Quantification reveals early dynamics inDrosophilamaternal gradients
Authors: Shlemov Alex , Alexandrov Theodore , Golyandina Nina , Holloway David , Baumgartner Stefan , Spirov Alexander V. 
Published: December 2021

The Bicoid (Bcd) protein is a primary determinant of early anterior-posterior (AP) axis specification inDrosophilaembryogenesis. This morphogen is spatially distributed in an anterior-high gradient, and affects particular AP cell fates in a concentration-dependent manner. The early distribution and dynamics of thebicoid(bcd) mRNA, the source for the Bcd protein gradient, is not well understood, leaving a number of open questions for how Bcd positional information develops and is regulated.

Confocal microscope images of whole early embryos, stained forbcdmRNA or the Staufen (Stau) protein involved in its transport, were processed to extract quantitative AP intensity profiles at two depths (apical - under the embryo surface but above the nuclear layer; and basal – below the nuclei). Each profile was quantified by a two- (or three-) exponential equation. The parameters of these equations were used to analyze the early developmental dynamics ofbcd. Analysis of 1D profiles was compared with 2D intensity surfaces from the same images. This approach reveals strong early changes inbcdand Stau, which appear to be coordinated. We can unambiguously discriminate three stages in early development using the exponential parameters: pre-blastoderm, syncytial blastoderm and cellularization. Key features which differ in this period are how fast the first exponential (anterior component) of the apical profile drops with distance and whether it is higher or lower than the basal first exponential. We can further discriminate early and late embryos within the pre-blastoderm stage, depending on how quickly the anterior exponential drops. This relates to the posterior-wards spread ofbcdin the first hour of development. Bothbcdand Stau show several redistributions in the head cytoplasm, quite probably related to nuclear activity: first shifting inwards towards the core plasm, forming either protrusions (early pre-blastoderm) or round aggregations (early nuclear cleavage cycles, cc, 13 and 14), then moving to the embryo surface and spreading posteriorly. These movements are seen both with the 2D surface study and the 1D profile analysis. The continued spreading ofbcdcan be tracked from the time of nuclear layer formation (later pre-blastoderm) to the later syncytial blastoderm stages by the progressive loss of steepness of the apical anterior exponential (for bothbcdand Stau). Finally, at the beginning of cc14 (cellularization stage) we see a distinctive flip from the basal anterior gradient being higher to the apical gradient being higher (for bothbcdand Stau).

Quantitative analysis reveals substantial (and correlated)bcdand Stau redistributions during early development, supporting that the distribution and dynamics ofbcdmRNA are key factors in the formation and maintenance of the Bcd protein morphogenetic gradient. This analysis reveals the complex and dynamic nature ofbcdredistribution, particularly in the head cytoplasm. These resemble observations in oogenesis; their role and significance have yet to be clarified. The observed co-localization during redistribution ofbcdand Stau may indicate the involvement of active transport.