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Gigascience. 2019 Jun 1;8(6). pii: giz072. doi: 10.1093/gigascience/giz072.

A chromosome-scale genome assembly of cucumber (Cucumis sativus L.).

Author information

1
Institute of Vegetables and Flowers, Chinese Academy of Agricultural Sciences, No.12, Haidian District, Beijing 100081, China.
2
Agricultural Genomics Institute at Shenzhen, Chinese Academy of Agricultural Sciences, No. 7, Pengfei Road, Dapeng District, Shenzhen 518124, China.
3
College of Horticulture, Northwest A&F University, Yangling, Shanxi 712100, China.

Abstract

BACKGROUND:

Accurate and complete reference genome assemblies are fundamental for biological research. Cucumber is an important vegetable crop and model system for sex determination and vascular biology. Low-coverage Sanger sequences and high-coverage short Illumina sequences have been used to assemble draft cucumber genomes, but the incompleteness and low quality of these genomes limit their use in comparative genomics and genetic research. A high-quality and complete cucumber genome assembly is therefore essential.

FINDINGS:

We assembled single-molecule real-time (SMRT) long reads to generate an improved cucumber reference genome. This version contains 174 contigs with a total length of 226.2 Mb and an N50 of 8.9 Mb, and provides 29.0 Mb more sequence data than previous versions. Using 10X Genomics and high-throughput chromosome conformation capture (Hi-C) data, 89 contigs (∼211.0 Mb) were directly linked into 7 pseudo-chromosome sequences. The newly assembled regions show much higher guanine-cytosine or adenine-thymine content than found previously, which is likely to have been inaccessible to Illumina sequencing. The new assembly contains 1,374 full-length long terminal retrotransposons and 1,078 novel genes including 239 tandemly duplicated genes. For example, we found 4 tandemly duplicated tyrosylprotein sulfotransferases, in contrast to the single copy of the gene found previously and in most other plants.

CONCLUSION:

This high-quality genome presents novel features of the cucumber genome and will serve as a valuable resource for genetic research in cucumber and plant comparative genomics.

KEYWORDS:

Hi-C; PacBio; chromosome-scale assembly; cucumber; genomics

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