Chromosomal evolution in Prospero autumnale complex

Cytological plasticity, including changes of chromosome number (polyploidy, dysploidy/aneuploidy), chromosome structure (inversions, translocations, centric shifts, etc.) and more subtle changes in sequence composition (expansion/reduction of repetitive DNA, distribution of eu- and heterochromatin, etc.), is a prominent feature of plant genomes contributing to race diversification and eventually to speciation. Recent methodological advances, including availability of whole genome sequences and molecular cytogenetic techniques, especially fluorescence in situ hybridisation (FISH), have greatly enhanced our understanding and appreciation of the frequency of such changes. A particularly informative research avenue is to investigate and interpret data on chromosomal and genomic rearrangements in a phylogenetic-evolutionary context. A unique and extraordinarily well suited system to address aspects of genome evolution is provided by the relatively young and morphologically little differentiated genus Prospero (Hyacinthaceae). This circum- Mediterranean group includes a stunning and unparalleled array of chromosome base numbers (x = 4, 5, 6, 7) and ploidy levels (2x to c. 20x) involving both auto- and allopolyploids. Additionally, inter- and intrapopulational variation and polymorphisms involving chromosome structure, supernumerary segments and B-chromosomes, and genome size, occur at a very high frequency. Essentially, every known form of chromosome mutation has been detected in this group. A thorough investigation of the cytological features of this remarkable group employing repetitive DNA and the interpretation of changes in a sound phylogenetic context will allow us to address significant evolutionary questions concerning the origins, mechanisms, directions and frequencies of chromosomal changes, and thus establish the role of chromosome change in race formation and speciation. These include: Is differentiation of chromosome base numbers following an ascending or a descending dysploidy series? What are the dynamics and mechanisms involved in chromosome base number and chromosome structure changes? What is the dynamics of genome size change, which sequence types are involved and how significant are these changes in the context of race differentiation and speciation? Is the genome diploidisation in polyploids directional, and which types of repetitive DNA are involved? Do karyotypic features and characteristics correlate with phylogenetic relationships, indicating a direct role in race formation?

The circum-Mediterranean bulbous monocot plant genus Prospero has exceptionally dynamic genomes, exhibiting extraordinary variation in chromosome numbers and structure. The numerical variation includes a dysploid series on the diploid level (chromosome base numbers x = 4, 5, 6, 7), the presence of various polyploids, and the occurrence of B- chromosomes.Of the three commonly recognized species (P. hanburyi, P. obtusifolium, and P. autumnale s. l.), the latter is particularly chromosomally variable and comprises four diploid cytotypes (AA, B7B7, B6B6, and B5B5); each characterized by a unique combination of basic chromosome number (x = 5, 6, and 7), genome size, and karyotype structure. Cytotype B7B7 is distributed in the whole Mediterranean region, while the other cytotypes have much smaller distributions being endemic to Libya (B5B5), Crete (B6B6), and the Iberian Peninsula and adjacent NW Africa (AA). Comparative analyses of chromosomes of all cytotypes and sequencing of several DNA regions allow establishing a model of chromosomal evolution in P. autumnale. Each cytotype forms a distinct evolutionary lineage, and different chromosome base numbers originated independently from the ancestral chromosome number x = 7 via chromosomal rearrangements. Each cytotype has unique chromosomal distributions of various DNA markers, including ribosomal genes and several tandemly repeated DNA sequences (satellite DNAs). Application of recently developed next generation sequencing methods, which allow sequencing of millions of short DNA fragments sampled randomly from whole genome, reveals that Prospero genomes are mostly composed of mobile genetic elements (retrotransposons). Despite high levels of chromosomal variation among cytotypes, the frequencies of different repetitive DNA families are proportional in all cytotypes. All diploid cytotypes except B5B5 participate in the formation of polyploids, without or with hybridization (auto- and allopolyploids, respectively). Autopolyploids are only found in genome B7 and are genomically stable. Allopolyploids show either near-complete additivity compared to diploid parents (polyploids of A and B7 origin) or are strongly rearranged with chromosome numbers (2n = 2528) and genome structure different from those expected (polyploids of B6 and B7 origin). The B6 and B7 allotetraploids experienced genome size changes and associated changes in copy number of various repetitive DNA types, enhanced by extensive hybridization on the polyploid level. Chromosomes in allopolyploids can be painted with DNAs of the parents and show low levels of inter-parental genome restructuring. In addition to regular chromosome complements, Prospero genomes contain also accessory B chromosomes, which originated multiple times as a by-product of chromosome restructuring and are evolutionary young. The high incidence of chromosomal changes in all cytotypes of P. autumnale strongly contrasts with morphological near-uniformity of all plants, suggesting that chromosomal restructuring is a major mechanism of diversification and eventually speciation in Prospero.