Comparative Cytological and Transcriptome Analyses of Anther Development in Nsa Cytoplasmic Male Sterile (1258A) and Maintainer Lines in Brassica napus Produced by Distant Hybridization

A Systematic Review and Developmental Perspective on Origin of CMS Genes in Crops

Cytoplasmic male sterility (CMS) arises from the incompatibility between the nucleus and cytoplasm as typical representatives of the chimeric structures in the mitochondrial genome (mitogenome), which has been extensively applied for hybrid seed production in various crops. The frequent occurrence of chimeric mitochondrial genes leading to CMS is consistent with the mitochondrial DNA (mtDNA) evolution. The sequence conservation resulting from faithfully maternal inheritance and the chimeric structure caused by frequent sequence recombination have been defined as two major features of the mitogenome. However, when and how these chimeric mitochondrial genes appear in the context of the highly conserved reproduction of mitochondria is an enigma. This review, therefore, presents the critical view of the research on CMS in plants to elucidate the mechanisms of this phenomenon. Generally, distant hybridization is the main mechanism to generate an original CMS source in natural populations and in breeding. Mitochondria and mitogenomes show pleomorphic and dynamic changes at key stages of the life cycle. The promitochondria in dry seeds develop into fully functioning mitochondria during seed imbibition, followed by massive mitochondria or mitogenome fusion and fission in the germination stage along with changes in the mtDNA structure and quantity. The mitogenome stability is controlled by nuclear loci, such as the nuclear gene Msh1. Its suppression leads to the rearrangement of mtDNA and the production of heritable CMS genes. An abundant recombination of mtDNA is also often found in distant hybrids and somatic/cybrid hybrids. Since mtDNA recombination is ubiquitous in distant hybridization, we put forward a hypothesis that the original CMS genes originated from mtDNA recombination during the germination of the hybrid seeds produced from distant hybridizations to solve the nucleo-cytoplasmic incompatibility resulting from the allogenic nuclear genome during seed germination.

Origin and early divergence of tandem duplicated sorbitol transporter genes in Rosaceae: insights from evolutionary analysis of the SOT gene family in angiosperms

Sorbitol is a critical photosynthate and storage substance in the Rosaceae family. Sorbitol transporters (SOTs) play a vital role in facilitating sorbitol allocation from source to sink organs and sugar accumulation in sink organs. While prior research has addressed gene duplications within the SOT gene family in Rosaceae, the precise origin and evolutionary dynamics of these duplications remain unclear, largely due to the complicated interplay of whole genome duplications and tandem duplications. Here, we investigated the synteny relationships among all identified Polyol/Monosaccharide Transporter (PLT) genes in 61 angiosperm genomes and SOT genes in representative genomes within the Rosaceae family. By integrating phylogenetic analyses, we elucidated the lineage-specific expansion and syntenic conservation of PLTs and SOTs across diverse plant lineages. We found that Rosaceae SOTs, as PLT family members, originated from a pair of tandemly duplicated PLT genes within Class III-A. Furthermore, our investigation highlights the role of lineage-specific and synergistic duplications in Amygdaloideae in contributing to the expansion of SOTs in Rosaceae plants. Collectively, our findings provide insights into the genomic origins, duplication events, and subsequent divergence of SOT gene family members. Such insights lay a crucial foundation for comprehensive functional characterizations in future studies.

An efficient approach to synthesize sterile allopolyploids through the combined reproduction mode of ameiotic oogenesis and sperm-egg fusion in the polyploid Carassius complex

The association between polyploidy and reproduction transition, which is an intriguing issue in evolutionary genetics, can also be exploited as an approach for genetic improvement in agriculture. Recently, we generated novel amphitriploids (NA3n) by integrating the genomes of the gynogenetic Carassius gibelio and sexual C. auratus, and found gynogenesis was recovered in most NA3n females (NA3n♀I). Here, we discovered a unique reproduction mode, termed ameio-fusiongenesis, which combines the abilities of both ameiotic oogenesis and sperm-egg fusion, in a few NA3n females (NA3n♀II). These females inherited ameiotic oogenesis to produce unreduced eggs from gynogenetic C. gibelio and sperm-egg fusion from sexual C. auratus. Subsequently, we utilized this unique reproduction mode to generate a group of synthetic alloheptaploids by crossing NA3n♀II with Megalobrama amblycephala. They contained all chromosomes of maternal NA3n♀II and a chromosomal set of paternal M. amblycephala. Intergenomic chromosome translocations between NA3n♀II and M. amblycephala were also observed in a few somatic cells. Primary oocytes of the alloheptaploid underwent severe apoptosis owing to incomplete double-strand break repair at prophase I. Although spermatocytes displayed similar chromosome behavior at prophase I, they underwent apoptosis due to chromosome separation failure at metaphase I. Therefore, the alloheptaploid females and males were all sterile. Finally, we established a sustainable clone for the large-scale production of NA3n♀II and developed an efficient approach to synthesize diverse allopolyploids containing genomes of different cyprinid species. These findings not only broaden our understanding of reproduction transition but also offer a practical strategy for polyploidy breeding and heterosis fixing.

High-temperature stress in crops: male sterility, yield loss and potential remedy approaches

Global food security is one of the utmost essential challenges in the 21st century in providing enough food for the growing population while coping with the already stressed environment. High temperature (HT) is one of the main factors affecting plant growth, development and reproduction and causes male sterility in plants. In male reproductive tissues, metabolic changes induced by HT involve carbohydrates, lipids, hormones, epigenetics and reactive oxygen species, leading to male sterility and ultimately reducing yield. Understanding the mechanism and genes involved in these pathways during the HT stress response will provide a new path to improve crops by using molecular breeding and biotechnological approaches. Moreover, this review provides insight into male sterility and integrates this with suggested strategies to enhance crop tolerance under HT stress conditions at the reproductive stage.

A Set of Molecular Markers to Accelerate Breeding and Determine Seed Purity of CMS Three-Line Hybrids in Brassica napus

Cytoplasmic male sterility (CMS) is the main mechanism employed to utilize the heterosis of Brassica napus. CMS three-line rapeseed hybrids have dramatically enhanced yield and brought about the global revolution of hybrid varieties, replacing conventional crop varieties. Over the last half century, China has led the development of hybrid Brassica napus varieties. Two sterile lines, polima (pol) and shaan 2A, were of particular importance for the establishment of three-line hybrid systems in rapeseed, which has opened up a new era of heterosis utilization. However, in current breeding practices, it takes up to three years to identify the restorer or maintainer relationship and the cytoplasmic type of any inbred material. This greatly affects the breeding speed of new varieties and inhibits the rapid development of the rapeseed industry. To address this problem, we developed a set of molecular markers for the identification of fertile cytoplasmic gene N and sterile cytoplasmic gene S, as well as for the fertile nucleus gene R and sterile nucleus gene r, based on differences in the gene sequences between the CMS line, maintainer line and restorer line of Brassica napus. Combining these markers can accurately identify the CMS line, maintainer and restorer of both the pol and shaan systems, as well as their hybrids. These markers can not only be used to identify of the maintainer and restorer relationship of inbred materials; they can also be used as general molecular markers to identify the CMS-type hybrid purity of pol and shaan systems.

Comparative study on abortion characteristics of Nsa CMS and Pol CMS and analysis of long non-coding RNAs related to pollen abortion in Brassica napus

Cytoplasmic male sterile system (CMS) is one of the important methods for the utilization of heterosisin Brassica napus. The involvement of long non-coding RNAs (lncRNAs) in anther and pollen development in B.napus has been recognized, but there is little data on the involvement of lncRNAs in pollen abortion in different types of rapeseed CMS. The present study compared the cytological, physiological and biochemical characteristics of Nsa CMS (1258A) and Pol CMS (P5A) during pollen abortion, and high-throughput sequencing of flower buds of different sizes before and after pollen abortion. The results showed that insufficient energy supply was an important physiological basis for 1258A and P5A pollen abortion, and 1258A had excessive ROS (reactive oxygen species) accumulation in the stage of pollen abortion. Functional analysis showed that Starch and sucrose metabolism and Sulfur metabolism were significantly enriched before and after pollen abortion in 1258A and P5A, and a large number of genes were down-regulated. In 1258A, 227 lncRNAs had cis-targeting regulation, and 240 cis-target genes of the lncRNAs were identified. In P5A, 116 lncRNAs had cis-targeting regulation, and 101 cis-target genes of the lncRNAs were identified. There were five lncRNAs cis-target genes in 1258A and P5A during pollen abortion, and LOC106445716 encodes β-D-glucopyranosyl abscisate β-glucosidase and could regulate pollen abortion. Taken together, this study, provides a new perspective for lncRNAs to participate in the regulation of Nsa CMS and Pol CMS pollen abortion.

Comparative transcriptome analysis provides insight into the important pathways and key genes related to the pollen abortion in the thermo-sensitive genic male sterile line 373S in Brassica napus L

The thermo-sensitive genic male sterility (TGMS) system plays a key role in the production of two-line hybrids in rapeseed (Brassica napus). To uncover key cellular events and genetic regulation associated with TGMS, a combined study using cytological methods and RNA-sequencing analysis was conducted for the rapeseed TGMS line 373S. Cytological studies showed that microspore cytoplasm of 373S plants was condensed, the microspore nucleus was degraded at an early stage, the exine was irregular, and the tapetum developed abnormally, eventually leading to male sterility. RNA-sequencing analysis identified 430 differentially expressed genes (298 upregulated and 132 downregulated) between the fertile and sterile samples. Gene ontology analysis demonstrated that the most highly represented biological processes included sporopollenin biosynthetic process, pollen exine formation, and extracellular matrix assembly. Kyoto encyclopedia of genes and genomes analysis indicated that the enriched pathways included amino acid metabolism, carbohydrate metabolism, and lipid metabolism. Moreover, 26 transcript factors were identified, which may be associated with abnormal tapetum degeneration and exine formation. Subsequently, 19 key genes were selected, which are considered to regulate pollen development and even participate in pollen exine formation. Our results will provide important insight into the molecular mechanisms underlying TGMS in rapeseed.