Comparative analyses of eight complete plastid genomes of two hemiparasitic Cassytha vines in the family Lauraceae

Plastome evolution in Santalales involves relaxed selection prior to loss of ndh genes and major boundary shifts of the inverted repeat

BACKGROUND AND AIMS

Biological aspects of haustorial parasitism have significant effects on the configuration of the plastid genome. Approximately half the diversity of haustorial parasites belongs to the order Santalales, where a clearer picture of plastome evolution in relation to parasitism is starting to emerge. However, in previous studies of plastome evolution there is still a notable under-representation of members from non-parasitic and deep-branching hemiparasitic lineages, limiting evolutionary inference around the time of transition to a parasitic lifestyle. To expand taxon sampling relevant to this transition we therefore targeted three families of non-parasites (Erythropalaceae, Strombosiaceae and Coulaceae), two families of root-feeding hemiparasites (Ximeniaceae and Olacaceae) and two families of uncertain parasitic status (Aptandraceae and Octoknemaceae). With data from these lineages we aimed to explore plastome evolution in relation to the evolution of parasitism.

METHODS

From 29 new samples we sequenced and annotated plastomes and the nuclear ribosomal cistron. We examined phylogenetic patterns, plastome evolution, and patterns of relaxed or intensified selection in plastid genes. Available transcriptome data were analysed to investigate potential transfer of infA to the nuclear genome.

RESULTS

Phylogenetic relationships indicate a single functional loss of all plastid ndh genes (ndhA-K) in a clade formed by confirmed parasites and Aptandraceae, and the loss coincides with major size and boundary shifts of the inverted repeat (IR) region. Depending on an autotrophic or heterotrophic lifestyle in Aptandraceae, plastome changes are either correlated with or pre-date the evolution of parasitism. Phylogenetic patterns also indicate repeated loss of infA from the plastome, and based on the presence of transcribed sequences with presequences corresponding to thylakoid luminal transit peptides, we infer that the genes were transferred to the nuclear genome.

CONCLUSIONS

Except for the loss of the ndh complex, relatively few genes have been lost from the plastome in deep-branching root parasites in Santalales. Prior to loss of the ndh genes, they show signs of relaxed selection indicative of their dispensability. To firmly establish a potential correlation between ndh gene loss, plastome instability and evolution of parasitism, it is pertinent to refute or confirm a parasitic lifestyle in all Santalales clades.

Comparative mitogenomic analysis reveals variations and evolution of ectomycorrhizal fungal Strobilomyces

The genus Strobilomyces, representing a diverse and widespread group of ectomycorrhizal mushroom-forming fungi, plays a crucial ecological and economical role. However, until now, a comprehensive description of its mitochondrial genome (mitogenome) has been lacking. In our current study, we have successfully assembled and analysed the mitogenomes of five Strobilomyces species. These mitogenomes span a range from 35,618 base pairs (bp) to 42,088 bp, exhibiting a higher nucleotide abundance of AT compared to GC. All five mitogenomes harbour 14 conserved protein-coding genes (PCGs), two ribosomal RNAs (rRNAs) and 24 transfer RNAs (tRNAs). Notably, the overall ratio of Ka/Ks for all PCGs was found to be less than 1.0, indicating that these genes have undergone purifying selection during evolution. Intriguingly, the mitogenomic comparison revealed two instances of gene re-arrangement, which were directly linked to the geographical distribution of the Strobilomyces species. The concatenated mitochondrial PCGs (mtPCGs) and nuclear ribosomal DNA (nrDNA) phylogenies displayed a robust congruent topology at the family level. Specifically, the Strobilomyces species clustered together and formed sister relationship with other Boletaceae species in the mtPCGs tree. In contrast, the Strobilomyces species grouped at the base of the nrDNA tree when concerning Boletaceae. This study represents the first report on the mitogenomes of the Strobilomyces genus, providing valuable insights into fungal evolution within Boletales.

Variations and reduction of plastome are associated with the evolution of parasitism in Convolvulaceae

Parasitic lifestyle can often relax the constraint on the plastome, leading to gene pseudogenization and loss, and resulting in diverse genomic structures and rampant genome degradation. Although several plastomes of parasitic Cuscuta have  been reported, the evolution of parasitism in the family Convolvulaceae which is linked to structural variations and reduction of plastome has not been well investigated. In this study, we assembled and collected 40 plastid genomes belonging to 23 species representing four subgenera of Cuscuta and ten species of autotrophic Convolvulaceae. Our findings revealed nine types of structural variations and six types of inverted repeat (IR) boundary variations in the plastome of Convolvulaceae spp. These structural variations were associated with the shift of parasitic lifestyle, and IR boundary shift, as well as the abundance of long repeats. Overall, the degradation of Cuscuta plastome proceeded gradually, with one clade exhibiting an accelerated degradation rate. We observed five stages of gene loss in Cuscuta, including NAD(P)H complex → PEP complex → Photosynthesis-related → Ribosomal protein subunits → ATP synthase complex. Based on our results, we speculated that the shift of parasitic lifestyle in early divergent time promoted relaxed selection on plastomes, leading to the accumulation of microvariations, which ultimately resulted in the plastome reduction. This study provides new evidence towards a better understanding of plastomic evolution, variation, and reduction in the genus Cuscuta.