Apposition of Dorsal and Ventral Cells Induces Outgrowth of Leaves

One of the most striking similarities between developmental mechanisms in plants and animals is how outgrowth of plant leaves and fly animal appendages depends on interactions between the dorsal and ventral surfaces along the margin of these structures. As you may recall from Chapter 4, when cells on the dorsal surface of the fly wing confront cells on the ventral surface, they send a signal via the Notch receptor to initiate formation of the wing margin, which is a necessary condition for...

Secreted Neuralinducing Factors Dj Vu

The classic organizer transplantation experiments in frogs performed by Hilde Mangold (see Chapter 1) demonstrated two important facts. First, there is a unique region of the frog embryo, the Spemann organizer, which can trigger formation of a secondary nervous system when transplanted to ectopic sites in another embryo. Recall that the Spemann organizer comprises the dorsal portion of the marginal zone in the animal hemisphere (see Fig. 1.11) and normally gives rise to a specialized dorsal...

The Central Dogma of Biology

The central premise of developmental biology is that all cells in a given organism contain the same genetic material. The difference between various cell types, such as those making up the nervous system, muscle, or skin, is that they employ, or express, overlapping but distinct subsets of the genetic information that all cells contain. This genetic information is stored in the molecular form of DNA (deoxyribonucleic acid), which is copied every time a cell divides, the result being that each...

AP and DV Information in the Embryo Determines the Identity of Imaginal Discs

Imaginal discs acquire their identities when they are first formed in the embryo (Fig. 4.1A). Because imaginal discs are generated after the primary body axes have been established, they form in response to preexisting A P and D V positional information. For example, the homeotic gene Antennapedia is responsible for specifying the second thoracic body segment. Like other structures derived from the second thoracic segment, formation of wing and leg imaginal discs depends on Antennapedia...

Access To Genetic Information Is Regulated

If different types of cells contain the same genetic information, then what distinguishes them from one another The answer is that each cell employs only a small fraction of the information carried in its DNA and that different cells access distinct subsets of their total genetic information. Thus, about 85 of the genetic information used in an adult cell of any given type (e.g., liver) is also used in other cell types (e.g., kidney). The remaining 15 of genetic information accessed by these...

Similarities between Patterning Vertebrate and Invertebrate Appendages

As mentioned above, all appendages have three primary patterning axes, A P, D V, and P D. It is worth backtracking momentarily into fly development for a refresher on the basic features of D V and P D patterning in wing and leg imaginal primordia, as it has recently become apparent that there are notable similarities in the mechanisms by which these axes are patterned in fly and vertebrate appendages (Fig. 6.3). The fly wing, like a human hand, has a dorsal and a ventral surface. A narrow strip...

Homeobox Genes Pattern the Apical Region of Plant Embryos

Genes involved in patterning the A B axis of plant embryos are just now beginning to be identified. Among this currently limited group of genes are those encoding transcription factors. For example, WUS and STM, two key genes required for determining shoot meristem identity, encode homeobox-type transcription factors distantly related to the homeotic Hox genes that determine segment identity in animal embryos. The field of embryonic plant development is still in its infancy, thus, patterning...

Did the Ancestor of Vertebrates and Invertebrates Have Appendages

Is it a coincidence that key genetic pathways involved in patterning the A P axis (Hedgehog and Dpp), D V axis (Notch), and P D axis (Distalless) of vertebrate and invertebrate appendages are the same Perhaps, there are important properties of each of these gene systems that make them optimally suited for carrying out particular developmental tasks. According to this hypothesis, two different lineages of animals could end up using the same genes to do the same jobs because of what one might...

Homeotic Genes in Plants

The now blossoming field of flower development was launched in the laboratories of Elliot Meyerowitz at Cal Tech and Enrico Coen at the John Innes Center. Meyerowitz was originally a fly guy who for some perplexing reason decided to turn his talents to another organism, the mustard plant. Over the last decade, his lab has conducted a series of classic experiments that have defined the genetic basis for flower development. As mentioned previously (see Fig. 7.1), flowers consist of four organ...

Fruit Organidentity Genes Subdivide the Carpel into Distinct Regions

The ovary comprises the bottom portion of the flower carpel and houses the eggs, which when fertilized by sperm, develop into embryos within seeds. After fertilization, the carpel develops into the fruit of the plant. The development of a fruit from the carpel is a separate patterning process unto itself, the end point of which is the release of seeds into the environment to begin life on their own. In many fruits, seed dispersal is accomplished by the fruits ripening, falling to the ground,...

An Overview Of Embryo Development In The Fruit

The fertilized fruit fly embryo, like all other embryos, starts off with a single diploid nucleus (i.e., a nucleus having two copies of every gene). This diploid nucleus is the product of a fusion of the nuclei from the egg and the sperm, which each supply a single copy of every gene. This initial diploid nucleus undergoes a series of 14 rounds of cell division to generate approximately 5,000 cells, forming what is known as the blastoderm embryo. Cells in a...

Signals Emanate from Organizing Centers in Developing Limbs

Classic transplantation experiments using ZPA grafts in vertebrates performed by Saunders and Gasseling in 1968, similar to those of Mangold and Spemann, led to the identification of an organizing factor present in early limb buds of gastrulating chick embryos. Saunders and Gasseling searched for organizing centers in developing limbs by grafting little patches of limb bud derived from different positions into various locations in a host limb bud. These experiments revealed that a region in the...

Genes Have A Bipartite Organization Coding Versus Regulatory Regions

The complete genetic blueprint of an organism is referred to as the organism's genome. The genome can be imagined as a linear string of DNA base pairs A T, C G, G C, or T A 100 million to 3 billion base pairs long, depending on the organism. To give a sense of the immensity of this genetic information, if a single base were represented by a printed letter on a single-spaced typed page, the genome of a human being would be an encyclopedia one million pages long and standing 12 stories high. In...

The Most Recent Ancestor Of Vertebrates And Invertebrates

Nearly two centuries of experimentation and the enormous flurry of recent work, which has been only been touched on in the previous chapters, can be distilled into two very important but simple lessons about development and evolution. It is difficult to convey the great implica tions of these points in the history of the science of biology without some hint of melodrama. Thus, let us now consider these points and their profound implications. The most visual of these two points is that we can...