The Mitochondrial Genome Assembly of Fennel (Foeniculum vulgare) Reveals Two Different atp6 Gene Sequences in Cytoplasmic Male Sterile Accessions

Abstract

<jats:p>Cytoplasmic male sterility (CMS) has always aroused interest among researchers and breeders, being a valuable resource widely exploited not only to breed F1 hybrid varieties but also to investigate genes that control stamen and pollen development. With the aim of identifying candidate genes for CMS in fennel, we adopted an effective strategy relying on the comparison between mitochondrial genomes (mtDNA) of both fertile and sterile genotypes. mtDNA raw reads derived from a CMS genotype were assembled in a single molecule (296,483 bp), while a draft mtDNA assembly (166,124 nucleotides, 94 contigs) was performed using male fertile sample (MF) sequences. From their annotation and alignment, two atp6-like sequences were identified. atp6−, the putative mutant copy with a 300 bp truncation at the 5’-end, was found only in the mtDNA of CMS samples, while the wild type copy (atp6+) was detected only in the MF mtDNA. Further analyses (i.e., reads mapping and Sanger sequencing), revealed an atp6+ copy also in CMS samples, probably in the nuclear DNA. However, qPCRs performed on different tissues proved that, despite its availability, atp6+ is expressed only in MF samples, while apt6− mRNA was always detected in CMS individuals. In the light of these findings, the energy deficiency model could explain the pollen deficiency observed in male sterile flower. atp6− could represent a gene whose mRNA is translated into a not-fully functional protein leading to suboptimal ATP production that guarantees essential cellular processes but not a high energy demand process such as pollen development. Our study provides novel insights into the fennel mtDNA genome and its atp6 genes, and paves the way for further studies aimed at understanding their functional roles in the determination of male sterility.</jats:p>

Molecular Characterization and Genetic Structure Evaluation of Breeding Populations of Fennel (Foeniculum vulgare Mill.)

“…The newly obtained line (strain CMS) will then be used as a mother plant (or seed plant) and crossed with strain P for F1 hybrid production, while strain M will be used to maintain strain CMS. By crossing highly dissimilar strains P and CMS, the resulting offspring are expected to exhibit a high level of heterozygosity, maximizing heterotic vigor [7,9,10]. Following F1 hybrid development, the registration process of the new plant variety is subject to compliance with rigorous and specific requirements concerning distinctness (D), uniformity (U) and stability (S).…”

Impact of Genomic and Transcriptomic Resources on Apiaceae Crop Breeding Strategies

“…On the contrary, MS seems to be more common within the Apiaceae family. Cytoplasmic male sterility (CMS), a maternally inherited trait that is often associated with single-gene mutations in the mitochondrial genome [ 39 ], has been reported in Daucus carota (for a review see [ 40 ]) but also in Foeniculum vulgare [ 41 ], Apium graveolens [ 42 ] and Pastinaca sativa [ 43 ]. In Apium graveolens , the discovery of CMS genotypes has been reported since the 1980s, and few F1 hybrids available on the market for celery and celeriac ( A. graveolens var.…”

“…Due to the economic interest behind this trait, the mechanisms and origin of male sterility are often contained by the breeding companies, and reports on CMS sources are elusive. Only in few cases are the genetics underlying the lack of pollen elucidated [ 40 , 41 ]. It is well established that one of the most effective strategies to identify the genes responsible for CMS is the comparison between mitochondrial genomes (mtDNA) of fertile and sterile genotypes to identify differences in terms of sequences and/or structure organization.…”

“…NMS has been described in Apium graveolens [ 42 ] and Daucus carota [ 53 ], but to date, the use of this type of male sterility for commercial purposes in Apiaceae is scarcely documented. In fact, when both CMS and NMS male sterility systems are available, such as in the case of carrot and celery, the first is far more convenient because of its inheritance, mode of preservation and restoration of fertility [ 41 , 53 ].…”

Induction of Male Sterility: A Boon for Plant Breeding

“…RAFTIN gene in rice and wheat (Wang et al, 2003), OsDEX1, a Ca 2+ binding protein in rice (Yu et al, 2016) also been reported to associate with male sterility. Palumbo et al, (2020) reported role of two atp6 gene sequences for CMS in Fennel (Foeniculum vulgare) by mitochondrial genome assembly of male sterile and fertile accessions. Similarly, the mechanism of abortion in CMS was studied and reported by transcriptome analysis in CMS tobacco and fertile lines .…”