After birth, recognising an organism is usually not difficult. Supposedly unique physiological features allow us to tell the difference between say, a dog and dolphin. During embryogenesis however, the task is not so easy. Early embryonic cell division, or cleavage, differs across various organisms. However, once the three germ layers are established (during gastrulation), embryonic development proceeds in a similar manner across a variety of species. Early development in the vertebrate classes; mammals, birds, reptiles etc can be so alike, it can be difficult to tell embryos of different species apart at this stage. The following pictures show two vertebrate embryos. Can you tell what organism they are?
As well as having a similar appearance, many embryos also share potential developmental fates. A snake embryo has the potential to grow legs; human embryos can grow tails and chickens have the ability to grow teeth. Many organisms possess genes which can form unnecessary structures or features, yet once the offspring is born, the chance of seeing these are rare. Some embryos go as far as growing these unnecessary features only to have them regress later in development, such as tails in human embryos.
Gene expression dictates which genes are expressed when and where. As an embryo progresses through development, specific patterns of gene expression cause the embryo to become more specialised and look as it should. A group of developmental genes called homeobox genes are the ‘toolkit’ responsible for the development of body structures and orientation in animals. They work by triggering the development of structures along the anterior-posterior axis of an organism. These include the formation of eyes, ears and limbs. Hox genes are collinear which means that the order in which they are found on a chromosome, matches the order in which they are expressed along the anterior-posterior axis. These genes are highly conserved among many organisms, and explain the similarities shown in the images above. They also suggest descent from a common ancestor.
Sometimes, gene expression fails to repress the development of these ancestral genes. As a result, organisms are born with mutations known as atavisms; a reappearance of an ancestral trait. Some examples include snakes with limbs, dolphins with hind fins and humans with tails.
Atavisms are traits that were once common in an ancestral lineage but have since not been seen in recent generations. In the past, these features may have been beneficial and helped the organism with survival. Changes in the environment or behaviour of these organisms could have meant that these once essential features were no longer needed and therefore not expressed. Dolphins are a good example to explain this evolutionary progression. Once land mammals, dolphins would have possessed limbs to move around on land. Over time however, they decided to take to the water. With little use for limbs, the dolphin developed two fins and a tail which made them more efficient in water. Dolphin embryos still show the beginnings of hind limb formation (circled in green) but these later regresses as development of the embryo continues. Despite evolutionary change, some ancestral genes are not lost but remain unexpressed. In some cases, traits begin to develop in the embryo as with the hind limbs of dolphins, but are removed later in development. These atavisms are a reminder of an ancestral past, and evidence for common descent as seen in the embryonic development of vertebrates.
Answers to embryo matching: 1= mouse, 2= dolphin
Brian K Hall, 1984, Developmental Mechanisms Underlying the Formation of Atavisms, Biological Reviews, 59, 89-122
P Z Myers, 2008, Hox Genes in Development: The Hox Code, Nature Education, 1, 1
J. G. M. Thewissen et al, 2006, Developmental Basis for Hind-limb Loss in Dolphins and Origin of the Cetacean Bodyplan, PNAS, 103, 22
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