Complete mitochondrial genomes of three Mangifera species, their genomic structure and gene transfer from chloroplast genomes

Background

Among the Mangifera species, mango (Mangifera indica) is an important commercial fruit crop. However, very few studies have been conducted on the Mangifera mitochondrial genome. This study reports and compares the newly sequenced mitochondrial genomes of three Mangifera species.

Results

Mangifera mitochondrial genomes showed partial similarities in the overall size, genomic structure, and gene content. Specifically, the genomes are circular and contain about 63–69 predicted functional genes, including five ribosomal RNA (rRNA) genes and 24–27 transfer RNA (tRNA) genes. The GC contents of the Mangifera mitochondrial genomes are similar, ranging from 44.42–44.66%. Leucine (Leu) and serine (Ser) are the most frequently used, while tryptophan (Trp) and cysteine (Cys) are the least used amino acids among the protein-coding genes in Mangifera mitochondrial genomes. We also identified 7–10 large chloroplast genomic fragments in the mitochondrial genome, ranging from 1407 to 6142 bp. Additionally, four intact mitochondrial tRNAs genes (tRNA-Cys, tRNA-Trp, tRNA-Pro, and tRNA-Met) and intergenic spacer regions were identified. Phylogenetic analysis based on the common protein-coding genes of most branches provided a high support value.

Conclusions

We sequenced and compared the mitochondrial genomes of three Mangifera species. The results showed that the gene content and the codon usage pattern of Mangifera mitochondrial genomes is similar across various species. Gene transfer from the chloroplast genome to the mitochondrial genome were identified. This study provides valuable information for evolutionary and molecular studies of Mangifera and a basis for further studies on genomic breeding of mango.

Large-scale phylogenomics reveals ancient introgression in Asian Hepatica and new insights into the origin of the insular endemic Hepatica maxima

Hepatica maxima is native to Ulleungdo, which is one of the oceanic islands in Korea, and it likely originated via anagenetic speciation from the Korean mainland species H. asiatica. However, the relationships among the Asian lineages remain unresolved. Phylogenomics based on plant genomes can provide new insights into the evolutionary history of plants. We first generated plastid, mitochondrial and transcriptome sequences of the insular endemic species H. maxima. Using the genomic data for H. maxima, we obtained a phylogenomic dataset consisting of 76 plastid, 37 mitochondrial and 413 nuclear genes from Asian Hepatica and two outgroups. Coalescent- and concatenation-based methods revealed cytonuclear and organellar discordance in the lineage. The presence of gynodioecy with cytoplasmic male sterility in Asian Hepatica suggests that the discordance is correlated with potential disruption of linkage disequilibrium between the organellar genomes. Species network analyses revealed a deep history of hybridization and introgression in Asian Hepatica. We discovered that ancient and recent introgression events occurred throughout the evolutionary history of the insular endemic species H. maxima. The introgression may serve as an important source of genetic variation to facilitate adaptation to the Ulleungdo environment.

Heteroplasmy and Patterns of Cytonuclear Linkage Disequilibrium in Wild Carrot

Organellar genomes are considered to be strictly uniparentally-inherited. Uniparental inheritance allows for cytonuclear coevolution and the development of highly coordinated cytonuclear interactions. Yet, instances of biparental inheritance have been documented across eukaryotes. Biparental inheritance in otherwise uniparentally-inherited organelles is termed leakage (maternal or paternal) and allows for the presence of multiple variants of the same organellar genome within an individual, called heteroplasmy. It is unclear what, if any, evolutionary consequences are placed on nuclear and/or organellar genomes due to heteroplasmy. One way of accessing cytonuclear interactions and potential coevolution is through calculating cytonuclear linkage disequilibrium (cnLD), or the non-random association of alleles between nuclear and organellar genomes. Patterns of cnLD can indicate positive or negative cytonuclear selection, coevolution between the nuclear and organellar genomes, non-traditional organellar inheritance, or instances of ancestral heteroplasmy. In plants, cytonuclear interactions have been shown to play a role in cytoplasmic male sterility which occurs in gynodioecious species and is associated with leakage. We used the gynodioecious species, Daucus carota L. spp. carota, or wild carrot, to investigate cnLD. We genotyped a total of 265 individuals from two regions of the USA at 15 nuclear microsatellites, the mitochondrial genes cox1 and atp9, and an intergenic region between trnS and trnG (StoG) in the plastid genome to calculate nuclear–nuclear LD (nucLD), cnLD, and organellar LD (i.e., within the mtDNA and between mtDNA and ptDNA) within the two regions. We were further able to identify cox1 and StoG heteroplasmy and calculate some of the same LD measures within heteroplasmic and homoplasmic (non-heteroplasmic) datasets. We used a Z-transformation test to demonstrate that heteroplasmic individuals display significantly higher levels of cnLD within both regions. In spite of this, within and between organellar LD is low to moderate. Given these patterns of LD in two regions of the USA in which gene flow has been shown to occur between crop and wild carrot, we suggest that heteroplasmy is an evolutionary mechanism which permits the maintenance of cnLD while also acting to disrupt organellar LD.

Comparison of nuclear, plastid, and mitochondrial phylogenies and the origin of wild octoploid strawberry species

PREMISE OF THE STUDY

Molecular phylogenies derived from all three plant genomes can provide insight into the evolutionary history of plant groups influenced by reticulation. We sought to reconstruct mitochondrial exome, chloroplast, and nuclear genome phylogenies for octoploid Fragaria and their diploid ancestors and to document patterns of incongruence between and within the cytoplasmic genomes and interpret these in the context of evolutionary origin of the octoploid strawberries.

METHODS

Using a genome-skimming approach, we assembled chloroplast genomes and mitochondrial exomes, and we used the POLiMAPS method to assemble nuclear sequence for octoploid species and constructed phylogenies from all three genomes. We assessed incongruence between and within cytoplasmic genomes using topology-based phylogenetic incongruence tests.

KEY RESULTS

The incongruent cytoplasmic genome phylogeny with respect to the placement of octoploids suggests potential breakage in linkage disequilibrium of cytoplasmic genomes during allopolyploid origin of the octoploids. Furthermore, a single mitochondrial chimeric gene with a putative role in cytoplasmic male sterility yields a phylogeny that is inconsistent with the rest of the mitochondrial genome but consistent with the chloroplast phylogeny, suggesting intracellular gene transfer between heteroplasmic mitochondria, possibly driven by selection to overcome the effects of mito-nuclear incompatibility in octoploid origins.

CONCLUSIONS

This work expands on the current understanding of evolutionary history of the octoploid ancestors of cultivated strawberry. It demonstrates phylogenetic incongruence between cytoplasmic genomes in octoploids with respect to diploid ancestors, indicating breakage in linkage disequilibrium of cytoplasmic genomes. We discuss potential organism-level processes that may have contributed to the observed incongruence in Fragaria.