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. -Pax-6 usually causes the production of a type of light-receptor pigment.In vertebrate eyes,though,a different gene (the rh gene family)is responsible for the light-receptor pigments of the retina.The rh gene,like Pax-6,is ancient.In the marine ragworm,for example,the rh gene causes production of c-opsin,which helps regulate the worm's biological clock.Which of these most likely accounts for vertebrate vision?
A)The Pax-6 gene mutated to become the rh gene among early mammals. B)During vertebrate evolution, the rh gene for biological clock opsin was co-opted as a gene for visual receptor pigments. C)In animals more ancient than ragworms, the rh gene(s)coded for visual receptor pigments; in lineages more recent than ragworms, rh has flip-flopped several times between producing biological clock opsins and visual receptor pigments. D)Pax-6 was lost from the mammalian genome, and replaced by the rh gene much later.