Chromosomal location of Ag/CMA3-NORs, 5S rDNA and telomeric repeats in two stickleback species

Abstract

During the last 10 years, the stickleback has emerged as a promising model system in several biological fields. In particular, it is one of the most investigated fish groups by evolutionary biologists, as a model for understanding the genetic and molecular basis in processes of adaptive radiation, microevolution and development, as well as the genetic basis of sex determination. To provide karyological information about stickleback fish we performed cytogenetic analysis of the threespine stickleback (Gasterosteus aculeatus L.) and the ninespine stickleback (Pungitius pungitius L.). Diploid chromosome number in both species equalled 42; however, the chromosome arm numbers were 62 and 70, respectively. In the threespine stickleback, multichromosomal location of silver (AgNO3) and chromomycin A3 (CMA3) positively stained Nucleolar Organizer Regions (Ag/CMA3-NORs) was observed. The number of silver-stained chromosomes varied among different individuals from two to six. The Ag/CMA3-NORs covered almost entire short arms of one pair of middle-sized submetacentric chromosome pairs in the ninespine stickleback. In one male, the only Ag-NOR was situated telomerically on the long arm of the Y chromosome. The 5S rDNA clusters were observed on short arms of one pair of subtelocentric chromosomes in the threespine stickleback and four pairs of subtelo-acrocentric chromosomes in the ninespine stickleback of both sexes. Chromosome rearrangements responsible for the stickleback karyotype polymorphisms did not leave any internally located telomeric sequences. The results obtained in two stickleback species are therefore of considerable interest and further karyological studies should be carried out on the other populations of the wide species range in order to provide a more general picture of karyoevolutive trends in Gasterosteidae.