Misexpression of the Pax6 gene was shown to promote eye development in both vertebrates and invertebrates. The authors of this study suggested that this was evidence that eyes evolved only once. What other evidence contradicts this view?

Question

Pax6 is a transcription factor noted for its unique role in specifying eye development across the animal kingdom. A homologous gene known as eyeless specifies eyes in Drosophila, and misexpression of mouse Pax6 can cause ectopic eyes in flies! This creates a quandary -- how can we explain the universality of the master regulatory gene for eye development, with the striking variability of eye anatomy in different phyla?

A.  Invertebrates normally have only simple eyespots.
B.  Many different transcription factors can initiate eye development when misexpressed.
C.  While the anatomy of all animal eyes is the same, lens proteins vary greatly.
D.  The anatomical structure of insect, mollusk, and vertebrate eyes are quite different.


Clarify Question
What is the key concept addressed by the question?
What type of thinking is required?

Gather Content
What do you already know about eyes? What other information is related to the question?

Choose Answer
Given what you now know, what information is most likely to produce the correct answer?

Reflect on Process
Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Answer 1

D.  The anatomical structure of insect, mollusk, and vertebrate eyes are quite different.


Clarify Question
What is the key concept addressed by the question?
        · How did eye development evolve?
What type of thinking is required?
            o This is an analyze question because you have to break down the role of Pax6 in the process of eye development and understand the process of convergent evolution.

Gather Content
What do you already know about eyes? What other information is related to the question?
        · You already know that Pax6 is involved in eye development and that it has been referred to as the master regulatory gene for eye development. You also know, from the chapter reading, that eyes have evolved independently in several distinct taxa of animals, with distinct features. For example, flies have eyes with hundreds of different lenses while mollusks and chordates have a single lens. This is an example of convergent evolution in which analogous structures (not homologous ones) have evolved the same function of vision in different taxa.  But something you may not know is that Pax6 regulates the development of a number of sensory organs and other neural tissues, not just eyes. Also, Pax6 may be an important regulatory gene, but it works in conjunction with a number of other developmental genes.

Choose Answer
Given what you now know, what information is most likely to produce the correct answer?
        · The statement that invertebrates normally have only simple eyespots is simply not true for all invertebrates. Mollusks like squid and octopi have well-developed eyes with a single large lens similar to those of vertebrates. Many different transcription factors can aid in the process of eye development, but there is no evidence of eye development being initiated by the misexpression of genes other than Pax6. The statement that all animal eyes have the same anatomy is also not true, as insects, mollusks, and vertebrate eyes are different in morphology.

Reflect on Process
Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?
        o Answering this question correctly depended not only on distinguishing between true and false statements about the anatomy of eyes from different taxa, but also understanding that Pax6 is the major, although not only, gene involved in eye development. Since Pax6 affects the development of sensory tissues in general, it makes sense that it could affect the development of eyes in different taxa even if these structures are analogous (evolved independently) instead of homologous (from the same ancestor).