The NSS Bulletin - ISSN 1090-6924
Volume 47 Number 2: 101-108 - December 1985
A publication of the National Speleological Society

The Evolution of Polygenic Systems, Studied on Epigean and Cave Populations of Astyanax fasciatus (Characidae, Pisces)
Horst Wilkens

Abstract

The epigean and cave populations of Astyanax fasciatus (Pisces) differ with respect to several structures.The regressive features of eye and melanophore/morphological color change as well as the constructive features of taste bud apparatus and the feeding behavior change are genetically analyzed. They are based on polygenic systems characterized by between at least 2 and at least 7 loci.

The phylogenetically older regressive and the more recently evolved contructive polygenic systems show no difference in the principles of manifestation. The most striking similarity is that after recombination of a minimum number of polygenes the amount of expression a newly added polygene manifests, is considerably increased. General importance in evolutionary progress is attributed to this phenomenon of varying expressivity. Only in phases of high expressivity is directional selection able to enlarge or even manifest the analagen of cryptogenic structures.

The morphological color change seems to be adaptive specialization in connection with high polygenic expression. In this case this specialization is developed by the fact that environmental influence can suppress high polygenic expression. This type of switch mechanism is probably wide-spread (e.g. phenotypic sex determination).

Monogenic systems are often nothing else but extremes of polygenic inheritance. Monogenic dominance in these cases is caused by loss of a single polygene of high expression. On account of little structural difference, the rest of the polygenic systems mostly remain hidden. This is exemplified by the sex determination system in some toothcarps in which the transition from a poly- to an apparently monogenic mode is found.

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