Molecular, phylogenetic and developmental analyses of Sall proteins in bilaterians.

BACKGROUND

Sall (Spalt-like) proteins are zinc-finger transcription factors involved in a number of biological processes. They have only been studied in a few model organisms, such as , , and some vertebrates. Further taxon sampling is critical to understand the evolution and diversification of this protein and its functional roles in animals.

RESULTS

Using genome and transcriptome mining, we confirmed the presence of genes in a range of additional animal taxa, for which their presence had not yet been described. We show that genes are broadly conserved across the Bilateria, and likely appeared in the bilaterian stem lineage. Our analysis of the protein domains shows that the characteristic arrangement of the multiple zinc-finger domains is conserved in bilaterians and may represent the ancient arrangement of this family of transcription factors. We also show the existence of a previously unknown zinc-finger domain. In situ hybridization was used to describe the gene expression patterns in embryonic and larval stages in two species of snails: and . In , presents maternal expression, although later on the expression is restricted to the A and B quadrants during gastrulation and larval stage. In , has no maternal expression and it is expressed mainly in the A, C and D quadrants during blastula stages and in an asymmetric fashion during the larval stage.

DISCUSSION

Our results suggest that the bilaterian common ancestor had a Sall protein with at least six zinc-finger domains. The evolution of Sall proteins in bilaterians might have occurred mostly as a result of the loss of protein domains and gene duplications leading to diversification. The new evidence complements previous studies in highlighting an important role of Sall proteins in bilaterian development. Our results show maternal expression of in the snail , but not . The asymmetric expression shown in the ectoderm of the trochophore larva of snails is probably related to shell/mantle development. The observed expression in cephalic tissue in snails and some other bilaterians suggests a possible ancestral role of in neural development in bilaterians.