Evaluation of Crossability between Nicotiana benthamiana and Nicotiana excelsior

Wild tobacco species in the Nicotiana section Suaveolentes are promising genetic resources to introduce their disease resistance to cultivated tobacco, Nicotiana tabacum. However, hybrid lethality is observed in hybrid seedlings from crosses between most Suaveolentes species and N. tabacum. In particular, N. benthamiana belonging to the section Suaveolentes produces only viable hybrids after crossing with N. tabacum. In the present study, crossability between N. benthamiana and N. excelsior (section Suaveolentes) was investigated to test the possible usefulness of N. benthamiana as the bridge parent to transfer desirable genes of N. excelsior to N. tabacum via bridge crossing. After reciprocal crosses using three accessions of N. benthamiana and N. excelsior each, several crossing barriers such as cross-incompatibility, seed abortion, and male and female hybrid sterility were observed. Although reciprocal hybrids between N. benthamiana and N. excelsior showed a high degree of chromosome pairing in meiosis, univalents and multivalents, as well as chromosome bridges and lagging chromosomes, were observed. These meiotic abnormalities were thought to cause hybrid sterility. The possible usefulness of reciprocal hybrids between N. benthamiana and N. excelsior is discussed.

Genetic Recombination of the Mantle Color Pattern of Two Boring Giant Clam (Tridacna crocea) Strains

According to the RGB law display, the polymorphism of the giant clam mantle color pattern is through four iridocytes. The boring giant clam (Tridacna crocea) exhibits diverse mantle colors, including blue, green, purple, gold, and orange. In order to evaluate the genetic laws driving these mantle color patterns, a complete diallel cross between two color strains [blue strain (only blue iridocyte) and the yellow-green strain (yellow and green iridocytes)] was performed. Using a single-to-single mating system, two intra-strain crosses (BB and YY) and two reciprocal inter-strain crosses (BY and YB) were produced in triplicates. Higher fertilization rate and hatching rate were observed in all experimental groups, suggesting that there was no sperm–egg recognition barrier between the two strains. In the grow-out stage, the size of the reciprocal hybrids was larger than that of the two pure strains with a degree of heterosis. In addition, compared with the two pure strains, the hybrids have higher larval metamorphosis rate and higher survival rate. At 1 year of age, the mantle color pattern of pure strains showed 100% stable inheritance, while the reciprocal hybrids exhibited colorful patterns (a combination of blue, yellow, and green), suggesting that there was a genetic recombination of the mantle colors during the stable expression period. These results provide a theoretical basis for the formation of the mantle color of giant clam and its genetic segregation law, as well as provide guidance for genetic breeding of giant clams.

Transcriptome-Wide Analysis of RNA m6A Methylation and Gene Expression Changes Among Two Arabidopsis Ecotypes and Their Reciprocal Hybrids

The remodeling of transcriptome, epigenome, proteome, and metabolome in hybrids plays an important role in heterosis. N(6)-methyladenosine (m6A) methylation is the most abundant type of post-transcriptional modification for mRNAs, but the pattern of inheritance from parents to hybrids and potential impact on heterosis are largely unknown. We constructed transcriptome-wide mRNA m6A methylation maps of Arabidopsis thaliana Col-0 and Landsberg erecta (Ler) and their reciprocal F1 hybrids. Generally, the transcriptome-wide pattern of m6A methylation tends to be conserved between accessions. Approximately 74% of m6A methylation peaks are consistent between the parents and hybrids, indicating that a majority of the m6A methylation is maintained after hybridization. We found a significant association between differential expression and differential m6A modification, and between non-additive expression and non-additive methylation on the same gene. The overall RNA m6A level between Col-0 and Ler is clearly different but tended to disappear at the allelic sites in the hybrids. Interestingly, many enriched biological functions of genes with differential m6A modification between parents and hybrids are also conserved, including many heterosis-related genes involved in biosynthetic processes of starch. Collectively, our study revealed the overall pattern of inheritance of mRNA m6A modifications from parents to hybrids and a potential new layer of regulatory mechanisms related to heterosis formation.

Polymorphism in the Chloroplast ATP Synthase Beta-Subunit Is Associated with a Maternally Inherited Enhanced Cold Recovery in Cucumber

Cucumber (Cucumis sativus L.) is a warm-season crop that is sensitive to chilling temperatures and a maternally inherited cold tolerance exists in the heirloom cultivar ‘Chipper’ (CH). Because the organelles of cucumber show differential transmission (maternal for chloroplast and paternal for mitochondrion), this cold tolerance is hypothesized to be chloroplast-associated. The goal of this research was to characterize the cold tolerant phenotype from CH and determine its genetic basis. Doubled haploid (DH) lines were produced from CH and cold susceptible cucumbers, reciprocal hybrids with identical nuclear genotypes were produced, and plants were subjected to cold treatments under lights at 4 °C for 5.5 h. Hybrid plants with CH as the maternal parent had significantly higher fresh and dry weights 14 days after cold treatment compared to the reciprocal hybrid, revealing an enhanced cold recovery phenotype maternally conferred by CH. Results from analyses of the nuclear transcriptome and reactive oxygen species (ROS) between reciprocal hybrids were consistent with the cold recovery phenotype. Sequencing of the chloroplast genome and transcriptome of the DH parents and reciprocal hybrids, respectively, revealed one maternally transmitted non-synonymous single nucleotide polymorphism (SNP) in the chloroplast F1FO-ATP synthase (CF1FO-ATPase) beta-subunit gene (atpB) of CH which confers an amino acid change from threonine to arginine. Protein modeling revealed that this change is located at the interface of the alpha- and beta-subunits in the CF1FO-ATPase complex. Polymorphisms in the CF1FO-ATPase complex have been associated with stress tolerances in other plants, and selection for or creation of polymorphic beta-subunit proteins by chloroplast transformation or gene editing could condition improved recovery from cold stress in plants.