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Pinus L. is the largest genus of conifers and provides a classical model for studying species divergence and phylogenetic evolution by gymnosperms. However, our poor understanding of sequence divergence in the whole plastid genomes of Pinus species severely hinders studies of their evolution and phylogeny. Thus, we analyzed the sequences of 97 Pinus plastid genomes, including four newly sequenced genomes and 93 previously published plastomes, to explore the evolution and phylogenetic relationships in the genus Pinus. The complete chloroplast genomes of Pinus species ranged in size from 109 640 bp (P. cembra L.) to 121 976 bp (P. glabra Walter), and these genomes comprised circular DNA molecules in a similar manner to those of most gymnosperms. We identified 9108 repeats where most of the repeats comprised the dispersed type with 3983 (44%), followed by tandem repeats with 2999 (33%), and then palindromic repeats with 2126 (23%). Sixteen divergence hotspot regions were identified in Pinus plastid genomes, which could be useful molecular markers for future population genetics studies. Phylogenetic analysis showed that Pinus species could be divided into two diverged clades comprising the subgenera Strobus (single needle section) and Pinus (double needles section). Molecular dating suggested that the genus Pinus originated approximately 130.38 Mya during the late Cretaceous. The two subgenera subsequently split 85.86 Mya, which was largely consistent with the other molecular results based on partial DNA markers. These findings provide important insights into the sequence variations and phylogenetic evolution of Pinus plastid genomes. [ABSTRACT FROM AUTHOR]

Cotton is one of the most important fiber and oil crops in the world. Chloroplast genomes harbor their own genetic materials and are considered to be highly conserved. Transfer RNAs (tRNAs) act as "bridges" in protein synthesis by carrying amino acids. Currently, the variation and evolutionary characteristics of tRNAs in the cotton chloroplast genome are poorly understood. Here, we analyzed the structural variation and evolution of chloroplast tRNA (cp tRNA) based on eight diploid and two allotetraploid cotton species. We also investigated the nucleotide evolution of chloroplast genomes in cotton species. We found that cp tRNAs in cotton encoded 36 or 37 tRNAs, and 28 or 29 anti-codon types with lengths ranging from 60 to 93 nucleotides. Cotton chloroplast tRNA sequences possessed specific conservation and, in particular, the Ψ-loop contained the conserved U-U-C-X3-U. The cp tRNAs of Gossypium L. contained introns, and cp tRNAIle contained the anti-codon (C-A-U), which was generally the anti-codon of tRNAMet. The transition and transversion analyses showed that cp tRNAs in cotton species were iso-acceptor specific and had undergone unequal rates of evolution. The intergenic region was more variable than coding regions, and non-synonymous mutations have been fixed in cotton cp genomes. On the other hand, phylogeny analyses indicated that cp tRNAs of cotton were derived from several inferred ancestors with greater gene duplications. This study provides new insights into the structural variation and evolution of chloroplast tRNAs in cotton plants. Our findings could contribute to understanding the detailed characteristics and evolutionary variation of the tRNA family. [ABSTRACT FROM AUTHOR]