Search

Abstract: Pax genes play an important role in networks of transcription factors that determine organogenesis, notably the development of sensory organs. Other members of this regulatory network include transcription factors encoded by the Six gene family. Sponges lack organs and a nervous system, possibly because they have not evolved a Pax/Six network. Here we show that the demosponge Chalinula loosanoffi encodes only one Pax and one Six gene, representatives of the PaxB and Six1/2 subfamilies. Analysis of their temporal transcription patterns during development shows no correlation of their mRNA levels while their spatial patterns show some overlap of expression in adult tissue, although cellular resolution was not achieved. These results do not suggest that these genes form a major network in this basal phylum, although its existence in a minor fraction of cells is not excluded. We further show that sponge PaxB can substitute for some of the Pax2, but not of the Pax6 functions in Drosophila. Finally, we have analyzed the phylogeny of Pax and Six genes and have derived a model of the evolution of the Pax gene subfamilies in metazoans. It illustrates a diversification of Pax genes into subfamilies mostly in triploblasts before the protostome–deuterostome split, whereas few subfamilies were lost in various phyla after the Cambrian explosion. [Copyright &y& Elsevier]

Abstract: Patterning of the Drosophila embryonic mesoderm requires the regulation of cell type-specific factors in response to dorsoventral and anteroposterior axis information. For the dorsoventral axis, the homeodomain gene, tinman, is a key patterning mediator for dorsal mesodermal fates like the heart. However, equivalent mediators for more ventral fates are unknown. We show that D-six4, which encodes a Six family transcription factor, is required for the appropriate development of most cell types deriving from the non-dorsal mesoderm — the fat body, somatic cells of the gonad, and a specific subset of somatic muscles. Misexpression analysis suggests that D-Six4 and its likely cofactor, Eyes absent, are sufficient to impose these fates on other mesodermal cells. At stage 10, the mesodermal expression patterns of D-six4 and tin are complementary, being restricted to the dorsal and non-dorsal regions respectively. Our data suggest that D-six4 is a key mesodermal patterning mediator at this stage that regulates a variety of cell-type-specific factors and hence plays an equivalent role to tin. At stage 9, however, D-six4 and tin are both expressed pan-mesodermally. At this stage, tin function is required for full D-six4 expression. This may explain the known requirement for tin in some non-dorsal cell types. [Copyright &y& Elsevier]

The development of visual organs is regulated in Bilateria by a network of genes where members of the Six and Pax gene families play a central role. To investigate the molecular aspects of eye evolution, we analyzed the structure and expression patterns of cognate members of the Six family genes in jellyfish (Cnidaria, Hydrozoa), representatives of a basal, non-bilaterian phylum where complex lens eyes with spherical lens, an epidermal cornea, and a retina appear for the first time in evolution.In the jellyfish Cladonema radiatum, a species with well-developed lens eyes in the tentacle bulbs, Six1/2-Cr and Six3/6-Cr, are expressed in the eye cup. Six4/5-Cr is mainly expressed in the manubrium, the feeding, and sex organ. All three Six genes are expressed in different subsets of epidermal nerve cells, possibly of the RFamide type which are part of a net connecting the different eyes with each other and the effector organs. Furthermore, expression is found in other tissues, notably in the striated muscle. During eye regeneration, expression of Six1/2-Cr and Six3/6-Cr is upregulated, but not of Six4/5-Cr. In Podocoryne carnea, a jellyfish without eyes, Six1/2-Pc and Six3/6-Pc are also expressed in the tentacle bulbs, Six1/2-Pc additionally in the manubrium and striated muscle, and Six3/6-Pc in the mechanosensory nematocytes of the tentacle.The conserved gene structure and expression patterns of all Cladonema Six genes suggest broad conservation of upstream regulatory mechanisms in eye development. [Copyright &y& Elsevier]