[Eye structure of cavernicolous populations ofAstyanax fasciatus : Contribution to the problems of degenerative evolution]

Abstract

1. The characidAstyanax fasciatus, which has good, fully pigmented eyes is the ancestral form of a series of cave descendents, whose melanin pigment and eyes are reduced almost completely. Eye rudiments of two cave populations ("Anoptichthysantrobius" = population of Pachon and "A. hubbsi" = population of Sabinos) were examined anatomically. 2. Variabilities of eye types of both populations do not differ. Those developed best show a retina, which consists of all layers typical for teleostian eyes, except for the outer nuclear and outer reticular layers. The optic nerve is connected with the brain. The most degenerate eyes of all are those which have lost the optic nerve and show some tissue of unidentified character. 3. Within the eye itself there are correlations: large ones are less degenerate than small ones. 4. Size and degree of differentiation of the eyes of these cave fish are obviously dependent on genetic and inductive correlations, but also on environmental factors, such as light. 5. The E1-hybrids of the two cave forms demonstrate that they are genetically different because of separate evolution. This difference cannot be seen in the morphology of eye rudiments: there are more similarities than differences. The phenotypical divergences of eye rudiments between the two populations as observed by Peters and Peters (1966) are non-existent, and were obviously caused by an unexpected degree of modificability. 6. The genetic basis of eyes results developmentally physiologically in a special pattern of organisation. Phenotypically it reacts on all degenerative mutations alike. Therefore eyes degenerate in a trend. This pattern of organisation seems to be different in every species. Cavernicolous Amblyopsidae, for example, show similarities of structures of degenerate eyes, but they differ notably from those ofAstyanax cave populations.