The following questions refer to the description below. All animals with eyes or eyespots that have been studied so far share a gene in common.When mutated,the gene Pax-6 causes lack of eyes in fruit flies,tiny eyes in mice,and missing irises (and other eye parts)in humans.The sequence of Pax-6 in humans and mice is identical.There are so few sequence differences with fruit fly Pax-6 that the human/mouse version can cause eye formation in eyeless fruit flies,even though vertebrates and invertebrates last shared a common ancestor more than 500 million years ago. -Fruit fly eyes are of the compound type,which is structurally very different from the camera-type eyes of mammals.Even the camera-type eyes of molluscs,such as octopi,are structurally quite different from those of mammals.Yet,fruit flies,octopi,and mammals possess very similar versions of Pax-6.The fact that the same gene helps produce very different types of eyes is most likely due to
A)the few differences in nucleotide sequence among the Pax-6 genes of these organisms. B)variations in the number of Pax-6 genes among these organisms. C)the independent evolution of this gene at many different times during animal evolution. D)differences in the control of Pax-6 expression among these organisms.