5S ribosomal gene family in the moray eel Gymnothorax unicolor (Anguilliformes: Muraenidae)

Moray eels are very common species, mainly in tropical and subtropical waters; nevertheless, they are very little studied. While the monophyly of the family is well-supported by phylogenetic studies, the species taxonomy is still controversial and some genera appear to be not-monophyletic. From cytogenetic point of view, only 20 of about 200 species have been studied and for most of them few karyological data are available. Beside a rather conserved diploid number of 42, morays display a greatly diversified karyotype structure and a large amount of heterochromatin. Chromosomal rearrangements as well as chromosome homologies have been reported in several species, resulting in divergent karyotype structures. To further investigate these cytogenetic features, the localization and distribution of specific sequences by FISH are essential for species with no genomic data but, to date, this molecular cytogenetic technique has been employed in very few moray species, with promising results. The karyotype of the Mediterranean brown moray eel Gymnothorax unicolor (Delaroche, 1809) was previous investigated and compared with the other Mediterranean species Muraena helena and with the congeneric species G. tile, by the use of classical techniques that pointed out many chromosomal banding similarities, as well as the occurrence of pericentric inversions and changes in heterochromatin amount. Furthermore, in this species FISH has been employed for the localization of centromeric, telomeric and 45S ribosomal sequences. In order to extend the study of ribosomal genes in this species, in the present study we carried out the isolation, molecular characterization and chromosomal mapping of the 5S sequences. These ribosomal genes have been interstitially localized in the acrocentric chromosome pair n. 10, different from the major ribosomal gene-bearing pair. Within the Muraenidae family, the minor ribosomal gene family has been recently mapped in other five species, pointing out with differences in the chromosomal distribution. G. unicolor is the only studied species with only one 5S cluster, but a similar location on an acrocentric chromosome has been found in almost all other species. Furthermore a comparative analysis of the distribution of the two ribosomal gene families among morays have been carried out. These results, even if preliminary, provide novel karyological information for comparative genomic investigations and could also help to understand the cytotaxonomy of this family.