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The early morphogenesis of the lens and the expression of the γs-crystallin gene was studied in epigean Astyanax fasciatus and its cave-dwelling derivative. At early stages, the lens of the cave fish develops in a way that is similar to the epigean form. Later, the developmental timing is delayed and growth ceases in the cave-fish lens. With the beginning of cytodifferentiation, the development of the lens breaks down. Crystallin lens fibres are not produced at any time and the γs-crystallin gene, which is transcribed during a limited period in the lens of epigean fishes, is not active in cave specimens. This study confirms earlier immunofluorescence observations that demonstrated the lack of crystallin proteins in the cave-fish lens, but is in contrast to results on the blind mole rat, which showed a persistence of functioning crystallins in the degenerated lens of this species. The significance of developmental constraints in regressive evolution is discussed. Zusammenfassung Entwicklungsbedingte Beschrankungen in der regressiven Evolution: Untersuchungen zur Expression des γs-Krystallin Gens in der Linsenentwicklung des Hohlenfisches Astyanax fasciatus (Teleostei, Characidae) durch in situ Hybridisierung Die fruhe Morphogenese der Linse und die Expression des γs-Kristallin Gens wurden in dem oberirdischen Fisch Astyanax fasciatus und seiner Hohlenform untersucht. In fruhen Stadien entwickelt sich die Linse des Hohlenfischauges ahnlich wie die der oberirdischen Form. Es setzt jedoch bald eine Verzogerung in der Entwicklung der Hohlenfischlinse ein, die spater zum Entwicklungsstillstand fuhrt. Mit Beginn der Zelldifferenzierung bricht die Linsenentwicklung ab. Kristalline Linsenfasern werden zu keinem Zeitpunkt entwickelt und die mRNA des γs-Kristallin Gen, das in der oberirdischen Form wahrend eines begrenzten Zeitraumes exprimiert wird, kann in der Linse der Hohlenfische zu keiner Zeit nachgewiesen werden. Diese Untersuchungen bestatigen fruhere immunohistochemische Arbeiten, in denen das Fehlen von Kristallin-Proteinen in der Hohlenfischlinse nachgewiesen wurde. Sie stehen jedoch im Gegensatz zu Untersuchungen an dem Blindmull Spalax ehrenbergi, in dessen degenerierten Augen funktionell intakte Kristalline nachgewiesen wurden.

Epigean and blind cave-dwelling populations of the characid Astyanax fasciatus differ in their agonistic behavior. Epigean specimens are characterized by intense aggressive behavior and territoriality in the laboratory, whereas the blind cave fishes are rarely aggressive and have totally lost the territorial behavior. These behavioral changes are genetically based. In the present study, a comparative analysis of the optical releasers for aggressive behavior has been performed in both epigean and cave-dwelling populations. It was expected that the regression of aggressive behavior should be accompanied by the regression of the genetically fixed optical releasers for aggression in the cave specimens. For the analysis of cave fishes, well-eyed F1-hybrids of cave and epigean specimens (Pachon × Teapao) as well as phylogenetically young cave specimens (Micos) selected for large eyes, have been used. In epigean specimens, aggression is most intensely released by a combination of the signals 'natural shape' and 'locomotion'. The cave populations exhibit a partial regression of the releaser 'natural shape'. 'Locomotion', however, has become a more effective releaser for aggression in cave fishes than it has in their epigean relatives. The results are discussed with respect to their evolutionary significance.

The epigean characid Astyanax fasciatus developed a series of cave-dwelling populations in Central Mexico. The dorsal light reaction of the epigean fish was compared with that of a phylogenetically young cave population (‘Micos’ fish) and with that of the F1 and F2 hybrids between the epigean and a phylogenetically old cave form (‘Pachon’ fish). The experiments revealed a slight regression of the dorsal light response in the Micos fish and an almost complete regression of this behaviour in the phylogenetically old Pachon population. A genetic analysis was performed. It suggests that the dorsal light behaviour of A. fasciatus is based on additive gene action, a minimum number of 3 to 6 genetic factors has been calculated to be involved in this system. The results are discussed with respect to the current controversy about the causes of regressive evolution. It is suggested that the observed regression of the dorsal light behaviour in the Pachon population is due to an accumulation of degenerative, selectiv...

The early morphogenesis of the degenerate eyes of the Mexican cave fish Astyanax fasciatus and of its conspecific epigean ancestor has been studied comparatively using light- and electron-microscopical techniques; the transcription of the opsin gene has been analysed during early ontogeny in both populations by in situ hybridization. The opsin protein is an integral component of the disk membranes of the photoreceptor cells. In epigean specimens, its expression is correlated with the beginning of outer segment formation on the third day of development. Morphogenesis of the cave fish eye is similar to that of the epigean eye until end of the third day. However, eye growth and morphogenesis of the retina are delayed and sporadic cell death occurs in all retinal layers at the beginning of the second day. Retinal cytodifferentiation breaks off at the point of outer segment formation. Cave specimens are not able to develop regular outer segments at any stage, but the opsin gene is nevertheless expressed in the outer nuclear layer of the developing retina for a limited period of time. On the basis of the comparative morphology and transcriptional studies of epigean and cave specimens, it is suggested that the eye regression of cave fish is primarily the result of mutations of developmental control genes and not of structural genes.

The Pax-6 gene encodes a transcription factor essential for eye development in a wide range of animal phyla. In order to elucidate a possible role of Pax-6 in the eye regression of a blind cave form of the freshwater fish Astyanax fasciatus (Characidae, Teleostei) we investigated the expression of Pax-6 in eyes and brains of different larval stages by in situ hybridization. Pattern, strength, and time course of Pax-6 expression were not altered in the tissues of the cave form when compared to the epigean form. Pax-6 was even expressed in the highly degenerated eyes of late larval stages of the cave form. Comparative sequence analysis of Pax-6 cDNA clones of both forms of Astyanax fasciatus showed the complete integrity of cave fish Pax-6 mRNA. These results suggest that Pax-6 is not involved in the evolutionary process of eye degeneration in this model system of cave-living fishes. Comparison of the Astyanax Pax-6 cDNA with the other available fish Pax-6 sequence from zebrafish revealed putative fish-specific regions of homology. A stretch of 19 N-terminal amino acids is nearly identical on the nucleotide and amino acid levels in both fish species but not present in all other known Pax-6 sequences.