Identificazione e caratterizzazione dei geni snRNA U1 di due specie di crostacei isopodi

The U1 small nuclear RNAs (snRNA) gene, together with the U2, U4 and U5 snRNA genes, constitutes a group of class II evolutionary conserved genes, the transcripts of which are involved in pre-mRNA splicing in the nucleus as RNA components of the spliceosome. U1 snRNA genes, isolated from a variety of eukaryotes, were reported to be clustered in a number of species and codified as tandemly repeated units, containing one or more types of U1 snRNA genes, or to be linked but not tandemly repeated. Different types of U1 snRNA were found in some organisms to be tissue-specific and developmentally regulated. Recently, using PCR amplification, in the genome of the crustacean isopod Asellus aquaticus, we found a U1 snRNA gene within a 1842 bp long tandemly repeated unit also containing a 5S rRNA gene. No other snRNA gene had been previously identified in any crustacean species. In the present work, using PCR amplification, we identified three further units containing U1 snDNA in the genome of A. aquaticus. The three units were sequenced and used as probes in FISH experiments to localize them on A. aquaticus chromosomes. Likewise, we investigated the snRNA genes in the genome of Proasellus coxalis and identified only one unit containing U1 snDNA in this organism. P. coxalis and A. aquaticus are two Asellidae species that cohabit in the fresh waters of central and southern Italy. The haploid DNA amount of A. aquaticus is 2.52 pg, its karyotype consists of 2n = 16 homomorphic chromosomes. The haploid DNA amount of P. coxalis is 1.30 pg, its karyotype consists of 2n = 12 homomorphic chromosomes. In A. aquaticus, two intercalary heterochromatic areas are also revealed by CMA on one chromosome, the Y chromosome, of a heteromorphic sex chromosome pair present in about 25% of males from a natural population collected in the Sarno river near Naples (Barzotti et al., 2000 and references cited therein). CONCLUSIONS In this work, we demonstrate that at least three variants of the U1 snRNA gene contained in four different fragments (A,B,C,D) are present in the genome of A. aquaticus, while once again, as for 5S rRNA genes (Barzotti et al., 2000; Pelliccia et al., 1998; Pelliccia et al., 2001), in the genome of P. coxalis we identified only one unit containing one U1 snRNA gene. The genome DNA amount of A. aquaticus (2.52 pg) is about twice that of P. coxalis (1.30 pg) (Rocchi et al., 1989). These values point to a mechanism of genomic duplication as a source of the diversity between the genomes of the two species, while the complex organization of the 5S rRNA genes and of the U1 snRNA genes in genome of A. aquaticus seems to indicate the occurrence of events of duplicative transposition. Undoubtedly a better knowledge of the genome of these organisms will be necessary for more reliable conclusions to be drawn on these matters.