Chromosomes of Cichlasoma istlanum (Perciformes: Cichlidae) and karyotype comparison of two presumed subspecies

" Cichlasoma istlanum (Jordan & Snyder, 1900) is a freshwater cichlid from the Balsas river province in the Pacific Basin. Two subspecies: C. istlana istlana, from the Ixtla river in the state of Morelos, Mexico, and C. istlana fusca, from the Huámito river near the town of La Huacana, in the state of Michoacán, were named half a century ago on the basis of meristic characters. In this work, the karyotype of the species was established by conventional and G-banding cytogenetic procedures and a comparative analysis of karyotypes from the two populations, previously proposed as subspecies, was performed. Ten females were collected in the Amacuzac river, and nine specimens (two females), were collected in the Huámito river. Based on the count of 264 mitotic fields of the former and 203 of the latter, the modal number of 2n=48 was established in every sample and considered as the diploid number of both populations. The karyo type analysis was based on ten karyotypes prepared from Morelos and eight from Michoacán, which included three from females and five from males. The chromosome formula thus established was of 8sm+40stt. The G-banding pattern was similar in both populations and the comparison of the mean lengths of the chromosome pairs did not reveal statistically significant differences between both populations. The presence of a practically identical karyotype does not support the subspecific division. The morphometric analyses made by other authors, which detected overlapping in the characters that were proposed as distinctive of the two subspecies, agree with the results of this study: not enough divergence has been found to substantiate the subdivision of C. istlanum. The lack of population divergence might have been brought about by an abated geographic isolation caused by gene flow among contiguous populations along their continuous distribution in the Balsas Basin regions, by the rela tively small divergence time since their distribution in these regions, and/or, less probably, by a recent historical replacement of one population by the other. The absence of karyotype differences might also be attributed to characteristics inherent to the genome organization in the genus Cichlasoma still to be identified and understood "