Cucumis hystrix Chakr. (2n = 2x = 24) is a wild species that can hybridize with cultivated cucumber (C. sativus L., 2n = 2x = 14), a globally important vegetable crop. However, cucumber breeding is hindered by its narrow genetic base. Therefore, introgression from C. hystrix has been anticipated to bring a breakthrough in cucumber improvement. Here, we report the chromosome-scale assembly of C. hystrix genome (289 Mb). Scaffold N50 reached 14.1 Mb. Over 90% of the sequences were anchored onto 12 chromosomes. A total of 23,864 genes were annotated using a hybrid method. Further, we conducted a comprehensive comparative genomic analysis of cucumber, C. hystrix, and melon (C. melo L., 2n = 2x = 24). Whole-genome comparisons revealed that C. hystrix is phylogenetically closer to cucumber than to melon, providing a molecular basis for the success of its hybridization with cucumber. Moreover, expanded gene families of C. hystrix were significantly enriched in “defense response,” and C. hystrix harbored 104 nucleotide-binding site–encoding disease resistance gene analogs. Furthermore, 121 genes were positively selected, and 12 (9.9%) of these were involved in responses to biotic stimuli, which might explain the high disease resistance of C. hystrix. The alignment of whole C. hystrix genome with cucumber genome and self-alignment revealed 45,417 chromosome-specific sequences evenly distributed on C. hystrix chromosomes. Finally, we developed four cucumber–C. hystrix alien addition lines and identified the exact introgressed chromosome using molecular and cytological methods. The assembled C. hystrix genome can serve as a valuable resource for studies on Cucumis evolution and interspecific introgression breeding of cucumber.

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Ovule-derived haploid culture is an effective and important method for genetic study and plant breeding. Gerbera hybrida is a highly heterozygous species, and the lack of homozygous lines presents a challenge for molecular genetic research. Therefore, we performed haploid induction through unpollinated ovule culture and evaluated the effects of several important factors on this culturing procedure in G. hybrida, including genotype, low temperature, and the development seasons of the ovules. Among 45 G. hybrida cultivars analyzed, 29 cultivars exhibited adventitious bud induction via in vitro unpollinated ovule culture with significant different responses, indicating that the genotype of donor plants was a vital factor for inducibility. Four cultivars with significantly different induction rates, including one non-induced cultivar, were selected to analyze seasonal effects. Ovules extracted in the summer consistently had the highest induction rates, and even the non-induced cultivar included in the analysis could be induced at low levels when ovules from summer were used. Low temperature treatment could also promote adventitious bud induction, and in particular, a strong and significant effect was detected after 7 days of cold treatment. Ploidy level measurements by flow cytometry revealed that 288 ovule-derived regenerants were haploid (55.17%) and 218 lines were diploid (41.76%). Moreover, genetic stability analysis of the regenerants indicated 100% similarity to the marker profile of the mother plant. This is the first report of ovule-derived haploids in G. hybrida, which may facilitate the development of homozygous lines for molecular research and plant breeding. Gerbera hybrida is one of the most popular ornamental plants worldwide 1. Since most of the commecial cultivars were bred from the crossing of wild-type G. jamesonii and G. viridifolia, the genome of G. hybrida is highly heterozygous 2. Hundreds of G. hybrida varieties with extremely rich flower color patterns are available in the flower market, including white, yellow, red, pink, purple, and brown, and G. hybrida ranks fourth in cut flowers after rose, chrysanthemum, and tulip 3. As a highly heterozygous species, G. hybrida naturally harbors genetic diversity, which is beneficial for hybridization breeding. However, the lack of a homozygous genome is challenging for molecular genetic research, including forward genetic screening and mutation mapping. Ploidy breeding, the modification of the number of chromosome sets in a plant genome, is frequently used in ornamental plant breeding to induce novel variation and to create homogenous lines 4. A commonly used protocol for ploidy breeding is haploid induction, which refers to haploid regeneration via a single gamete cell under specific conditions, using either pollen (anther or microspore culture) or egg cells (ovule culture) 5. Essentially, the applicability of haploid induction is the main trigger for ornamental breeding, and some haploids of ornamentals have been commercially developed, including the ...

Although whole genome sequencing, genetic variation mapping, and pan-genome studies have been done on a large group of cucumber nuclear genomes, organelle genome information is largely unclear. As an important component of the organelle genome, the chloroplast genome is highly conserved, which makes it a useful tool for studying plant phylogeny, crop domestication, and species adaptation. Here, we have constructed the first cucumber chloroplast pan-genome based on 121 cucumber germplasms, and investigated the genetic variations of the cucumber chloroplast genome through comparative genomic, phylogenetic, haplotype, and population genetic structure analysis. Meanwhile, we explored the changes in expression of cucumber chloroplast genes under high- and low-temperature stimulation via transcriptome analysis. As a result, a total of 50 complete chloroplast genomes were successfully assembled from 121 cucumber resequencing data, ranging in size from 156,616–157,641 bp. The 50 cucumber chloroplast genomes have typical quadripartite structures, consisting of a large single copy (LSC, 86,339–86,883 bp), a small single copy (SSC, 18,069–18,363 bp), and two inverted repeats (IRs, 25,166–25,797 bp). Comparative genomic, haplotype, and population genetic structure results showed that there is more genetic variation in Indian ecotype cucumbers compared to other cucumber cultivars, which means that many genetic resources remain to be explored in Indian ecotype cucumbers. Phylogenetic analysis showed that the 50 cucumber germplasms could be classified into 3 types: East Asian, Eurasian + Indian, and Xishuangbanna + Indian. The transcriptomic analysis showed that matK were significantly up-regulated under high- and low-temperature stresses, further demonstrating that cucumber chloroplasts respond to temperature adversity by regulating lipid metabolism and ribosome metabolism. Further, accD has higher editing efficiency under high-temperature stress, which may contribute to the heat tolerance. These studies provide useful insight into genetic variation in the chloroplast genome, and established the foundation for exploring the mechanisms of temperature-stimulated chloroplast adaptation.