Genome complexity: starts with a single fertilized egg (Introduction)

by David Turell , Friday, April 27, 2018, 01:55 (2403 days ago) @ David Turell

We are starting to follow and ledarn the development of an embryo by studying how individual cells from the first division onward progress to become aggregates of different organ cells:

https://phys.org/news/2018-04-scientists-reveal-genetic-roadmap-entire.html

"From this solitary cell emerges the galaxy of others needed to build an organism, with each new cell developing in the right place at the right time to carry out a precise function in coordination with its neighbors.

"This feat is one of the most remarkable in the natural world, and despite decades of study, a complete understanding of the process has eluded biologists.

***

"Using single-cell sequencing technology, the research teams traced the fates of individual cells over the first 24 hours of the life of an embryo. Their analyses reveal the comprehensive landscape of which genes are switched on or off, and when, as embryonic cells transition into new cell states and types.

"Together, the findings represent a catalog of genetic "recipes" for generating different cell types in two important model species and provide an unprecedented resource for the study of developmental biology and disease.

***

"'Understanding how an organism is made requires knowing which genes are turned on or off as cells make fate decisions, not just the static sequence of a genome," Megason said. "This is the first technological approach that has allowed us to systematically and quantitatively address this question."

***

"In both species, the teams' findings mirrored much of what was previously known about the progression of embryonic development, a result that underscored the power of the new approaches. But the analyses were unprecedented in revealing in comprehensive detail the cascades of events that take cells from early progenitor or "generalist" states to more specialized states with narrowly defined functions.

"The teams identified otherwise difficult-to-detect details such as rare cell types and subtypes and linked new and highly specific gene expression patterns to different cell lineages. In several cases, they found cell types emerging far earlier than was previously thought.

***

"When Klein, Kirschner, Megason and colleagues compared cell-state landscapes between zebrafish and frog embryos, they observed mostly similarities. But their analyses revealed numerous surprises as well. One such observation was that genes marking cell states in one species were often poor gene markers for the same cell state in the other species.In several instances, they found that the DNA sequence of a gene—and the structure of the protein it encodes—could be nearly identical between species but have very different expression patterns.

"'This really shocked us, because it goes against all the intuition we had about development and biology," Klein said. "It was a really uncomfortable observation. It directly challenges our idea of what it means to be a certain 'cell type.'"

***

"In another striking finding, the teams observed that the process of cell differentiation into distinct cell types—which is commonly thought to occur in a tree-like structure where different cell types branch off from a common ancestor cell—can form "loops" as well as branches.

"For example, the neural crest—a group of cells that give rise to diverse tissue types including smooth muscle, certain neurons and craniofacial bone—initially emerges from neural and skin precursors, but is well-known to generate cells that appear almost identical to bone and cartilage precursors.

"The new results suggest that similar loops might occur in other situations. That cells in the same state can have very different developmental histories suggests that our hierarchical view of development as a "tree" is far too simplified, Klein said.

"All three teams also identified certain cell populations that existed in a kind of intermediate "decision making" state. Schier and colleagues found that, at certain key developmental branch points, cells appeared to go down one developmental trajectory but then changed their fate to another trajectory.

"Klein, Megason, Kirschner and colleagues made a related observation that, early in development, some cells activated two distinct developmental programs. Though those intermediate cells would eventually adopt a single identity, these discoveries add to the picture of how cells develop their eventual fate and hint that there may be factors beyond genes involved in directing cell fate.

"'With multilineage cells, we have to start wondering if their final fate is being determined by some selective force or interaction with the environment, rather than just genetic programs," Kirschner said."

Comment: Darwin ignored embryology, and there is no answer for it in his theory. Note how complex the problem is to understand, and recognize that a fertilized egg, one cell, makes the whole individual. Not by chance. To me this is the equivalent of trying to explain consciousness. A whole human in nine months!