Abstracts (first author)
A tale of two ends: axis polarity in the scuttle fly Megaselia abdita
In dipterans (flies, midges and mosquitoes), segment determination occurs early in embryogenesis through a hierarchy of genes activated by maternal factors. These include the anterior determinant bicoid (bcd), found only in Cyclorrhapha or "higher" flies, and also caudal (cad), hunchback (hb), and nanos (nos). Loss of any one of these genes in Drosophila results in the loss of segments, and in some cases duplication of anterior or posterior structures at the opposite pole of the egg. However, in none of these cases is global polarity lost. In contrast, removal of both maternal bcd and hb results in symmetrical embryos (mirror-abdomen or bicaudal phenotypes) in which global anterior-posterior polarity is lost. Similar bicaudal phenotypes can be generated in two other fly species, the hover fly Episyrphus balteatus and the scuttle fly Megaselia abdita, by knocking down only maternal bcd. In Episyrphus, this has been explained by a lack of maternal hb. However, in Megaselia this factor is present so another explanation must be found. We have generated gene knock-downs for each of the maternal factors in Megaselia and analysed the resulting gene expression patterns. We identify loss of Kr activation by maternal hb as the likely cause of the loss of polarity. Furthermore, we identify differences in cad and gap gene expression that account for the more anterior plane of symmetry observed in Megaselia bicaudal phenotypes. These differences suggest that a number of regulatory changes in the gap gene network have occurred since the divergence of these dipteran lineages. We are testing these inferred changes by performing more knock-down experiments, combined with data-driven modeling of the gap gene system in this species. Finally, we revisit the question of Kr activation in Drosophila and suggest an evolutionary scenario for the development of anterior-posterior polarity in dipterans.