Genic identity and differentiation of Ilyodon (Cyprinodontoidea: goodeidae): morphological polymorphism and chromosomal divergence

Certain populations of the goodeid fish genus Ilyodon occur as bimodal assemblages of broad and narrow-mouthed individuals which have been regarded as sympatric species. Allozyme surveys of four bimodal populations indicated no significant differences in allele frequencies between the presumed species in seven to fifteen polymorphic loci, but substantial interpopulation genic differentiation. These results suggested regular gene flow between the supposed species at each of four independent localities and conspecificity of two mouth width morphs.

Progeny of field inseminated females yielded some offspring with morphs opposite to those of their mother and some progeny of laboratory crosses were of morphs different from their parents. The analysis of Ilyodon broods conclusively proved that the two presumed species were conspecific and displayed a morphological polymorphism in mouth width.

Variation in the number of metacentric chromosomes has been reported for populations of Ilyodon along the length of the Rio Coahuayana. Allozyme analyses of fifteen populations including three of the known cytotypes showed that the chromosomally diverse populations of the Rio Coahuayana were half as genetically diverse as chromosomally non-divergent populations from the adjacent Rio Armeria. Evolutionary events which fixed the various cytotypes did not produce detectable differences in either levels of polymorphic loci or average heterozygosity. Genetic similarity indices produced phenograms of genetic relationships which clustered different cytotypes together; chromosomal variation and allozymic differentiation are uncoupled processes in Ilyodon.