XB-ART-56863Dev Biol June 15, 2020; 462 (2): 165-179.
Natural size variation among embryos leads to the corresponding scaling in gene expression.
Xenopus laevis frogs from laboratory stocks normally lay eggs exhibiting extensive size variability. We find that these initial size differences subsequently affect the size of the embryos prior to the onset of growth, and the size of tadpoles during the growth period. Even though these tadpoles differ in size, their tissues, organs, and structures always seem to be properly proportioned, i.e. they display static allometry. Initial axial patterning events in Xenopus occur in a spherical embryo, allowing easy documentation of their size-dependent features. We examined the size distribution of early Xenopus laevis embryos and measured diameters that differed by about 38% with a median of about 1.43 mm. This range of embryo sizes corresponds to about a 1.9-fold difference in surface area and a 2.6-fold difference in volume. We examined the relationship between embryo size and gene expression and observed a significant correlation between diameter and RNA content during gastrula stages. In addition, we investigated the expression levels of genes that pattern the mesoderm, induce the nervous system and mediate the progression of ectodermal cells to neural precursors in large and small embryos. We found that most of these factors were expressed at levels that scaled with the different embryo sizes and total embryo RNA content. In agreement with the changes in transcript levels, the expression domains in larger embryos increased proportionally with the increase in surface area, maintaining their relative expression domain size in relation to the total size of the embryo. Thus, our study identified a mechanism for adapting gene expression domains to embryo size by adjusting the transcript levels of the genes regulating mesoderm induction and patterning. In the neural plate, besides the scaling of the expression domains, we observed similar cell sizes and cell densities in small and large embryos suggesting that additional cell divisions took place in large embryos to compensate for the increased size. Our results show in detail the size variability among Xenopus laevis embryos and the transcriptional adaptation to scale gene expression with size. The observations further support the involvement of BMP/ADMP signaling in the scaling process.
PubMed ID: 32259520
Article link: Dev Biol
Genes referenced: acvr1 acvrl1 admp bmp2 bmp4 bmp7.1 bmpr1a bmpr1b cer1 chrd.1 foxd4l1.1 fst gapdh gmnn gsc lhx1 meis3 myod1 nodal3.1 nog not sia1 slc35b1 sox11 sox2 sox3 szl ventx1.2 ventx2.2 ventx3.2 wnt8a zic1 zic2 zic3
GO keywords: BMP signaling pathway
Article Images: [+] show captions
|Fig. 2. RNA content as a function of size during early development. Groups of embryos from multiple clutches were collected during (A) early gastrula, (B) late gastrula, and (C) early neurula. The average embryo diameter of the clutch was determined and RNA was extracted. The volume and the average amount of RNA/embryo were calculated. At each developmental stage, the RNA content/embryo was plotted as a function of volume. N, number of clutches/RNA samples analyzed; n, number of embryos used for RNA extraction and in parenthesis the number of embryos measured to determine the average volume; r2, coefficient of determination; p-value provided is the significance of a non-zero slope.|
|Fig. 5. Temporal expression pattern of the neural genes. The temporal pattern of expression of the neural genes was determined by quantitative real-time RT-PCR (qPCR) with primers specific for each gene. The relative expression level was calculated normalizing to the stage 19 sample. (A) Temporal expression pattern for nog, sox3, zic2, and gmnn (A); for foxd4l1.1, zic1, and zic3 (B); and for fst, meis3, sox11, and sox2 (C).|
|Fig. 8. Cell density and size in small and large embryos. (A) Phalloidin stained neural plate. (B) DAPI stained neural plate. (C) Cell density in a comparable area of the anterior neural plate was calculated in phalloidin and DAPI stained large and small embryos at early neurula stages (st. 14). (D) The long axis of cells within the neural plate of large and small embryos was measured in phalloidin stained embryos. ns, not significant (p > 0.05, two-sided t-test).|
|Fig. 9. Consequences of scaling of the BMP morphogen gradient with size. Schematic representation of the size-scaled BMP gradient and the effects on the organizer domain as determined by the arc length along the circumference and the induced neural domain.|