Molecular Cytogenetics in Digenean Parasites: Linked and Unlinked Major and 5S rDNAs, B Chromosomes and Karyotype Diversification

Abstract

Digenetic trematodes are the largest group of internal metazoan parasites, but their chromosomes are poorly studied. Although chromosome numbers and/or karyotypes are known for about 300 of the 18,000 described species, molecular cytogenetic knowledge is mostly limited to the mapping of telomeric sequences and/or of major rDNA clusters in 9 species. In this work we mapped major and 5S rDNA clusters and telomeric sequences in chromosomes of Bucephalus minimus, B. australis, Prosorhynchoides carvajali (Bucephaloidea), Monascus filiformis (Gymnophalloidea), Parorchis acanthus (Echinostomatoidea), Cryptocotyle lingua (Opisthorchioidea), Cercaria longicaudata, Monorchis parvus (Monorchioidea), Diphterostomum brusinae, and Bacciger bacciger (Microphalloidea). Whilst single major and minor rDNA clusters were mapped to different chromosome pairs in B. minimus and P. acanthus, overlapping signals were detected on a single chromosome pair in the remaining taxa. FISH experiments using major rDNA and telomeric probes clearly demonstrated the presence of highly stretched NORs in most of the digenean taxa analyzed. B chromosomes were detected in the B. bacciger samples hosted by Ruditapes decussatus. Although the cercariae specimens obtained from Donax trunculus, Tellina tenuis, and R. decussatus were in agreement with B. bacciger, their karyotypes showed striking morphological differences in agreement with the proposed assignation of these cercariae to different species of the genus Bacciger. Results are discussed in comparison with previous data on digenean chromosomes.

Karyotype features of trematode Himasthla elongata

“…H. elongata karyotype consists of two pairs of large chromosomes and 10 pairs of smaller-sized chromosomes. In contrast with a previous reports collected and analysed by Garcia-Souto and J. Pasantes [ 41 ] where single loci of major rDNA clusters are described in several digeneans, we revealed multiple signals of rDNA at 3 chromosome pairs in H. elongata karyotype. The lack of karyological data on the other Himasthla species doesn’t allow any speculation about chromosome number characteristic for this genus.…”

Chromosome analysis and the occurrence of B chromosomes in fish parasite Acanthocephalus anguillae (Palaeacanthocephala: Echinorhynchida)

“…The B chromosomes are known to be non-essential supernumerary elements present in thousands of plant, animal, and fungal species [ 32 ] and have also been described in some digenean parasites [ 21 , 69 ]. Comparison of their molecular composition has shown that most DNA sequences found on B chromosomes are also present on A chromosomes [ 12 ], and although mainly repetitive DNA is found on them [ 8 ], protein-coding genes have also been discovered [ 39 ].…”

Karyotype Characterization of Nine Periwinkle Species (Gastropoda, Littorinidae)

“…Although those differences could indicate variations in chromosome numbers among different genera in the family, as the divergent diploid numbers (30 and 36) for three of these species were determined from paraffinned material in a study performed in the early 1960s [ 5 ], it would be necessary to confirm them using modern techniques of chromosome preparation. In regard to the highly divergent diploid chromosome number of 2n = 16 proposed for E. subnodosa [ 13 ], it is important to take into account that gastropod mollusks are usually intermediary hosts for digenean parasites, many of which present chromosome numbers and karyotypes [ 30 ] similar to the one reported for E. subnodosa [ 13 ]. Thus, future studies are needed to confirm the chromosome number in this species.…”