Disentangling the complex karyotype evolution of crocuses

• Applicant: Dr. Dörte Harpke
• Subject Area: Evolution and Systematics of Plants and Fungi
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 465449547

Dysploidy is assumed to be a main driver for re-diploidization after polyploidization and it could also be a driver of diversification by creating karyotype differences functioning as reproductive barriers. Our knowledge regarding these processes is, however, based on very few well-investigated examples and mechanistic explanations are yet lacking. The high frequency of dysploid chromosome number changes in the genus Crocus is unusual for plants with monocentric chromosomes. In extreme cases, the basic chromosome number was reduced from n = 15 to n = 4 within a few million years while doubling genome size at the same time. The sympatric and parapatric occurrence of closely related Crocus taxa without morphologically recognizable hybrids suggests that their different chromosome numbers might represent crossing barriers. However, molecular analysis already revealed the presence of homoploid hybridization in some Crocus groups despite parental species differing in their karyotypes. Therefore, in-detail population- and cytogenetic analyses are necessary to understand the role that different karyotypes and chromosome numbers might play in reproductive isolation and will add to clarify what mechanisms act at chromosome level. It is assumed that whole-genome duplication (WGD) triggers dysploidy. Thus, the initial identification of WGD events across the genus is crucial to correlate them with changed dysploidy frequencies. However, polyploidization cannot be deduced from mere chromosome numbers or genome sizes in Crocus. Instead, we will use k-mer frequencies of NGS data and identification of paralogous genes in transcriptome or shotgun sequences data of representatives of the relevant Crocus clades. (i) Once the clades with WGD events in the genus are identified, we can infer if and how fast dysploidy follows polyploidization and if single WGD events trigger multiple chromosomal rearrangements resulting eventually in species with different chromosome numbers.(ii) To trace dysploid karyotype changes, we will follow the fate of chromosomes/chromosome parts through dysploidy events by developing and employing chromosomal landscape markers in three selected species groups of Crocus.(iii) Crossing experiments will be set up in parallel to infer the influence of different (dysploid) karyotypes on the strengths of reproductive isolation or lack thereof. The proposed project will unravel the complex chromosome evolution of crocuses, give important insights in the role of dysploidy in plant evolution, and is aimed to establish the genus as model for mechanisms and regulation of dysploidy in monocentric plants.

• DFG Programme: Research Grants
• Co-Investigator: Dr. Frank R. Blattner