Karyotypic diversification in Mytilus mussels (Bivalvia: Mytilidae) inferred from chromosomal mapping of rRNA and histone gene clusters

Abstract

BackgroundMussels of the genus Mytilus present morphologically similar karyotypes that are presumably conserved. The absence of chromosome painting probes in bivalves makes difficult verifying this hypothesis. In this context, we comparatively mapped ribosomal RNA and histone gene families on the chromosomes of Mytilus edulis, M. galloprovincialis, M. trossulus and M. californianus by fluorescent in situ hybridization (FISH).ResultsMajor rRNA, core and linker histone gene clusters mapped to different chromosome pairs in the four taxa. In contrast, minor rRNA gene clusters showed a different behavior. In all Mytilus two of the 5S rDNA clusters mapped to the same chromosome pair and one of them showed overlapping signals with those corresponding to one of the histone H1 gene clusters. The overlapping signals on mitotic chromosomes became a pattern of alternate 5S rRNA and linker histone gene signals on extended chromatin fibers. Additionally, M. trossulus showed minor and major rDNA clusters on the same chromosome pair.ConclusionThe results obtained suggest that at least some of the chromosomes bearing these sequences are orthologous and that chromosomal mapping of rRNA and histone gene clusters could be a good tool to help deciphering some of the many unsolved questions in the systematic classification of Mytilidae.

Divergent evolutionary behavior of H3 histone gene and rDNA clusters in venerid clams

“…These differences in the number of core histone gene clusters in related species are coincident with results reported for other bivalve families. While three species of scallops and one mussel showed a single core histone gene cluster [ 17 , 21 ], one scallop and six mussels presented two [ 17 , 18 , 20 , 21 ] and one mussel had four [ 19 ]. This behavior differs with those described for other invertebrate groups in which histone gene clusters have been reported to be a highly conservative cytogenetic marker [ 5 – 10 ].…”

“…In contrast with the variability in number, 75 % of the H3 histone gene clusters detected in venerid clams (15 of a total of 20) were located at subterminal chromosome positions. This is unusual for bivalves and also for other invertebrates; in fact, barely a 30 % of the core histone gene clusters detected in mussels of the family Mytilidae (5 of 17) [ 17 – 20 ] and only a 40 % of those reported in the scallops of the family Pectinidae (2 of 5) [ 21 ] were subterminal. For other invertebrates, the subterminal position of the histone gene signal has only been described in three grasshoppers [ 5 ].…”

“…These results partially differ with those found in other bivalve families. While mytilid mussels showed one to four major rDNA and two to five 5S rDNA clusters [ 17 – 20 , 42 ], both Pectinidae and Ostreidae species presented one or two major and 5S rDNA clusters [ 31 , 43 – 47 ].…”

“…Usually, the clusters comprise only core histone genes but repeated clusters including linker histone and/or other genes have also been reported. In these organisms histone H3 genes have been mapped to chromosomes in only eight mussels [ 13 , 14 , 17 – 20 ], four scallops [ 21 ], one oyster [ 22 ] and two clams [ 23 ]. In comparison with 45S and 5S rDNA clusters, surprisingly remarkable differences in number and location of the histone H3 gene clusters were found, more outstandingly among mussels and clams.…”

Methylation profile of a satellite DNA constituting the intercalary G+C-rich heterochromatin of the cut trough shell Spisula subtruncata (Bivalvia, Mactridae)

“…Fluorescent in situ hybridisation (FISH) with digoxigenin-labelled SSUsat probes was performed as described previously 38 , 42 , 59 , 61 . Chromosome spreads were treated with RNase and pepsin before denaturation in 70% formamide (70 °C, 2 min).…”