Structural variation and evolution of chloroplast tRNAs in green algae

As one of the important groups of the core Chlorophyta (Green algae), Chlorophyceae plays an important role in the evolution of plants. As a carrier of amino acids, tRNA plays an indispensable role in life activities. However, the structural variation of chloroplast tRNA and its evolutionary characteristics in Chlorophyta species have not been well studied. In this study, we analyzed the chloroplast genome tRNAs of 14 species in five categories in the green algae. We found that the number of chloroplasts tRNAs of Chlorophyceae is maintained between 28–32, and the length of the gene sequence ranges from 71 nt to 91 nt. There are 23–27 anticodon types of tRNAs, and some tRNAs have missing anticodons that are compensated for by other types of anticodons of that tRNA. In addition, three tRNAs were found to contain introns in the anti-codon loop of the tRNA, but the analysis scored poorly and it is presumed that these introns are not functional. After multiple sequence alignment, the Ψ-loop is the most conserved structural unit in the tRNA secondary structure, containing mostly U-U-C-x-A-x-U conserved sequences. The number of transitions in tRNA is higher than the number of transversions. In the replication loss analysis, it was found that green algal chloroplast tRNAs may have undergone substantial gene loss during the course of evolution. Based on the constructed phylogenetic tree, mutations were found to accompany the evolution of the Green algae chloroplast tRNA. Moreover, chloroplast tRNAs of Chlorophyceae are consistent with those of monocotyledons and gymnosperms in terms of evolutionary patterns, sharing a common multi-phylogenetic pattern and rooted in a rich common ancestor. Sequence alignment and systematic analysis of tRNA in chloroplast genome of Chlorophyceae, clarified the characteristics and rules of tRNA changes, which will promote the evolutionary relationship of tRNA and the origin and evolution of chloroplast.


INTRODUCTION
As autotrophic lineages with primary plastids, Viridiplantae, Rhodophyta and Cyanobacteria all originated from an endosymbiotic event, which also marked the origin of oxidative photosynthesis in eukaryotes (Keeling, 2010;Leliaert, Verbruggen & Zechman, 2011). Chlorophyta and Streptophyta are two major lineages that split early in the evolution of green plant (O'Kelly, 2007). The two taxa have subsequently produced the widespread and diverse groups. Among them, green algae in different environments have always maintained relatively independent evolution, not only shows obvious differences in biological characteristics, but also has great differences in genome genetic characteristics (Barsanti & Gualtieri, 2006). It is generally accepted that the Streptophyta branched out into the Land plants and Charophyta. The Chlorophyta gradually formed the extremely widespread core Chlorophyta and other green algae (Leliaert et al., 2012).
The core Chlorophyta including Ulvophyceae, Trebouxiophyceae, Chlorophyceae (UTC) and two small groups namely Chlorodendrophyceae and Pedinophyceae (Fučíková et al., 2014). In the core Chlorophyta, the evolutionary relationships between the various branches and between the UTC branches have been increasingly studied (Lemieux et al., 2015;Leliaert et al., 2016). Chlorodendrophyceae, as a small group of the core Chlorophyceae, is traditionally attributed to prasinophyceae and is considered to be an earlier branch of the core Chlorophyceae. Based on 18s rDNA data, a close relationship between Chlorodendrophyceae and UTC is confirmed (Cocquyt et al., 2010). As the main branch of the core Chlorophyta, UTC has abundant species, diverse living environments and the complicated evolutionary relationships. The monophyletic origin of the Chlorophyceae has been confirmed by analysis of a large number of genome sequences (Luo et al., 2010).
Because of the maternal genetic characteristics of chloroplasts, the chloroplast genome is not only structurally stable, but also relatively conservative in number, composition and arrangement (Palmer & Genetics, 1985;Raubeson & Jansen, 2005;Fučíková, Lewis & Lewis, 2016). Thus, studying the evolutionary relationships of species taxa from the perspective of the chloroplast genome can compensate for the deficiencies caused by the analysis of data such as 18S as well as 26S rDNA.
The chloroplast genome structure is usually circular, rarely linear, and consists of four parts, the large single copy region (LSC), the small single copy region (SSC) and the inverted repeat regions A and B (IRA, IRB) (Zhang et al., 2012). In most of angiosperm chloroplast genomes, there are generally about 120 coding genes, of which about 80 genes encode proteins involved in photosynthesis and gene expression, another 30 or so genes encode tRNAs (Greiner, Lehwark & Bock, 2019) and a few genes encode rRNAs (Masanori et al., 2003;Gao, Ying-Juan & Wang, 2010) (Fig. 1).
tRNA is essential for linking and matching mRNA and amino acids during protein synthesis and is functionally stable. The number of nucleotides of tRNA is also relatively conservative, usually 75-95 nt nucleotides (Goodenbour & Pan, 2006). Further, the cloverlike secondary structure formed by nucleotide pairing and the inverted L-like tertiary structure formed by further folding have been relatively stable throughout evolution (Holley et al., 1965). So, it has been highly conserved in genetic evolution. Finally, based on the distribution of tRNAs in the eukaryotic chloroplast genome, little is known about the structure and its evolutionary mechanisms. We selected chloroplast tRNAs from green algae to investigate the structural variation and evolutionary features of chloroplast tRNAs.
In our study, we analyzed 14 species from five orders of the core Chlorophyceae to infer the possible evolutionary relationships of tRNAs in the chloroplast genome of Chlorophyceae. From the analysis results, we have not only obtained the sequence characteristics and structural features of tRNAs in the chloroplast genome of Chlorophyceae, but also discovered that chloroplast tRNAs of Chlorophyceae are consistent with those of monocotyledons and gymnosperms in terms of evolutionary patterns, sharing a common multi-phylogenetic pattern and rooted in a multiple common ancestor.

Sequence analysis of chloroplast tRNA genome
We selected 14 representative species from the NCBI genome database in five classes of the core green algae order.  . The specific parameters set by the tRNAscan-SE software are: sequence source: mixed (general tRNA model); Search mode: default; Query sequence: formatted (FASTA); Genetic Code for tRNA Isotype Prediction: universal. From the output of tRNAscan-SE, we can get the overall statistical information as well as the summary data of the whole search . Through the analysis of the data, the total number of tRNAs in the chloroplast genome of each species, the distribution of anticodons of the chloroplast tRNA, the number of tRNAs with introns, and the gene sequence length of each subtype tRNA were recorded.

Multiple sequence alignment
We grouped the tRNA nucleotide sequences of 20 isoforms sequentially and performed tRNA isoform multiple sequence alignment using Multalin program to analyze the conserved tRNA isoform nucleotide sequences. Multalin's parameter settings: Sequence input format: Auto, Result page format: a coloured image, Symbol comparison

Phylogenetic tree construction
We constructed a phylogenetic tree based on the genomic sequences of tRNAs using MEGA X. Clustal format files of all tRNAs were then created by Clustal X 2.0 software. We used MEGA X software to convert the generated tRNA Clustal files to MEGA format. For the model selection for constructing a phylogenetic tree, the specific parameters are as follows:

tRNA transition /transversion analysis
We used the tRNA files in MEGA format in the MEGA software which further analyzed the transition /transversion rate of all tRNAs (Tamura, Nei & Kumar, 2004;Sudhir et al., 2018). The specific analysis process and parameters are as follows

Gene loss and duplication analysis
We analyzed chloroplast genomic tRNA gene duplication/loss events in chlorophyceae by Notung2.9 software (Chen, Durand & Farach-Colton, 2000;Vernot et al., 2008 cardiacum in the oedogoniales and F. terrestris in the chaetopeltidales. After constructing the species tree, import it into Notung2.9 software, and reconcile and analyze the 20 subtype tRNA gene trees and species tree one by one to obtain gene duplication and deletion nodes.

Analysis of the number and sequence length of tRNA in chloroplast genome
For a more comprehensive analysis of the evolution of chloroplast tRNA genomes in Chlorophyceae (the core Chlorophyceae), fourteen species from the NCBI Genome Database were randomly selected from the main five orders of Chlorophyceae. Genome Database of 14 species. Subsequently, the tRNAscan-SE software was used to perform detailed sequence analysis of the chloroplast genomes of 14 species. Through preliminary analysis, the results show that the chloroplast genomes of all species contain 28-32 tRNAs.  (Table 1). For the predicted 406 chloroplast tRNA gene sequences (except for 3 sequences containing introns), the length of the chloroplast tRNA gene sequence of all species ranges from 71 nt (NC_005353.1). tRNA 29-CysGCA) to 91 nt, and maintains an average of 75 nt, of which 73 nt and 74 nt length contain 32% and 28% respectively. Not only that, among the 20 subtypes of tRNAs, tRNA Glu , tRNA Lys and tRNA Val contain 73 nt, 72 nt and 73 nt nucleotides, respectively. The nucleotides contained in tRNA Asn , tRNA Asp , tRNA Cys , tRNA Gln , tRNA Gly , tRNA His , tRNA Phe , tRNA Pro , tRNA Thr and tRNA Trp were all below the mean value (75 nt), while the nucleotides contained in tRNA Leu , tRNA Ser and tRNA Tyr are all above 80 nt nucleotides ( Fig. 2) (Data S1).

Multiple sequence alignment analysis of chloroplast tRNA
After categorizing the 20 subtypes of tRNA, the corresponding nucleotide sequence of each was analyzed by sequence alignment ( Table 2). The results show that the -loop contains a conserved sequence of U-U-C-x-A in the secondary structure of the 20 tRNA isoforms. And the first position of most tRNA nucleotide sequence has G nucleotide, while tRNA Asn and tRNA Gln have U in the first position, tRNA Trp and tRNA Val have A in the first position, and tRNA Pro has C in the first position. Subsequently, no identical nucleotide sequence was found at positions 2 to 7 of the acceptor arm. In the D-arm, 7 subtypes of tRNA were found to have a conserved sequence of GCNN (N represents any nucleotide), and there is a conserved sequence of AGU-(-represents any nucleotide or no nucleotide) in the D-loop. This is consistent with the reported results of the conserved consensus sequence 7 GUGGCNNAGU 16 -starting with the 7th nucleotide of the acceptor arm in a typical tRNA (Laslett & Canback, 2004). In addition, it was found that tRNA Ala , tRNA Arg , tRNA Asn , tRNA Gly , tRNA Lle and tRNA phe have the same nucleotide sequence as G-C-U-C in the D-arm.
No conserved sequences were found in the anticodon arms and variable loops, but in the anticodon loops, a conserved U nucleotide was found at the second nucleotide. In addition to tRNA Cys , tRNA Glu , tRNA Gly and tRNA phe , it was found that there are conserved nucleotide sequences of GG at positions 52 and 53 (that is, positions 4 and 5 of the -arm); for positions 54, 55 and 56, the 58th and 60th nucleotides (that is, the 1, 2, 3, 5, and 7 positions of the -arm), except for tRNA His and tRNA phe , all have the conserved nucleotide sequence of U-U-C-x-A-x-U. Among them, the nucleotide sequence of tRNA His at positions 54-60 is U-U-C-G-A-A-C; the nucleotide sequence of tRNA phe at positions 54-60 is U-U-C-A-A-U-C.

Analysis of the number of each structure of chloroplast tRNA
After analyzing and sorting out the chloroplast tRNAs of 14 species, a total of 406 tRNA sequences (without 3 containing a group of introns) tRNA sequences were obtained. After statistical analysis of the number of nucleotides in each secondary structure of 406 tRNAs, we found that in the acceptor arm, the number of nucleotides is 6-7, of which 6-nucleotide tRNA accounts for 0.32%, 7-nucleotide tRNA accounted for 96.8%, which is consistent with the result that the number of acceptor arm nucleotides in the study is 7; In the D-arm, the number of nucleotides is 2-4, of which 3 nucleotides tRNA accounted for 28.3%, 4 nucleotides tRNA accounted for 71.4%, only NC_005353.1.tRNA Cys D-arm is 2 nucleotides; In the D-ring, the number of nucleotides is 7-12, accounting for 12.8%, 24.6%, 44.6%, 7.1%, 11.6% and 0.25% respectively. Only NC_008372.1.tRNA Tyr has 12 nucleotides. In the anticodon arm, the number of nucleotides is 4-5, of which tRNA containing 4 nucleotides accounted for 8.9%, and the proportion of tRNA containing 5 nucleotides was 91.1%. In the anticodon loop, the number of nucleotides is 7-9, 99.0% of the tRNA nucleotides are 7, and 0.73% of the tRNA nucleotides are 9. Similarly, only one tRNA has an anticodon ring structure with a nucleotide number of 8. For the variable loop, as the structure with the largest number of nucleotide changes in the tRNA secondary structure, among the 406 tRNAs tested, the number of nucleotides in the variable loop  ranged from 3-22 nt, but 77.44% tRNA, the number of nucleotides in the variable loop is 5. In addition, we enumerate the secondary structures of three different tRNAs with variable loops/stems (Fig. S1). In the -arm, only one tRNA has 4 nucleotides, and the remaining 405 tRNAs have 5 nucleotides; in the -ring, the number of tRNA nucleotides is all 7, and there is no special structure of the variation. Therefore, it is speculated that the -loop in the tRNA secondary structure is the most conserved structural unit, which is consistent with the results of the previous multiple sequence alignments (Fig. 3).

Analysis of introns in chloroplast tRNA
After analyzing the chloroplast tRNA genome sequence of all Chlorophyta, we found that the chloroplast tRNA of M. homosphaera (NC_029670.1) .tRNA Leu (TAA)) in the sphaeropleales, S. leibleinii (NC_015645.1). tRNA Leu (TAA)) and S. helveticum (NC_008372.1). tRNA Leu (TAA)) in the chaetophorales contain introns (Fig. 4). These three groups of introns are all located in the anticodon loop of tRNA, with nucleotide lengths of 174, 253, and 243, respectively. The starting points in the tRNA structure are 38, 39, 39, and from the analysis results, the scores (Internal scores) of these three tRNAs are 35.9 bits, 20.7 bits and 33.4 bits, respectively, indicating that they may not be functional. Subsequently, we selected the chloroplast genome sequences of five plants from the NCBI genome database, streptophytina Chaetosphaeridium globosum (NC_004115), Mesostigma viride (NC_002186), angiospermae Liriodendron tulipifera (NC_008326), monocotyledon Zea mays (NC_001666), and dicotyledons Vitis vinifera (NC_007957.1). The chloroplast tRNA gene sequences of these 5 plants were predicted by tRNAscan-SE software. It was found that tRNA Leu (TAA), which also contains introns, was found in streptophytina, but this tRNA was not found in the chloroplast genome sequence of angiospermae L. tulipifera, monocotyledon Z. mays, and dicotyledons A. thaliana. Therefore, it is speculated that tRNA Leu (TAA) containing introns in the chloroplast genome of green plants first appeared in Chlorophyta and Streptophytina. In addition, the chloroplast genome tRNA Leu of angiosperms, monocots, and dicots was used as a peripheral group to construct a phylogenetic tree of tRNA Leu (Fig. 5). It is clear from tree that the tRNA Leu containing introns are close to each other on the evolutionary branch.

Transition, transversion bias analysis
To better understand tRNA evolution, we performed MCL conversion and escape bias analyses on 20 tRNA isoforms measured in chloroplasts and compared the results with those of tRNA analysis in cyanobacteria and monocot chloroplasts (Mohanta et al., 2017;Mohanta et al., 2019). The results showed that although green algae and monocotyledon belong to eukaryotes, the transition and transversion results were consistent with the analysis results of cyanobacteria tRNA, and the chloroplast tRNA conversion rate of green algae was higher than the transversion rate. On the contrary, it is inconsistent with the chloroplast tRNA analysis results of monocotyledonous plants, because we found that in the chloroplast tRNA analysis results of monocotyledonous plants, the transformation rate and transversion rate of tRNA Ala , tRNA Asn , tRNA Asp , tRNA His , tRNA Phe and tRNA Pro were almost equal, but did not appear in the chloroplast tRNA analysis results of green algae. In addition, from the data analysis, we found that the transition rate of tRNA Asp and tRNA Val is significantly higher than the transition rate. The transition rate between A ↔G in tRNA Val is as high as 30.95%, second only to the transition rate of C ↔U (33.32%) in tRNA Trp . Not only that, the transition rate between A ↔C in tRNA Val is the lowest among all tRNAs analyzed, only 0.58% (Table 3).

Phylogenetic tree analysis of Chlorophyta chloroplast tRNA
After aligning and analyzing 406 tRNA sequences, a phylogenetic tree was constructed using all tRNA genome sequences. After sorting and analyzing, according to the evolutionary relationship of the phylogenetic tree, all tRNAs are divided into four clusters, the first cluster of tRNAs are: tRNA Met , tRNA Arg , tRNA Tyr , tRNA Leu , tRNA Ser ; the second cluster of tRNAs are: tRNA His , tRNA Met , tRNA Gln , tRNA Cys , tRNA Phe , tRNA Thr ; the third cluster of tRNAs are: tRNA Arg , tRNA His , tRNA Gln , tRNA Glu , tRNA Trp , tRNA Val , tRNA lle , tRNA Asn , tRNA Lys ; the fourth cluster of tRNAs are: tRNA Ala , tRNA Asp , tRNA Gly , tRNA Pro , tRNA Met (Fig. 6). In addition, we can find several interesting phenomena from the evolutionary tree: firstly, there are multiple branches of tRNA Arg and tRNA Met in the entire phylogenetic tree. It is speculated that this may be due to multiple mutations of these two tRNAs during the evolution process. Secondly, in the third cluster, tRNA Arg (TCT) is the closest to tRNA Asn (GTT) and tRNA Trp (CCA). From the sequence comparison results, the sequence similarity between tRNA Arg (TCT) and tRNA Asn (GTT) was very high, reaching 80%. Similarly, the sequence similarity between tRNA Arg (TCT) and tRNA Trp (CCA) was also very high, reaching 77.36% (Figs. 7 and 8). The evolutionary relationship between tRNA Arg (TCT) and tRNA Arg (ACG) (TCG) is closer than that of other tRNA Arg in the evolutionary tree, so we speculate that tRNA Arg (TCT) is evolved from tRNA Arg (ACG) (TCG); tRNA Arg (CCT) has two points branches, one of which is similar to tRNA Met in genetic relationship, and the other branch is likely to be present in the branches of tRNA Ser due to mutations during evolution. In addition, in the evolutionary tree, the branches of tRNA Ser are obviously denser, which indicates that tRNA Ser seems to be susceptible to mutations. The secondary structure prediction results of tRNA Ser by tRNAscan-SE software also support it. There are three branches of tRNA Met (CAT), which are closely related to tRNA Arg (CCT), tRNA Thr (TGT) and tRNA Pro (TGG).

Analysis of duplication and loss of chloroplast tRNA
In the evolution of tRNA, there are not only transformation and transversion events, but also duplication and loss events. Therefore, studying the duplication and loss of chloroplast tRNA is very important for its evolution. After analysis, we've got the loss and duplication tree (Fig. S2). And the results showed that among all the tRNAs tested, duplications events occurred a total of 151 times, conditional duplications events occurred a total of 101 times,   and losses events occurred a total of 311 times (Table 4) (Data S2). Therefore, we speculate that a large number of genes may be lost during the evolution of the chloroplast tRNA.
In addition, we found that tRNA Met had a higher D/L (duplication/loss) score than other tRNAs, while tRNA Pro and tRNA Try had a lower D/L score, which may be due to the fact that tRNA Pro and tRNA Try are more conserved during evolution.

DISCUSSION
No tRNAs of the two subtypes selenocysteine and possible suppressor were found to be present in the analyzed chloroplast genomes of the Chlorophyceae. There are 11 subtypes of tRNAs that encoded only one tRNA in 14 species, and 9 subtypes of tRNAs encoded different numbers of tRNAs. Among these 9 subtypes of tRNAs, the tRNA Glu was not found in F. terrestris. It is speculated that in F. terrestris, the codon recognition of glutamate may rely on the introduction of tRNA from the cytoplasm. To prove that the chloroplast is the evidence that tRNA can be introduced from the cytoplasm (Wolfe, Morden & Palmer, 1992;Smith, 2009). tRNA Arg appeared 5 times in O. cardiacum, tRNA Ser appeared 4 times in S. leibleinii, and tRNA Met appeared 4 times in P. lenticularis. Therefore, based on the fact that tRNA Arg , tRNA Ser and tRNA Met occur frequently in the species, we speculate that it may be because these three groups of tRNA are more prone to mutation during evolution, which is consistent with the results of the constructed phylogenetic tree. The characteristic of tRNA is a clover secondary structure composed of three hairpin loops and a terminal spiral stem, and the length of tRNA is generally between 75-95 nt (Goodenbour & Pan, 2006). In our study, it was found that 73.4% of the sequence lengths of tRNA of the 14 chloroplast species were less than 75 nt, which may have a great relationship with the tRNA has been in a conserved evolutionary stage. Anticodons are of great significance in the process of protein synthesis. Anticodons and codons can introduce specific amino acids into the ribosome through base pairing (Sciarrino & Sorba, 2012). At the same time, anticodons and codons also differ greatly in number and evolutionary conservation. Anticodons also have variability in different organisms and organelles (Tong & Wong, 2004). In the 14 species analyzed, the anticodons were missing to varying degrees. At the same time, it was discovered that the missing tRNA Gly (GCC) anticodon in N. aquatica was compensated by the tRNA Gly (TCC) anticodon. The missing anticodon tRNA Leu (TAA) in B. giganteus, T. obliquus, F. terrestris, S. leibleinii and O. cardiacum is compensated by the anticodon tRNA Leu (TAG). But only in F. terrestris neither the anticodon tRNA Glu (TTC) nor the anticodon tRNA Glu (CTC) was found. In the chloroplast genome of Chlorophyceae, the gene sequences of tRNA Asn , tRNA Asp , tRNA Cys , tRNA Glu , tRNA His , tRNA Lys and tRNA Thr of the 20 subtypes of tRNAs of 14 species maintain relative conservation. In addition, it was also found that the variable loops of the three tRNAs were significantly longer than those of other tRNAs, which may be due to the fact that the variable loops have a certain influence on the orientation of the L-shaped tertiary structure of tRNAs (Bessho et al., 2007). Among them, the variable loop of tRNA Ser is the longest and is not conserved, which results in that tRNA Ser is susceptible to mutation in the phylogenetic tree.
As an important feature to distinguish prokaryotic genes, introns are widely present in eukaryotic genes, and a small amount are present in prokaryotes (Dai et al., 2003). The existence of introns in genes and their conservation information are closely related to the evolutionary origin and evolutionary relationship of species genes. For a long time, the main functions and research directions of introns have focused on the direction of gene expression regulation and selective splicing mechanism (Toor et al., 2008;Li et al., 2015;Swagata et al., 2018), but studies on introns as indicators of species evolution are relatively rare. The size and number of introns are not fixed in eukaryotic genes (Wu et al., 2013). The different ways in which repetitive sequences accumulate in plants and animals give rise to the fact that the length and number of introns in plants is much smaller than in higher animals (Wendel et al., 2002). Intron-containing tRNA genes are found in all kingdoms of life (Schmidt & Matera, 2020), and are rare in higher plants, but in Chlorophyta, more than half of tRNA genes contain an intron (Michaud et al., 2011). The distribution of introns in tRNA genes is also diverse, and can be located in different positions of tRNA genes, while the tRNAscan-SE server is difficult to accurately predict tRNA genes whose introns are not 37/38 and split tRNA genes (Sugahara et al., 2006). Therefore, in the process of analyzing the tRNA of the chloroplast genome of Chlorophyta, we only found that part of the tRNA genome contains a set of introns in the anti-codon ring. At the same time, based on the analysis scores of the tRNAscan-SE server, it is found that the scores of these three groups of tRNA are relatively low. It is speculated that these three groups of introns are not functional.
In the constructed tRNA phylogenetic tree, we found that the chloroplast tRNA of Chlorophyta was consistent with the tRNA of monocotyledonous and gymnospermatic chloroplasts in evolutionary pattern, with a common multi-phylogenetic pattern, and was rooted in a multiple common ancestor (Fig. 6). However, in the phylogenetic map of chloroplast genome constructed by us (Fig. S3), it was found that the branches of other species were not consistent with the branches of species tree except S. Leibleinii and S. helveticum in Chaetophorales, which indicated that the chloroplast evolution of chlorophyta was not inconsistent with the evolution pattern and direction of species evolution.
The conversion between nucleotides (U ↔C, A ↔G) and transversion (U ↔A, U ↔G, C ↔A, C ↔G) are important indicators for analyzing the evolution of tRNA genes. Research usually uses MAGE X software analysis and statistics. The MAGE X analysis in the data shows that the frequency of the nuclear genome of drosophilid is twice as high as the frequency of transversion (Begun et al., 2007). In the DNA sequences of many genomes, cyanobacteria tRNA and some monocot chloroplast tRNA genes, transition occurs more frequently than transversion occurs (Gojobori, Li & Dan, 1982;Wakeley, 1994;Mohanta et al., 2017;Mohanta et al., 2019). In this study, the MAGE X software measured the rate of transition/transversion greater than k1 (Purines) and K2 (Pyrimidines), indicating a higher frequency of transition than transversion in the chloroplast tRNA gene of Chlorophyta, which is similar to the results of the above analysis (Table 5). In the process of species evolution, gene duplication and loss affect the change of species genes. Gene duplication is an effective way to generate new gene functions in the genome, and gene loss can effectively shape gene families (Rasmussen & Kellis, 2012;Teufel, Liu & Liberles, 2016). In the event of gene sequence duplication loss in Chlorophyta chloroplast tRNA, tRNA Met , tRNA Arg and tRNA Ser all occurred at a high level, which may lead to multiple appearances of these three tRNAs in the chloroplast genome and active evolution in the phylogenetic trees constructed.

CONCLUSIONS
Through the sequence variation and evolutionary analysis of the chloroplast genome tRNA of chlorophyceae, we found that the chloroplast genome contains 28-32 tRNAs, and the length of the gene sequence ranges from 71 nt to 91 nt. There are 23-27 anticodon types of tRNAs, and some tRNAs have missing anticodons that are compensated for by other types of anticodons of that tRNA. In addition, three tRNAs were found to contain introns in the anticodon loop of the tRNA, but scored poorly when analyzed by the tRNAscan-SE software. Therefore, we speculate that these introns are not functional. After multiple sequence alignment, the -loop is the most conserved structural unit in the tRNA secondary structure, containing mostly U-U-C-x-A-x-U conserved sequences. The number of transitions in tRNA is higher than the number of transversions. In the gene duplication and gene loss analysis, it was found that green algal chloroplast tRNAs may have undergone substantial gene loss during the course of evolution. According to the constructed phylogenetic tree, it was found that there were mutations in the evolution of chlorophyta chloroplast tRNA, with tRNA Met , tRNA Arg and tRNA Ser being the most obvious.

ADDITIONAL INFORMATION AND DECLARATIONS Funding
This study was supported by the National Natural Science Foundation of China (31572665), the Shannxi Provincial Department of Education Research Project (Key Laboratory Project) (18JS112), and the Shaanxi Science and Technology Innovation Team (2019TD-012). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Grant Disclosures
The following grant information was disclosed by the authors: National Natural Science Foundation