chromosomal mapping of repetitive dnas in gobionellus oceanicus and g stomatus gobiidae perciformes a shared xxxy system and an unusual distribution of 5s rdna sites on the y chromosome with nearly 2000 species gobiidae is the most specious family of the vertebrates this high level of speciation is accompanied by conspicuous karyotypic modifications where the role of repetitive sequences remains largely unknown this study analyzed the karyotype of 2 species of the genus gobionellus and mapped 18s and 5s ribosomal rna genes and ca15 microsatellite sequences onto their chromosomes g oceanicus 2n 56 12 metacentrics m 4 submetacentrics sm 1 subtelocentric st 39 acrocentrics a 12m 4sm 2st 38a and g stomatus 2n 56 20m 14sm 1st 21a 20m 14sm 2st 20a possess the highest diploid chromosome number among the gobiidae and have different karyotypes both species share an xxxy sex chromosome system with a large subtelocentric x and a small acrocentric y chromosome which is rich in ca15 sequences and bears 5s rrna sites although coding and noncoding repetitive dna sequences may be involved in the genesis or differentiation of the sex chromosomes the exclusive presence of 5s rdna sites on the y but not on the x chromosome of both species represents a novelty in fishes in summary the karyotypic differences as well as new data on the sex chromosome systems in these 2 gobiidae species confirm the high chromosomal dynamism observed in this family