Karyotype differentiation in Chromaphyosemion killifishes (Cyprinodontiformes, Nothobranchiidae). II: cytogenetic and mitochondrial DNA analyses demonstrate karyotype differentiation and its evolutionary direction in C. riggenbachi.

African killifishes of the genus Chromaphyosemion show a high degree of phenotypic and karyotypic diversity. The latter is especially pronounced in C. riggenbachi, a morphologically defined species restricted to a small distribution area in Cameroon. This study presents a detailed reconstruction of karyotype differentiation within C. riggenbachi using conventional Giemsa staining and sequential chromosome banding as well as a phylogenetic analysis based on part of the mitochondrial (mt) cytochrome b gene from eleven populations. The cytogenetic analysis revealed differences in chromosome morphology, banding patterns and/or diploid chromosome number (2n) among all populations examined. Diploid number ranged from 2n = 20 to 2n = 36 and varied mainly among populations, while C-banding patterns and NOR phenotypes showed fixed differences among populations as well as some variability within populations. The mtDNA analysis disclosed five clearly differentiated haplotype groups. Mapping the karyotype data onto the mtDNA dendrogram revealed a decrease in 2n from the most basal to the most derived groups, thus demonstrating a reduction of 2n during their evolutionary history. Our results indicate that karyotype differentiation involved Robertsonian fusions as well as non-Robertsonian processes. Causes of the high karyotypic variability may include an elevated chromosomal mutation rate as well as certain features of the ecology and mating system that could facilitate the fixation of chromosomal rearrangements. The pattern of karyotype and haplotype differentiation and the results of previous crossing experiments suggest incipient speciation in C. riggenbachi.