A variety of research organisms have been used for for developmental genetic studies. Yeast are single celled eukaryotes with a genome of 12.5 Mb that has been fully sequenced They have mainly been used to study cell division cycle genes and mating type switching. The nematode worm Caenorhabditis elegans has a gnome of 100 Mb which has been fully sequenced. There is detailed knowledge of their development and the exact numbers of cells in adults (about 1000) are known. The plant Arabidopsis thaliana has a 100 Mb genome which has been fully sequenced. This model plant species is from the mustard family. Drosophila has a 165 Mb genome that has been fully sequenced. It has many advantages, including the ability to use the P element to induce mutations. Zebrafish are an experimentally tractable vertebrate, with a genome of about 1200 Mb, and sequencing in progress. The mouse is a representative mammal, with a 3000 Mb genome that has been sequenced. Knock-out mice are providing insights into gene function and development.
A variety of developmental mutants are known from humans. The mutation progeria is known to result in rapid aging. Children with this condition only live to about 13 years of age. The defect has now been identified as being in a gene Lamin A which affectes nuclear membrane structure. Study of human developmental abnormalities can provide general insights into development.
Examples of interesting mutants mutants studied in model organisms were reviewed.
The nematode mutant lin-12 changes cells that particular cells develop into. It is thought that this gene normally functions to receive a developmental signal. A recessive mutation in the gene fails to receive the signal, and anchor cells develop from the precursor cells. A dominant mutation in the gene has excess activity and vulva cells develop from the precursor cells.
Arabidopsis has several mutants in the process of flower development. Flowers have four developmental layers. One mutant interferes with development of the outer two layers (sepal, petal), two interfere with development of the middle two layers (petal, stamen) and one interferes with development of the inner two layers (stamen, pistil). These results imply that there are common control mechanisms across developmental layers.
Drosophila has many interesting developmental mutants. A classic example are the homeotic mutants, which lead to substitution of one body part for another. Bithorax leads to an extra wing on each side, while Antennapedia leads to legs in the place of the antennae. Homeoboxes are DNA sequences associated with these genes. They are 180 bp in length and represent a DNA binding region of transcriptional activator proteins involved in development.
General principles of development elucidated from developmental genetics studies. (1) Development can be described in terms of cell lineages, cell death and fate maps. Studies in nematodes have illustrated the ability to track the pathways of cell division for specific cells (cell lineages). Apoptosis (programmed cell death) is one normal outcome of development. In Drosophila, fate maps showing what adult parts develop from specific embryonic regions have been developed.