Evo-Devo

Evolutionary-Developmental Biology

What is evolutionary developmental biology? While the question is good, the answer remains somewhat unsatisfying. In the broadest sense, evolutionary developmental biology—or evo-devo—is the study of the interaction between developmental processes and evolutionary factors.

However, this definition does not speak to the overarching aims of evo-devo, or to the type of research that falls under its umbrella. The reason for this vagueness stems from conflicting opinions on the aims and scope of evo-devo research. T

his conflict is well demonstrated by the many names that are often used synonymously with evolutionary developmental biology: evolution of development, developmental evolution, comparative development, and microevolution of development

Homeotic Genes and Body Patterns

Every organism has a unique body pattern. Although specialized body structures, such as arms and legs, may be similar in makeup (both are made of muscle and bone), their shapes and details are different. While an embryo grows, arms and legs develop differently due to the actions of homeotic genes, which specify how structures develop in different segments of the body.

Homeotic genes control the development of whole body segments or structures.

HOX GENES

How to Build a Body

Homeotic genes are master regulator genes that direct the development of particular body segments or structures.
When homeotic genes are overactivated or inactivated by mutations, body structures may develop in the wrong place—sometimes dramatically so!
Most animal homeotic genes encode transcription factor proteins that contain a region called the homeodomain and are called Hox genes.
Hox genes are turned on by a cascade of regulatory genes; the proteins encoded by early genes regulate the expression of later genes.
Hox genes are found in many animals, including fruit flies, mice, and humans. Mutations in human Hox genes can cause genetic disorders

The diagram above shows eight major homeotic genes in flies. The upper part of the diagram shows where each gene is most strongly expressed in the mature fly, while the lower part of the diagram shows where the genes are located on the chromosome. The order of the genes on the chromosome more or less mirrors their order of expression along the head-tail axis of the fly.

What exactly are these homeotic genes? Each gene encodes a transcription factor that is expressed in a specific region of the fly starting early in its development as an embryo. The transcription factors change the expression of target genes to enact the genetic “program” that's right for each segment.

Fertilization

Fertilization is the process in which gametes (an egg and sperm) fuse to form a zygote. The egg and sperm each contain one set of chromosomes. To ensure that the offspring has only one complete diploid set of chromosomes, only one sperm must fuse with one egg. In mammals, the egg is protected by a layer of an extracellular matrix consisting mainly of glycoproteins called the zona pellucida.

a. When a sperm binds to the zona pellucida, a series of biochemical events, called the acrosomal reactions, take place. In placental mammals, the acrosome contains digestive enzymes that initiate the degradation of the glycoprotein matrix protecting the egg and allowing the sperm plasma membrane to fuse with the egg plasma membrane.

b. The fusion of these two membranes creates an opening through which the sperm nucleus is transferred into the ovum. The nuclear membranes of the egg and sperm break down and the two haploid genomes condense to form a diploid genome.

Cleavage and Blastula Stage

The development of multi-cellular organisms begins from a single-celled zygote, which undergoes rapid cell division to form the blastula. The rapid, multiple rounds of cell division are termed cleavage.

a. After the cleavage has produced over 100 cells, the embryo is called a blastula. The blastula is usually a spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-filled cavity (the blastocoel). Mammals at this stage form a structure called the blastocyst, characterized by an inner cell mass that is distinct from the surrounding blastula.

b. During cleavage, the cells divide without an increase in mass; that is, one large single-celled zygote divides into multiple smaller cells. Each cell within the blastula is called a blastomere.

Gastrulation

The typical blastula is a ball of cells. The next stage in embryonic development is the formation of the body plan. The cells in the blastula rearrange themselves spatially to form three layers of cells. This process is called gastrulation. During gastrulation, the blastula folds upon itself to form the three layers of cells. Each of these layers is called a germ layer and each germ layer differentiates into different organ systems.

Not only are Hox genes found in many different animal species, but they also tend to have the same order on the chromosome in all of these species. As in flies, this order roughly maps to the parts of the body whose development is controlled by each gene. Because this is so consistently the case, scientists think it is likely not a coincidence and may have functional importance

In vertebrates like humans and mice, Hox genes have been duplicated over evolutionary history and now exist as four similar gene clusters labeled A through D:

However, gene duplication has allowed some Hox genes to take on more specialized roles. For instance, many Hox genes towards the end of the cluster act specifically in the development of vertebrate limbs—arms, legs, or wings—as shown in the diagram of the woman above. Mutations in HoxD13 in humans can cause a genetic condition called synpolydactyly, in which people are born with extra fingers or toes that may also be fused together.

Page updated

Report abuse