Euchromatic Supernumerary Chromosomal Segments—Remnants of Ongoing Karyotype Restructuring in the Prospero autumnale Complex?

Supernumerary chromosomal segments (SCSs) represent additional chromosomal material that, unlike B chromosomes, is attached to the standard chromosome complement. The Prospero autumnale complex (Hyacinthaceae) is polymorphic for euchromatic large terminal SCSs located on the short arm of chromosome 1 in diploid cytotypes AA and B7B7, and tetraploid AAB7B7 and B6B6B7B7, in addition to on the short arm of chromosome 4 in polyploid B7B7B7B7 and B7B7B7B7B7B7 cytotypes. The genomic composition and evolutionary relationships among these SCSs have been assessed using fluorescence in situ hybridisation (FISH) with 5S and 35S ribosomal DNAs (rDNAs), satellite DNA PaB6, and a vertebrate-type telomeric repeat TTAGGG. Neither of the rDNA repeats were detected in SCSs, but most contained PaB6 and telomeric repeats, although these never spanned whole SCSs. Genomic in situ hybridisation (GISH) using A, B6, and B7 diploid genomic parental DNAs as probes revealed the consistently higher genomic affinity of SCSs in diploid hybrid B6B7 and allopolyploids AAB7B7 and B6B6B7B7 to genomic DNA of the B7 diploid cytotype. GISH results suggest a possible early origin of SCSs, especially that on chromosome 1, as by-products of the extensive genome restructuring within a putative ancestral P. autumnale B7 genome, predating the complex diversification at the diploid level and perhaps linked to B-chromosome evolution