Universität zu Köln

Zhang, Meng ORCID: 0000-0003-4792-9979 (2026). Unconventional Meiosis in Plants: Holocentric chromosome recombination in beak-sedges and asymmetric inheritance in pentaploid dogroses. PhD thesis, Universität zu Köln.

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Abstract

Meiosis is an essential cell division process for sexual reproduced eukaryotes, during which independent chromosome segregation and homologous recombination are usually indispensible, yet some species exhibit unconventional mechanisms to overcome challenges posed by holocentric chromosomes, achiasmy, or polyploidy. Recombination landscapes are typically shaped by centromere positioning and genomic features. However, the recombination patterns of holocentric species, that typically have multiple centromere units dipersed along the entire chromosomes, are understudied. Similarly, the mechanisms enabling stable reproduction in achiasmatic or polyploid species are poorly understood. Here, we investigate these exceptions through genomic, cytological, and evolutionary analyses in holocentric Rhynchospora and pentaploid dogroses. Using single-cell sequencing of pollen grains, we first constructed a recombination landscape for Rhynchospora breviuscula, which presents a distal bias pattern despite uniformly dispersed (epi)genetic signals. Telomere-led chromosome pairing and synapsis were identified as key drivers of this pattern. Comparative analyses across multiple Rhynchospora species revealed that chromosome fusions and fissions drive the karyotype evolution in this genus, and further demonstrated that recombination landscapes are shaped by meiotic dynamics rather than genomic architecture. Strikingly, we discovered that Rhynchospora tenuis achieves efficient sexual reproduction without crossovers in both maternal and paternal meiosis, yielding clonal offspring. Finally, in pentaploid dogroses, we proposed a mechanism for its stabilised polyploid inheritance: 14 chromosomes form bivalents (transmitted biparentally), while 21 univalents are maternally retained. Genome assembly and centromere analysis revealed that bivalent-forming subgenomes tend to have ATHILA retrotransposons- based centromeres, whereas univalent subgenomes carry expanded tandem-repeat-based centromeres (CANR4) with higher CENH3 enrichment, suggesting centromere divergence drives their asymmetric segregation. This work presents how meiosis progresses uncanonically in two different plant genus. These systems highlight nature’s versatility in overcoming meiotic constraints, offering insights for genome evolution, sexual reproduction, crop engineering, and the preservation of genetic diversity in challenging genomic contexts.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Zhang, Meng mzhang@mpipz.mpg.de https://orcid.org/0000-0003-4792-9979 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-798100
Date:	2026
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Außeruniversitäre Forschungseinrichtungen > MPI for Plant Breeding Research
Subjects:	Natural sciences and mathematics
Uncontrolled Keywords:	Keywords Language meiosis English holocentric English polyploid English plant UNSPECIFIED Rhynchospora UNSPECIFIED Rosa UNSPECIFIED oddploidy UNSPECIFIED pentaploids UNSPECIFIED dog rose UNSPECIFIED inverted meiosis UNSPECIFIED achiasmy UNSPECIFIED karyotype evolution UNSPECIFIED
Date of oral exam:	28 January 2026
Referee:	Name Academic Title Marques, André Dr. Stetter, Markus Prof. Dr. Mason, Annaliese Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/79810