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Zusammenfassung:
Chromosome fusions are a fundamental structural variation in the genome. Although they were often observed even between closely related animal and plant species, the role of chromosome fusions in speciation remains unclear. Recently, we established a new model system for studying speciation, Pristionchus nematodes, which are a soil nematode associated with beetles. The model species, Pristionchus pacificus, has been used in comparative studies with C. elegans and in studies for phenotypic plasticity. Worldwide field sampling over the last 15 years has resulted in the discovery of more than 50 new Pristionchus species, including the most closely related species group of nematodes. In the present study, we comprehensively analyzed reproductive isolation and genomic divergence between two closely related Pristionchus species, P. pacificus and Pristionchus exspectatus. Using single-molecule and Hi-C sequencing, we generated a chromosome-level de-novo assembly of P. exspectatus genome and performed comparative synteny analysis and FISH analysis, suggesting occurrence of two independent chromosome fusions between the two species, in which the same chromosome is fused to a different chromosome. The fused chromosomes have a conserved chromosomal pattern of two ancestral chromosomes but altered recombination patterns that form long low-recombination regions containing approximately 15% of the genes across the genome. Quantitatie trait locus (QTL) analysis for hybrid sterility indicated that the QTLs were accumulated on the fused chromosome. Subsequent genetic analysis suggested that aneuploidy induced by the abnormal chromosome segregation explains the female sterility, while hybrid-specific recombination that breaks the linkage of the low-recombination regions explains the male sterility. Thus, the present study clearly indicates multiple evolutionary effects of chromosome fusions on the evolution of reproductive isolations between nascent species pair of the nematodes.
