Abstract
BackgroundThe low-numbered 14-chromosome karyotype of marsupials has falsified the fusion hypothesis claiming ancestrality from a 22-chromosome karyotype. Since the 14-chromosome condition of the relict Dromiciops gliroides is reminecent of ancestrality, its interstitial traces of past putative fusions and heterochromatin banding patterns were studied and added to available marsupials’ cytogenetic data. Fluorescent in situ hybridization (FISH) and self-genomic in situ hybridization (self-GISH) were used to detect telomeric and repetitive sequences, respectively. These were complemented with C-, fluorescent banding, and centromere immunodetection over mitotic spreads. The presence of interstitial telomeric sequences (ITS) and diploid numbers were reconstructed and mapped onto the marsupial phylogenetic tree.ResultsNo interstitial, fluorescent signals, but clearly stained telomeric regions were detected by FISH and self-GISH. Heterochromatin distribution was sparse in the telomeric/subtelomeric regions of large submetacentric chromosomes. Large AT-rich blocks were detected in the long arm of four submetacentrics and CG-rich block in the telomeric regions of all chromosomes. The ancestral reconstructions both ITS presence and diploid numbers suggested that ITS are unrelated to fusion events.ConclusionAlthough the lack of interstitial signals in D. gliroides’ karyotype does not prove absence of past fusions, our data suggests its non-rearranged plesiomorphic condition.Electronic supplementary materialThe online version of this article (doi:10.1186/s13039-016-0270-8) contains supplementary material, which is available to authorized users.
Highlights
The low-numbered 14-chromosome karyotype of marsupials has falsified the fusion hypothesis claiming ancestrality from a 22-chromosome karyotype
Since major structural chromosomal rearrangements are associated with cytogenetically detectable heterochromatic regions and repetitive sequences [24,25,26,27], we explored both issues by C-banding and self-genomic in situ hybridization [28, 29]
Fluorescent in situ hybridization (FISH), Self-GISH, and immunofluorescence Telomeric sequence detection by FISH on metaphase chromosomes was performed with the universal telomeric probe (TTAGGG)n, generated by PCR and labeled with fluorescein 12-deoxyuridine Triphosphate (dUTP) (Roche Applied Science) [33, 34]
Summary
The low-numbered 14-chromosome karyotype of marsupials has falsified the fusion hypothesis claiming ancestrality from a 22-chromosome karyotype. Chromosome numbers among marsupials ranges from 2n = 10 to 32, with a modal 14-chromosome number (followed by 2n = 22) for the Australian and South American radiation [1,2,3] These karyotypes share extensive resemblance in chromosome morphology [4, 5] and G-banding patterns [6], leading to the hypothesis that 2n = 14 is ancestral for marsupials [6, 7]. Recent comparative metatherian and eutherian genome assemblies have falsified the fusion hypothesis, supporting the ancestrality of the 14-chromosome karyotype [20] This putative ancestral karyotype is shared by microbiotherians, caenolestids, peramelemorphians, vombatids, and pygmy possums [10].
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