发育遗传学导论

The development of every organism is guided by the genetic information encoded in its DNA. By studying how genes control developmental processes, such as cell migration and differentiation, scientists in the field of developmental genetics are trying to better understand how the complex structures of multicellular organisms are formed.

This video will present some of the major discoveries in this field, a number of fundamental questions asked by developmental geneticists, major tools that scientists use to answer these questions, and finally, specific studies being conducted on developmental genetics today.

Let’s begin by reviewing some of the important discoveries that have shaped the field of developmental genetics.

In 1865, an Austrian monk, Gregor Mendel, performed breeding experiments with peas. He observed that the peas’ visible traits or “phenotypes,” such as seed color, were inherited according to consistent rules. By proposing that these phenotypes are actually controlled by some invisible, discrete heredity factors, Mendel planted the seeds of the field of genetics.

These heredity factors were named “genes” by Danish botanist Wilhelm Johannsen in 1909. Then, in 1910, Thomas Hunt Morgan and his students used the fruit fly Drosophila as a model organism to discover that genes are found on physical structures in the cell nucleus called chromosomes.

In 1938, Salome Gluecksohn-Waelsch showed that a specific gene was needed for the development of an embryonic structure known as the notochord. This was among the earliest evidence that genes control early developmental processes.

In 1940, Conrad Hal Waddington proposed that cells in an embryo differentiate along paths, or “fates,” that are controlled by genes. He formulated a metaphor for this process, refined over the next 17 years, called the “epigenetic landscape,” where a cell is seen as a marble rolling down a hillside towards different cell fates. The paths taken by the cell follow the ridges and valleys in the landscape, which in turn are controlled by genes and their expression patterns.

In 1952, Wolfgang Beermann confirmed that while different cells in an organism have the same genetic content, different regions of the chromosomes are active, and this differential gene expression defines cell identity.

Once it was determined that gene expression influences development, the next question was, which genes? To answer this, in the 1970s, Edward B. Lewis, Christiane Nusslein-Volhard and Eric Weischaus used chemicals to randomly mutate genes in fruit flies. Through these mutation screens, the scientists identified a large number of genes controlling every step of the development process.

In 2007, an international consortium of scientists began work on creating a collection of mice in which every single gene, one in each mouse, is deleted or “knocked out.” The phenotype of each of these mice is currently being characterized, and will give us the first catalogue of the function of all genes in a mammal.

Now that we’ve reviewed the roots of the field, let’s look at a few key questions that developmental geneticists are trying to answer.

Some researchers are focusing on the early events during the transformation of fertilized eggs, or zygotes, into multicellular embryos. These events depend on RNAs and proteins that are deposited in the egg by the mother, in a phenomenon known as “maternal contribution” or “maternal effect.” Scientists are interested in learning how a mother’s genotype influences an embryo’s phenotype.

Another central question in developmental genetics is: how do genetically identical cells adopt different cell fates? Scientists are identifying the many factors that control differential gene expression among different cells, including the signaling pathways that tell the cell what genes to express, and when to express them, during development.

Finally, scientists are also asking how does the early embryo, an amorphous mass of cells, transform into a complex organism with distinct, functional parts. The formation of this body plan is called morphogenesis, and scientists are trying to identify the genes and pathways that govern this process.

Now that you know some of the questions that developmental geneticists are asking, let’s review the techniques they are using to answer these questions.

Scientists can study the role of specific genes in development by disrupting their expression. One way to do this is by “knocking out” the gene in the organism’s DNA by introducing mutations, or replacing it with nonfunctional DNA. Alternatively, gene expression can be “knocked down” by introducing oligonucleotides that will bind to the target mRNA sequences and prevent the production of functional proteins.

To identify which genes are responsible for particular phenotypes, scientists can carry out genetic screens. In a forward genetic screen, mutations are randomly generated in organisms by either radiation or chemicals known as mutagens. When a mutant is found to display a phenotype of interest, the unknown gene that was mutated can then be identified. The opposite approach is a reverse genetic screen, where scientists first target a large number of specific candidate genes for disruption, and then look at the resultant phenotypes of the mutants.

Finally, biologists are also interested in determining gene expression at different developmental stages. One tool for measuring gene expression is the microarray, which is a chip dotted with oligonucleotides containing sequences of the genes to be tested. In a typical experiment, RNA extracted from organisms at two different developmental stages is used to generate two different sets of fluorescently labeled probes, which are then hybridized to the microarray. Changes in gene expression can then be interpreted from the fluorescent signal at each dot on the array.

With these experimental techniques in mind, let’s take a look at how researchers are applying them to study developmental genetics.

Scientists are performing large-scale genetic screens in model organisms, such as C. elegans, to look for genes that affect development. This is usually done through RNA interference, or RNAi, a process whereby genes are silenced using small RNA molecules. Here, scientists fed worms with bacteria containing an RNAi library designed against a large number of worm genes, and analyzed the effect of gene knockdown on the animals’ development.

Other researchers are performing forward genetic screens using random mutagenesis to identify developmental phenotypes. In this experiment, researchers used the gene-trap technique to mutagenize zebrafish embryos, where a reporter construct is randomly targeted to introns of genes and render them nonfunctional. Scientists can then easily identify the animals in which the gene is successfully disrupted by looking for the reporter signal, and those that exhibit a developmental defect can have the responsible gene identified.

Finally, the gene expression of different cell types in a developing organism can be profiled by microarrays to identify which genes are turned on or off during cell differentiation and specialization. In this study, single neuronal cells of different cell types were isolated from the developing retina. RNA was then extracted from these cells for microarray analysis to identify genes that play a role in the development of each specific cell type.

You’ve just watched JoVE’s introduction to developmental genetics. This video reviewed some historical highlights of this field, the big questions asked by developmental geneticists, a few of the prominent methods currently being used in labs, and specific applications of these approaches to studying developmental biology. As always, thanks for watching!