Comparative analysis of chloroplast genomes of endangered heterostylous species Primula wilsonii and its closely related species

New Insights into Phylogenetic Relationship of Hydrocotyle (Araliaceae) Based on Plastid Genomes

Hydrocotyle, belonging to the Hydrocotyloideae of Araliaceae, consists of 95 perennial and 35 annual species. Due to the lack of stable diagnostic morphological characteristics and high-resolution molecular markers, the phylogenetic relationships of Hydrocotyle need to be further investigated. In this study, we newly sequenced and assembled 13 whole plastid genomes of Hydrocotyle and performed comparative plastid genomic analyses with four previously published Hydrocotyle plastomes and phylogenomic analyses within Araliaceae. The plastid genomes of Hydrocotyle exhibited typical quadripartite structures with lengths from 152,659 bp to 153,669 bp, comprising a large single-copy (LSC) region (83,958-84,792 bp), a small single-copy (SSC) region (18,585-18,768 bp), and a pair of inverted repeats (IRs) (25,058-25,145 bp). Each plastome encoded 113 unique genes, containing 79 protein-coding genes, 30 tRNA genes, and four rRNA genes. Comparative analyses showed that the IR boundaries of Hydrocotyle plastomes were highly similar, and the coding and IR regions exhibited more conserved than non-coding and single-copy (SC) regions. A total of 2932 simple sequence repeats and 520 long sequence repeats were identified, with specificity in the number and distribution of repeat sequences. Six hypervariable regions were screened from the SC region, including four intergenic spacers (IGS) (ycf3-trnS, trnS-rps4, petA-psbJ, and ndhF-rpl32) and two coding genes (rpl16 and ycf1). Three protein-coding genes (atpE, rpl16, and ycf2) were subjected to positive selection only in a few species, implying that most protein-coding genes were relatively conserved during the plastid evolutionary process. Plastid phylogenomic analyses supported the treatment of Hydrocotyle from Apiaceae to Araliaceae, and topologies with a high resolution indicated that plastome data can be further used in the comprehensive phylogenetic research of Hydrocotyle. The diagnostic characteristics currently used in Hydrocotyle may not accurately reflect the phylogenetic relationships of this genus, and new taxonomic characteristics may need to be evaluated and selected in combination with more comprehensive molecular phylogenetic results.

Molecular Structure and Variation Characteristics of the Plastomes from Six Malus baccata (L.) Borkh. Individuals and Comparative Genomic Analysis with Other Malus Species

Malus baccata (L.) Borkh. is an important wild species of Malus. Its rich variation types and population history are not well understood. Chloroplast genome mining plays an active role in germplasm identification and genetic evolution. In this study, by assembly and annotation, six complete cp genome sequences, ranging in size from 160,083 to 160,295 bp, were obtained. The GC content of stable IR regions (42.7%) was significantly higher than that of full length (36.5%) and SC regions (LSC-34.2%, SSC-30.4%). Compared with other Malus species, it was found that there were more sites of polymorphisms and hotspots of variation in LSC and SSC regions, with high variation sites including trnR/UCU-atpA, trnT/UGU-trnL/UAA, ndhF-rpl32 and ccsA-ndhD. The intraspecific and interspecific collinearity was good, and no structural rearrangement was observed. A large number of repeating elements and different boundary expansions may be involved in shaping the cp genome size. Up to 77 or 78 coding genes were annotated in the cp genomes of M. baccata, and high frequency codons such as UUA (Leu), GCU (Ala) and AGA (Arg) were identified by relative synonymous codon usage analysis. Phylogeographic analysis showed that 12 individuals of M. baccata clustered into three different groups with complex structure, whereas variant xiaojinensis (M.H. Cheng & N.G. Jiang) was not closely related to M. baccata evolutionarily. The phylogenetic analysis suggested that two main clades of different M. baccata in the genus Malus were formed and that I and II diverged about 9.7 MYA. In conclusion, through cp genome assembly and comparison, the interspecific relationships and molecular variations of M. baccata were further elucidated, and the results of this study provide valuable information for the phylogenetic evolution and germplasm conservation of M. baccata and Malus.