Genes Required for Making Eyes

One of the most exciting insights into vertebrate eye development came from identification of the gene that is disrupted in a mouse mutant known as Small eye. Some of the most severely affected Small eye mutant individuals entirely lack eyes. There also is a human disease known as aniridia, which is associated with eye defects similar to those in Small eye mutant mice. It came as quite a surprise when it was discovered that the same gene—pax6—was disrupted in mouse Small eye mutants and human aniridia patients, and that this gene was none other than the vertebrate version of the eyeless gene described in Chapter 4, which is required to initiate eye development in fruit flies.

The vertebrate pax6 gene is expressed in the early neuroectoderm in a stripe running perpendicular to the body axis at the level of the eyes. This pax6 expression precedes any morphological hint of eye development such as out-pocketing of the optic vesicle. The expression of pax6 in the eye primordium and the loss of eyes in mouse and human mutants that have reduced pax6 activity argue strongly that pax6, like its fly counterpart, plays a critical role in specifying the eye primordium. It is important to note, however, that pax6 is also expressed in other parts of the brain and that total loss of pax6 activity leads to widespread defects

B. Vertebrates

Epidermis

Invaginating eye imaginal disc

Anterior

Anterior

Posterior

Morphogenetic furrow

Posterior

Morphogenetic furrow

Furrow

Furrow

Photoreceptors induce lens cells

Photoreceptors induce lens cells

Lens

TWWf

B. Vertebrates

Invaginating eye imaginal disc

Optic nerve

C. Squid

Eye primordium

Lens fold

Optic nerve

Photoreceptors

FIGURE 6.5. Comparison of eye development in different species. (Based in part on summary figure in Harris 1997.)

Optic nerve

Photoreceptors

FIGURE 6.5. Comparison of eye development in different species. (Based in part on summary figure in Harris 1997.)

in these regions of the brain as well as to the loss of eyes. Thus, whereas pax6 is clearly essential for initiating vertebrate eye formation, it has other roles in brain development as well.

It also has been found that vertebrate counterparts of several other genes involved in early fly eye specification, such as sine oculus, are involved in early vertebrate eye formation. The regulatory interactions among these genes, however, appear not to be the same in flies and vertebrates. The potential role of the ancestral version of the pax6/eyeless gene in defining eyes is discussed further below.

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