Evolutionary Reorganization of
Embryonic Development in Flies
This project uses extraembryonic epithelia as a model to study the genetic and developmental basis morphological change in evolution. The amnioserosa of Drosophila combines characteristics of the amnion and the serosa found in almost all other insects. The genetic mechanism that underlies amnioserosa specification has been studied in great molecular detail and provides an ideal reference system to study the evolution of developmental mechanisms. Phorids such as Meagselia are among the closest relatives of Drosophila known to develop distinct amniotic and serosal epithelia. By studying the genetic mechanism of amnion and serosa specification in Megaselia, we expect to better understand how mechanisms of cell fate specification and tissue formation evolve.
Morphological transitions of extraembryonic development in dipteran evolution:
Previously, we obtained evidence for a two-step morphological transition that led to the origin of Drosophila’s amnioserosa, a unique extraembryonic epithelium that controls vital morphogenetic movements of the embryo. In the evolutionary order of events, the first transition presumably occurred in the stem lineage of cyclorrhaphan flies more than 145 milion years ago, and the second occurred probably in the stem lineage of higher cyclorrhaphan flies (Schizophora (Fig. 1). Currently, our research focuses on the second transition because it occurred more recently and involved with the genetic merger of two tissue types the more fundamental change.
Previously, we provided evidence that postgastrular expression of the homeobox gene zerknüllt (zen) is required to develop morphologically separate amniotic and serosal epithelia. We are now addressing the question of how sersoa and amnion tissues are specified. As amnioserosa specification is part of the dorsoventral patterning process in the blastoderm embryo, and is achieved through a well-studied Bone Morphogenetic Protein (BMP) signaling process, we hypothesize that differences in BMP-mediated dorsoventral patterning between lower cyclorrhaphan flies and Drosophila are important for the specification of two versus one extraembryonic tissue type.
BMP signaling specifies a single extraembryonic tissue in Drosophila:
In D. melanogaster, high levels of BMP activity are required to induce amnioserosa formation. This activity is induced by two extracellular ligands, Decapentaplegic (Dpp) and Screw (Scw), which are secreted into the perivitelline space and transported in the presence of antagonists towards the dorsal midline.
Dorsally, BMP dimers are released from their antagonists to the transmembrane receptor proteins Thickveins (Tkv) and Saxophone (Sax), triggering phosphorylation of the transcription factor Mad and thereby BMP-dependent transcriptional gene regulation. The BMP transport and release mechanism results in a shallow gradient of BMP activity, which broadly influences patterning in the dorsal ectoderm. Through a positive feedback loop, the initially shallow gradient of BMP activity is transformed into a narrow and sharply delineated domain of high BMP activity, which, at this stage, becomes critical for the expression of zen and hence amnioserosa specification.
Dpp is essential for BMP activity and, therefore, controls the specification of all tissues that develop under the control of the BMP pathway in the early embryo, including the amnioserosa and dorsal ectoderm. Scw, a diverged paralogue of glass bottom boat (gbb), boosts BMP activity along the dorsal midline and is therefore required for tissues whose specification depends on high BMP activity, such as the amnioserosa.
Fig. 2. Four models of how BMP signaling might specify serosal and amniotic tissue types along the DV perimeter. Cross sections of the cellular blastoderm with prospective serosa (red), amnion (orange) and dorsal epidermis (green) are depicted above BMP gradient profiles.
Potential mechanisms to specify two extraembryonic tissues:
We envision four different models of how two distinct extraembryonic epithelia could be specified in response to the dorsalizing activity of BMP signaling. In the blastoderm embryo, the two tissues could be specified by a single gradient of BMP activity, such that two thresholds of one morphogen differentiate between future serosa and amnion development (Fig. 2A), or by two gradients of distinct BMP ligands, so that one threshold for each gradient defines the extension of serosa and amnion anlage, respectively (Fig. 2B).
Alternatively, the two tissues could be specified by sequential gradients of BMP activity, such that peak levels of early activity define the serosa boundaries during blastoderm stages, while peak levels during gastrulation would define the amnion anlage (Fig. 2C).
If BMP signaling alone is not sufficient to create two distinct extraembryonic epithelia, a second dorso-ventral signaling center could be required to subdivide an undifferentiated, BMP-created extraembryonic primordium into amnion and serosa (Fig. 2D).
These potential mechanisms are not mutually exclusive and could as well occur in combination with each other. To differentiate between these possibilities, we have isolated candidate components of BMP signaling and BMP signaling targets in Megaselia. The goal of this research project is it to determine, which of them are required for early development and affect BMP signaling activity (Aim 1). We will then determine how these genes affect dorsal pattern formation and extraembryonic development (Aim 2).