Deciphering the high-quality genome sequence of coriander that causes controversial feelings.

Coriander (Coriandrum sativum L. 2n = 2x = 22), a plant from the Apiaceae family, also called cilantro or Chinese parsley, is a globally important crop used as vegetable, spice, fragrance and traditional medicine. Here, we report a high-quality assembly and analysis of its genome sequence, anchored to 11 chromosomes, with total length of 2118.68 Mb and N50 scaffold length of 160.99 Mb. We found that two whole-genome duplication events, respectively, dated to ~45-52 and ~54-61 million years ago, were shared by the Apiaceae family after their split from lettuce. Unbalanced gene loss and expression are observed between duplicated copies produced by these two events. Gene retention, expression, metabolomics and comparative genomic analyses of terpene synthase (TPS) gene family, involved in terpenoid biosynthesis pathway contributing to coriander's special flavour, revealed that tandem duplication contributed to coriander TPS gene family expansion, especially compared to their carrot counterparts. Notably, a TPS gene highly expressed in all 4 tissues and 3 development stages studied is likely a major-effect gene encoding linalool synthase and myrcene synthase. The present genome sequencing, transcriptome, metabolome and comparative genomic efforts provide valuable insights into the genome evolution and spice trait biology of Apiaceae and other related plants, and facilitated further research into important gene functions and crop improvement.

An Overlooked Paleotetraploidization in Cucurbitaceae.

Cucurbitaceae plants are of considerable biological and economic importance, and genomes of cucumber, watermelon, and melon have been sequenced. However, a comparative genomics exploration of their genome structures and evolution has not been available. Here, we aimed at performing a hierarchical inference of genomic homology resulted from recursive paleopolyploidizations. Unexpectedly, we found that, shortly after a core-eudicot-common hexaploidy, a cucurbit-common tetraploidization (CCT) occurred, overlooked by previous reports. Moreover, we characterized gene loss (and retention) after these respective events, which were significantly unbalanced between inferred subgenomes, and between plants after their split. The inference of a dominant subgenome and a sensitive one suggested an allotetraploid nature of the CCT. Besides, we found divergent evolutionary rates among cucurbits, and after doing rate correction, we dated the CCT to be 90-102 Ma, likely common to all Cucurbitaceae plants, showing its important role in the establishment of the plant family.

Hierarchically Aligning 10 Legume Genomes Establishes a Family-Level Genomics Platform.

Mainly due to their economic importance, genomes of 10 legumes, including soybean (Glycine max), wild peanut (Arachis duranensis and Arachis ipaensis), and barrel medic (Medicago truncatula), have been sequenced. However, a family-level comparative genomics analysis has been unavailable. With grape (Vitis vinifera) and selected legume genomes as outgroups, we managed to perform a hierarchical and event-related alignment of these genomes and deconvoluted layers of homologous regions produced by ancestral polyploidizations or speciations. Consequently, we illustrated genomic fractionation characterized by widespread gene losses after the polyploidizations. Notably, high similarity in gene retention between recently duplicated chromosomes in soybean supported the likely autopolyploidy nature of its tetraploid ancestor. Moreover, although most gene losses were nearly random, largely but not fully described by geometric distribution, we showed that polyploidization contributed divergently to the copy number variation of important gene families. Besides, we showed significantly divergent evolutionary levels among legumes and, by performing synonymous nucleotide substitutions at synonymous sites correction, redated major evolutionary events during their expansion. This effort laid a solid foundation for further genomics exploration in the legume research community and beyond. We describe only a tiny fraction of legume comparative genomics analysis that we performed; more information was stored in the newly constructed Legume Comparative Genomics Research Platform (www.legumegrp.org).

Paleo-polyploidization in Lycophytes.

Lycophytes and seed plants constitute the typical vascular plants. Lycophytes have been thought to have no paleo-polyploidization although the event is known to be critical for the fast expansion of seed plants. Here, genomic analyses including the homologous gene dot plot analysis detected multiple paleo-polyploidization events, with one occurring approximately 13-15 million years ago (MYA) and another about 125-142 MYA, during the evolution of the genome of Selaginella moellendorffii, a model lycophyte. In addition, comparative analysis of reconstructed ancestral genomes of lycophytes and angiosperms suggested that lycophytes were affected by more paleo-polyploidization events than seed plants. Results from the present genomic analyses indicate that paleo-polyploidization has contributed to the successful establishment of both lineages-lycophytes and seed plants-of vascular plants.

Alignment of Rutaceae Genomes Reveals Lower Genome Fractionation Level Than Eudicot Genomes Affected by Extra Polyploidization.

Owing to their nutritional and commercial values, the genomes of several citrus plants have been sequenced, and the genome of one close relative in the Rutaceae family, atalantia (Atalantia buxifolia), has also been sequenced. Here, we show a family-level comparative analysis of Rutaceae genomes. By using grape as the outgroup and checking cross-genome gene collinearity, we systematically performed a hierarchical and event-related alignment of Rutaceae genomes, and produced a gene list defining homologous regions based on ancestral polyploidization or speciation. We characterized genome fractionation resulting from gene loss or relocation, and found that erosion of gene collinearity could largely be described by a geometric distribution. Moreover, we found that well-assembled Rutaceae genomes retained significantly more genes (65-82%) than other eudicots affected by recursive polyploidization. Additionally, we showed divergent evolutionary rates among Rutaceae plants, with sweet orange evolving faster than others, and by performing evolutionary rate correction, re-dated major evolutionary events during their evolution. We deduced that the divergence between the Rutaceae family and grape occurred about 81.15-91.74 million years ago (mya), while the split between citrus and atalantia plants occurred <10 mya. In addition, we showed that polyploidization led to a copy number expansion of key gene families contributing to the biosynthesis of vitamin C. Overall, the present effort provides an important comparative genomics resource and lays a foundation to understand the evolution and functional innovation of Rutaceae genomes.

Two Likely Auto-Tetraploidization Events Shaped Kiwifruit Genome and Contributed to Establishment of the Actinidiaceae Family.

The genome of kiwifruit (Actinidia chinensis) was sequenced previously, the first in the Actinidiaceae family. It was shown to have been affected by polyploidization events, the nature of which has been elusive. Here, we performed a reanalysis of the genome and found clear evidence of 2 tetraploidization events, with one occurring ∼50-57 million years ago (Mya) and the other ∼18-20 Mya. Two subgenomes produced by each event have been under balanced fractionation. Moreover, genes were revealed to express in a balanced way between duplicated copies of chromosomes. Besides, lowered evolutionary rates of kiwifruit genes were observed. These findings could be explained by the likely auto-tetraploidization nature of the polyploidization events. Besides, we found that polyploidy contributed to the expansion of key functional genes, e.g., vitamin C biosynthesis genes. The present work also provided an important comparative genomics resource in the Actinidiaceae and related families.