Model organisms: Frogs and chickens

28/8/02. By Richard Twyman

Frogs and chickens provide good experimental models of vertebrate development and have therefore been adopted as model organisms even though they are not suitable for genetic analysis.

The African clawed frog (Xenopus laevis) and the chicken (Gallus gallus) are important model organisms representing the amphibians and birds respectively. Both have similar genome sizes (about 1500 million base pairs). Unlike the other species described in this section, neither the frog nor the chicken is favoured for its genetic amenability. Xenopus produces large numbers of eggs but the breeding cycle is very long (it takes about four months to produce sexually mature adults). Chickens produce fewer eggs and the breeding cycle is even longer (over six months). Therefore, it is difficult and time-consuming to carry out large-scale crosses.

There are few well-characterised mutants for either species, but gene transfer is possible so transgenic frogs and transgenic chickens can be produced if necessary (see Transgenic mice ).

An additional disadvantage of Xenopus laevis for genetic analysis is that the species is tetraploid. This means there are four copies of each chromosome, rather than the two copies found in most vertebrates, including humans. The closely related species Xenopus tropicalis is therefore favoured where genetics is important since this frog is diploid and also has a shorter breeding cycle.

The African clawed frog (Xenopus laevis) is often used for studies of early development.

The predominant reason for the use of Xenopus and chickens as models is that they produce large, robust embryos whose development occurs outside the body of the mother. Since they are both vertebrates, their developmental processes have much in common with each other, and with humans. However, the embryos are much more accessible than those of mammals. Frog development occurs in water and can be observed at all stages. Chickens develop inside an opaque egg, but the shell can be cut open and covered with clear plastic or the embryos can be cultured outside the egg.

The accessibility of the embryos means they can be surgically manipulated or treated with proteins and chemicals that interfere with normal development. In many cases, the results from such experiments can be extrapolated to mammals.

Xenopus has been particularly useful for the analysis of events occurring very early in development, such as the formation of the neural plate, which gives rise to the entire nervous system.

Chickens have been invaluable for elucidating the molecular basis of limb development and cell migration in the nervous system. They have helped to uncover the molecular basis of many limb and neural defects in humans.