Haplotype-resolved genomes of wild octoploid progenitors illuminate genomic diversifications from wild relatives to cultivated strawberry.

Strawberry is an emerging model for studying polyploid genome evolution and rapid domestication of fruit crops. Here we report haplotype-resolved genomes of two wild octoploids (Fragaria chiloensis and Fragaria virginiana), the progenitor species of cultivated strawberry. Substantial variation is identified between species and between haplotypes. We redefine the four subgenomes and track the genetic contributions of diploid species by additional sequencing of the diploid F. nipponica genome. We provide multiple lines of evidence that F. vesca and F. iinumae, rather than other described extant species, are the closest living relatives of these wild and cultivated octoploids. In response to coexistence with quadruplicate gene copies, the octoploid strawberries have experienced subgenome dominance, homoeologous exchanges and coordinated expression of homoeologous genes. However, some homoeologues have substantially altered expression bias after speciation and during domestication. These findings enhance our understanding of the origin, genome evolution and domestication of strawberries.

The chromosome-level genome of Gypsophila paniculata reveals the molecular mechanism of floral development and ethylene insensitivity.

Gypsophila paniculata, belonging to the Caryophyllaceae of the Caryophyllales, is one of the most famous worldwide cut flowers. It is commonly used as dried flowers, whereas the underlying mechanism of flower senescence has not yet been addressed. Here, we present a chromosome-scale genome assembly for G. paniculata with a total size of 749.58 Mb. Whole-genome duplication signatures unveil two major duplication events in its evolutionary history: an ancient one occurring before the divergence of Caryophyllaceae and a more recent one shared with Dianthus caryophyllus. The integrative analyses combining genomic and transcriptomic data reveal the mechanisms regulating floral development and ethylene response of G. paniculata. The reduction of AGAMOUS expression probably caused by sequence polymorphism and the mutation in miR172 binding site of PETALOSA are associated with the double flower formation in G. paniculata. The low expression of ETHYLENE RESPONSE SENSOR (ERS) and the reduction of downstream ETHYLENE RESPONSE FACTOR (ERF) gene copy number collectively lead to the ethylene insensitivity of G. paniculata, affecting flower senescence and making it capable of making dried flowers. This study provides a cornerstone for understanding the underlying principles governing floral development and flower senescence, which could accelerate the molecular breeding of the Caryophyllaceae species.

Fragaria mitogenomes evolve rapidly in structure but slowly in sequence and incur frequent multinucleotide mutations mediated by microinversions.

Plant mitochondrial DNA has been described as evolving rapidly in structure but slowly in sequence. However, many of the noncoding portions of plant mitogenomes are not homologous among species, raising questions about the rate and spectrum of mutations in noncoding regions. Recent studies have suggested that the lack of homology in noncoding regions could be due to increased sequence divergence. We compared 30 kb of coding and 200 kb of noncoding DNA from 13 sequenced Fragaria mitogenomes, followed by analysis of the rate of sequence divergence, microinversion events and structural variations. Substitution rates in synonymous sites and nongenic sites are nearly identical, suggesting that the genome-wide point mutation rate is generally consistent. A surprisingly high number of large multinucleotide substitutions were detected in Fragaria mitogenomes, which may have resulted from microinversion events and could affect phylogenetic signal and local rate estimates. Fragaria mitogenomes preferentially accumulate deletions relative to insertions and substantial genomic arrangements, whereas mutation rates could positively associate with these sequence and structural changes among species. Together, these observations suggest that plant mitogenomes exhibit low point mutations genome-wide but exceptionally high structural variations, and our results favour a gain-and-loss model for the rapid loss of homology among plant mitogenomes.