The Role of Pax Group III Genes in Short-Germ Segmentation

The Drosophila Pax group III (PgIII) genes paired, gooseberry, and gooseberry-neuro encode transcription factors, each containing both a DNA-binding paired domain and a paired-type homeodomain. In Drosophila embryogenesis, paired acts as one of several pair-rule genes to define the boundaries of future segments via the regulation of segment-polarity genes, including gooseberry. The function of gooseberry-neuro is not yet defined, but the gene is expressed later in the developing nervous system. The three genes are similar in sequence and are thought to have resulted from two duplication events in the lineage leading to Drosophila. Because of PgIII genes play a role in Drosophila segmentation, we screened cDNA pools from early embryos of the short-germ insects Tribolium and Schistocerca in order to isolate PgIII orthologs. We have identified distinct genes of high sequence identity to the fly PgIII genes and used phylogenetic methods to assign orthology. We have also raised polyclonal antisera that crossreact to all three Drosophila proteins. The antisera also crossreact with presumptive PgIII orthologs in Tribolium and Schistocerca. The role of these genes in the segmentation of short-germ insects, as inferred from expression data, sheds light on both the evolution of PgIII gene function and pair-rule patterning in insect segmentation.