 and Lim1 are expressed in Hatschek's nephridium [32,33], an excretory, kidney-like structure derived from the 1st left somite [34]. This position along the AP axis contrasts with that of the amniote pronephros, which develops posterior to the level of mid-somite 6.…”

It takes Two: Discovery of Spider Pax2 Duplicates Indicates Prominent Role in Chelicerate Central Nervous System, Eye, as Well as External Sense Organ Precursor Formation and Diversification After Neo- and Subfunctionalization

“…Since its role in nervous system development seems to be well conserved throughout metazoans, this raises the question, whether Pax2 plays a role in excretory system development in invertebrate animals as well. Expression studies in the lancelet Branchiostoma floridae, a non-vertebrate chordate, have confirmed Pax2 expression in the developing metanephridia (Kozmik et al, 1999). Tunicates are now known to be more closely related to vertebrates than lancelets, but do not possess an elaborate excretory system, and hence it is unsurprising that studies on Pax2 expression in the tunicates Ciona intestinalis and Oikopleura dioica mostly focus on nervous system expression (Mazet et al, 2003;Bassham et al, 2008).…”

Hedgehog signaling controls mouth opening in the amphioxus

“…This result showed that the initial specification of the mouth in the amphioxus may not depend on Hh signaling. To verify this, we also examined the expression of Pax2/5/8 at the primordial oral site before mouth opening (T1 stage) [ 31 ]. In agreement with Pou4 expression at the T1 stage, Pax 2/5/8 expression was not affected at the T1 stage in embryos with loss of Hh activation (Fig.…”