Molecular Cytogenetics in Trough Shells (Mactridae, Bivalvia): Divergent GC-Rich Heterochromatin Content

Abstract

The family Mactridae is composed of a diverse group of marine organisms, commonly known as trough shells or surf clams, which illustrate a global distribution. Although this family includes some of the most fished and cultured bivalve species, their chromosomes are poorly studied. In this work, we analyzed the chromosomes of Spisula solida, Spisula subtruncata and Mactra stultorum by means of fluorochrome staining, C-banding and fluorescent in situ hybridization using 28S ribosomal DNA (rDNA), 5S rDNA, H3 histone gene and telomeric probes. All three trough shells presented 2n = 38 chromosomes but different karyotype compositions. As happens in most bivalves, GC-rich regions were limited to the nucleolus organizing regions in Spisula solida. In contrast, many GC-rich heterochromatic bands were detected in both Spisula subtruncata and Mactra stultorum. Although the three trough shells presented single 5S rDNA and H3 histone gene clusters, their chromosomal locations differed. Regarding major rDNA clusters, while Spisula subtruncata presented a single cluster, both Spisula solida and Mactra stultorum showed two. No evidence of intercalary telomeric signals was detected in these species. The molecular cytogenetic characterization of these taxa will contribute to understanding the role played by chromosome changes in the evolution of trough shells.

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

“…6a,b ). As previously shown 42 , the karyotype of S. subtruncata is composed of 2n = 38 chromosomes and presents CMA positive/DAPI negative, intercalary G + C-rich bands on the long arms of chromosome pairs 4, 5, 7, 8, 9, 10, and 12, and a subterminal band, coincident with the NOR, on the long arm of pair 18 (Fig. 6g ).…”

“…6g ). As determined by barium hydroxide C-banding, these G + C-rich bands were heterochromatic 42 . Although most of the FISH signals obtained with the SSUsat probes overlapped the heterochromatic regions situated in those chromosome pairs (Fig.…”

“…As in many other invertebrates, G + C contents of bivalve genomes are relatively low: 31.65% in the mussel Mytilus galloprovincialis 3 , 33.69% in the pearl oyster Pinctada imbricata fucata 1 , 35.30% in the freshwater clam Corbicula fluminea ( https://www.ncbi.nlm.nih.gov/genome/15808 ), and 36.60% in the oyster Crassostrea gigas 2 . Heterochromatin content is also scarce in bivalves and only the wedge shell D. trunculus 30 , 39 , the zebra mussel Dreissena polymorpha 40 , 41 and the trough shells S. subtruncata and M.stultorum 42 characteristically present relatively high amounts of C-banded heterochromatin. Differential staining with base-specific fluorochromes demonstrated that these heterochromatic bands were G + C-rich, i.e.…”

“…Differential staining with base-specific fluorochromes demonstrated that these heterochromatic bands were G + C-rich, i.e. showed higher G + C content than the surrounding euchromatin 30 , 42 . As the SSUsat is the main component of the heterochromatic bands in S. subtruncata and its G + C-richness is around 44%, it can be inferred that the overall G + C-content of S. subtruncata must be below that value as it is in other bivalves.…”

“…In bivalves, G + C-rich regions are relatively scarce, mostly coincident with nucleolus organizing regions (NORs) 33 – 38 . Exceptions to this pattern are the wedge shell Donax trunculus 30 , 39 , the zebra mussel Dreisena polymorpha 40 , 41 and the trough shells Mactra stultorum and Spisula subtruncata 42 showing many G + C-rich bands non-coincident with the NORs. Furthermore, C-banding demonstrated that these regions were heterochromatic in both wedge shells 30 and trough shells 42 .…”