How two extraembryonic epithelia became one: serosa and amnion features and functions of 's amnioserosa.

The conservation of gene networks that specify and differentiate distinct tissues has long been a subject of great interest to evolutionary developmental biologists, but the question of how pre-existing tissue-specific developmental trajectories merge is rarely asked. During the radiation of flies, two extraembryonic epithelia, known as serosa and amnion, evolved into one, called amnioserosa. This unique extraembryonic epithelium is found in fly species of the group Schizophora, including the genetic model organism , and has been studied in depth.

Autonomous epithelial folding induced by an intracellular mechano-polarity feedback loop.

Epithelial tissues form folded structures during embryonic development and organogenesis. Whereas substantial efforts have been devoted to identifying mechanical and biochemical mechanisms that induce folding, whether and how their interplay synergistically shapes epithelial folds remains poorly understood. Here we propose a mechano-biochemical model for dorsal fold formation in the early Drosophila embryo, an epithelial folding event induced by shifts of cell polarity.

Functional evolution of a morphogenetic gradient.

Bone Morphogenetic Proteins (BMPs) pattern the dorsal-ventral axis of bilaterian embryos; however, their roles in the evolution of body plan are largely unknown. We examined their functional evolution in fly embryos. BMP signaling specifies two extraembryonic tissues, the serosa and amnion, in basal-branching flies such as , but only one, the amnioserosa, in .

Morphogenetic functions of extraembryonic membranes in insects.

Morphogenetic functions of the amnioserosa, the serosa, the amnion, and the yolk sac are reviewed on the basis of recent studies in flies (Drosophila, Megaselia), beetles (Tribolium), and hemipteran bugs (Oncopeltus). Three hypotheses are presented. First, it is suggested that the amnioserosa of Drosophila and the dorsal amnion of other fly species function in a similar manner.

Embryo development. A cysteine-clamp gene drives embryo polarity in the midge Chironomus.

In the fruit fly Drosophila, head formation is driven by a single gene, bicoid, which generates head-to-tail polarity of the main embryonic axis. Bicoid deficiency results in embryos with tail-to-tail polarity and no head. However, most insects lack bicoid, and the molecular mechanism for establishing head-to-tail polarity is poorly understood.

BMP-dependent serosa and amnion specification in the scuttle fly Megaselia abdita.

Bone morphogenetic protein (BMP) signaling is an essential factor in dorsoventral patterning of animal embryos but how BMP signaling evolved with fundamental changes in dorsoventral tissue differentiation is unclear. Flies experienced an evolutionary reduction of extra-embryonic tissue types from two (amniotic and serosal tissue) to one (amnionserosal tissue). BMP-dependent amnioserosa specification has been studied in Drosophila melanogaster.