Evidence of karyotypic stasis during the evolutionary divergence between Paralabrax maculatofasciatus and P. nebulifer (Perciformes: Serranidae)

Information on chromosome structure is basic within the biological knowledge of any speciesand fundamental to the understanding of the cytogenetic phenomena underlying physiological, ontogenic and evolutionary processes. Knowledge on the degree of similarity of chromosomes in phylogenetically close species supplements the necessary information to raise causal hypotheses on the processes of evolutionary divergence.The objective of this study was to determine the cytogenetic similarity between Paralabrax maculatofasciatus and P. nebulifer, closely (phylogenetically) related species distributed in the Eastern Pacific, by comparing number, type and size of chromosomes of these taxa. The examination of mitotic cells from eleutheroembryos of both species showed a karyotype consisting of 48 acrocentric chromosomes (2n = 48A; fundamental number = 48), without the presence of heteromorphic chromosomes. No significant differences between species were observed in size and relative length (RL) of homeologous chromosome pairs which showed maximum and minimum values of 4 µm (RL = 5.5 %; pair 1) and 1.7 µm (RL = 2.3 %; pair 24), respectively. This karyotype is considered ancestral within the Order Perciformes and is present in most of the species of the Family Serranidae. The results found in this study supported the hypothesis that evolutionary divergence between P. maculatofasciatus and P.nebulifer occurred through the establishment of pre-mating reproductive barriers, without karyotype modification or formation of post-mating barriers, either pre-zygotic or post-zygotic.