Chromosomal rearrangements drive differentiation and speciation in Prospero (Hyacinthaceae)

Hanna SCHNEEWEISS (Wien): The circum-Mediterranean Prospero autumnale is exceptionally dynamic with respect to karyotype evolution involving numerical (dysploidy, polyploidy, B-chromosomes) and structural variation. It comprises four evolutionary distinct diploid cytotypes (AA, B7B7, B6B6, and B5B5), each characterized by unique combination of basic chromosome number (x = 5, 6, and 7), genome size, karyotype structure, as well as rDNA and satDNA loci distribution. Comparative cytogenetic analyses, interpreted in a phylogenetic context, allowed establishing a model of chromosomal evolution in P. autumnale with descending dysploidy from x = 7 to x = 6 and independently from x = 7 to x = 5 via chromosomal fusions. All cytotypes except B5B5 participate in hybridization and/or polyploidization resulting in auto- and allopolyploids, and diploid hybrids. Whereas diploid homoploid hybrids and autopolyploids (only found in genome B7) are genomically stable, allopolyploids are very dynamic. Depending on their parental genomes divergence, allopolyploids can be either stable (near complete additivity; A/B7 polyploids), or numerically (subgenome compensating aneuploidy) and structurally variable (B6/B7 polyploids). In the latter, the variation blurs the boundaries of parental genomes and results in genetically balanced but genomically unique and restructured individuals with different chromosome numbers (4x: 2n = 25-28; skewed towards “autopolyploid” 2n = 28), high variation of genome sizes, and non-additive rDNA and satellite DNA loci numbers and localization, including spread of satDNA from genome B6 into B7. Frequently occurring and highly variable (structure and repeat composition) B-chromosomes and supernumerary chromosomal segments are likely a byproduct of elevated rates of chromosomal rearrangements of the regular chromosome complement. High incidence of chromosomal changes in all cytotypes of P. autumnale strongly contrasts with the relative stability of genome-wide repeat composition (NGS), and with morphological near-uniformity of cytotypes. All data suggest chromosomal restructuring as a major mechanism of diversification and eventually speciation in Prospero.