Discussion
Characteristics of Tenebrionoidea mtgenomes
These 90 mtgenomes investigated in the present study in the superfamily Tenebrionoidea have a length variation from 14 777 bp to 16 861 bp, and the length variation mainly stems from CR, intergenic overlap and spacers, which is consistent with earlier reports in Tenebrionoids (Burger et al. , 2003). The nucleotide composition for all species exhibits obvious AT bias with high A+T content, similar as earlier reports in Tenebrionoids (Jie et al. , 2016). All tRNA genes can form a complete clover secondary structure, except fortRNA -Ser (AGN) that lacks the DHU arm, which seem to be a common feature of Tenebrionoidea (Zhang et al. , 2016; Songet al. , 2018). There are some rearrangement events in some species of the family Mordellidae, Lagriidae and Pyrochroidae. The shuffling of the trnR and trnN genes (trnA-trnR-trnN-trnS-trnE-trnF gene cluster) is found in all specials in the family Mordellidae; the translocation of thetrnD and ATP8 genes is found in the A. rugipennisof Lagriidae for the first time; the shuffling of the trnC andtrnW genes (trnW-trnC-trnY gene cluster) is found in Pyrochroidae and Lagriidae. These gene rearrangement may be produced by abnormal priming of mitochondrial replication by a tRNA molecule or tandem duplications, which can provide an important reference for Tenebrionoidea phylogeny inference (Boore & Brown, 1998; Boore et al. , 1998; Timmermans & Vogler, 2012; Cameron, 2014). The ATN and TTG are mainly used as the start codon, and TAA and TAG as the stop codon for the 13 PCGs, which is similar as other mtgenome sequences in Tenebrionoidea (Du et al. , 2017). The Ka/Ks ratio is lower than one for all PCGs, which is consistent with earlier studies in Tenebrionoidea. The COX1 gene has experienced strong evolutionary pressure in order to maintain its own functional requirements, whereas ATP8 has experienced weak evolutionary pressures with allowing more mutations to accumulate in the mtgenome (Ou et al. , 2016; Bai et al. , 2018).
Overview of phylogenetic relationships
A total of 16 families are included in the phylogenetics and evolution analysis in the Tenebrionoidea, in which there are 10 families with at least two representative species included. The family Ciidae seems to be earliest derived in these families, followed Mordellidae + Ripiphoridae, and Aderidae + “Meloidae clade” + “Tenebrionidae clade”. Ciidae was historily placed in the Cucujoidea (Crowson, 1955) and then to the superfamily Tenebrionoidea mainly based on characteristics of the aedeagus and the larval abdomen (Crowson, 1960). It was proposed to be a monophyly based on 18S and COX1 genes using ML and BI methods, and demonstrated to be a either sister to Nitidulidae based on the reduced sample or at the base of the cucujoid-tenebrionoid assemblage based on the entire sample (Buder et al. , 2008). It was considered basal tenebrionoids based on 516 adult and larval morphological characteristics from 359 beetle taxa (Lawrence et al ., 2011). The present study also suggests the family to be the basal tenebrionoids, but further investigation is necessary to elucidate its place with the inclusion of more species.
Mordellidae + Ripiphoridae, Mordellidae and Ripiphoridae are all proposed to be monophyletic, and the two families demonstrate to be sister groups each other in the present study. The Mordellidae + Ripiphoridae was also proposed monophyletic in earlier molecular phylogeny inference based on five nuclear and mitochondrial genes with 300 genera in Tenebrionoidea using ML (Gunter et al. , 2014). The Mordellidae was also proposed to be monophyletic in the study based on four molecular genes (18SrRNA, 28S rRNA, rrnL and COX1 ) with 128 species in Tenebrionoidea using ML (Batelka et al. , 2016). The Ripiphoridae was proposed to be monophyletic from a molecular phylogenetic analysis based on eight nuclear genes with 367 species in Tenebrionoidea using Bayesian method (Mckenna et al. , 2015). However, it was proposed to be paraphyletic in the molecular phylogenetic study based on four mitochondrial and four nuclear gene fragments across 404 taxa (including 250 tenebrionid species) using ML (Kergoat et al. , 2014a), which suggests that the monophyletic status of the Ripiphoridae remains uncertain. The two families were not proposed to be sister group in the earlier molecular phylogenetic study (Gunter et al. , 2014; Kergoat et al. , 2014a), which due to the monophyletic status of Ripiphoridae remains uncertain.
Aderidae was proposed to be a monophyletic lineage in the earlier molecular phylogenetic study (Gunter et al. , 2014). There is only species in Aderidae to be included in the present study, which be formed a monophyly with “Meloidae clade” + “Tenebrionidae clade”, and its position and monophyletic status are yet to be resolved with more species to involved. The “Meloidae clade” + “Tenebrionidae clade”, “Meloidae clade” and “Tenebrionidae clade” are all proposed to be monophyletic, which are consistent with earlier studies based on five nuclear and mitochondrial genes with 300 genera in Tenebrionoidea using ML (Gunter et al. , 2014) and eight mitochondrial and nuclear gene with 404 taxa in Coleoptera using ML (Kergoat et al. , 2014a).
Phylogenetic relationships of “Meloidae clade”
The Meloidae + Anthicidae, Meloidae and Anthicidae are all proposed to be monophyletic, and the two families demonstrate to be sister groups each other in the present study. These results are consistent with earlier studies. The monophyly of Meloidae + Anthicidae was proposed based on 245 mitochondrial sequences in Coleoptera, including 159 newly sequenced full or partial mtgenomes using PhyloBayes (Timmermanset al. , 2015). The monophyly of Meloidae was proposed based on 4 818 nuclear genes in 146 species in beetles using ML (Mckenna et al. , 2019). The monophyly of Anthicidae was proposed based on18S rRNA, 16S rRNA and COX1 gene sequences from 340-taxa using BI (Hunt et al ., 2007), and also based on other molecular phylogenetic studies (Kergoat et al. , 2014a; Timmermanset al. , 2015; Mckenna et al. , 2019). The sister relationship of Anthicidae and Meloidae was proposed based on the morphology characteristics of mesothoracic glands (Hemp and Dettner, 1997), and also based on mitochondrial and nuclear genes (Timmermanset al. , 2015; Mckenna et al. , 2019).
In the “Oedemeridae clade”, the family Prostomidae seems to be located at the base of “Oedemeridae clade”, followed Oedemeridae and Trictenotomidae + Tetratomidae. Prostomidae was proposed a sister to the “pythid-pyrochroid-lineage” (including Trictenomatidae, Pyrochroidae, Salpingidae and so on) based on morphology characteristics of the maxillary articulatory area, the abdominal tergite IX extending to the ventral side of the segment, and the strongly pronounced prognathous condition (Schunger et al. , 2003). However, Prostomidae and Tetratomidae are clustered as one clade in ML tree, and the phylogenetic position of Prostomidae remained unresolved in the present analyses.
The family Oedemeridae is proposed to be monophyletic, which is consistent with the earlier phylogenetic study based on four gene (16S rRNA, COX1, 28S and 18S rRNA) sequences from 8441 taxa of Coleoptera, which removed misplaced single specimens and minor clades (Bocak et al. , 2014). However, it was proposed to be paraphyletic in the phylogenetic study based on mitochondrial and nuclear genes (Gunter et al. , 2014; Zhang et al. , 2018), which suggests that the monophyletic status of the Oedemeridae need be further determined with more species included. Trictenotomidae and Tetratomidae are clustered as one clade using BI in this study, whereas Trictenotomidae was a sister to Boridae in earlier phylogenetic studies based on nuclear genes (Mckennaet al. , 2019). There is only species in Trictenotomidae and Tetratomidae to be included in the present study, which suggests that the monophyletic status of the Trictenotomidae and Tetratomidae remains uncertain, and its position and monophyletic status are yet to be resolved with more species to involved.
Zopheridae and Pyrochroidae are clustered as one clade, and Pyrochroidae seems monophyletic in the present study. The monophyly of Pyrochroidae was also proposed in the earlier phylogenetic study based on 95 nuclear protein-coding genes in 373 beetle species using ML and BI (Zhang et al. , 2018), and in other phylogenetic studies based on mitochondrial and nuclear genes (Gunter et al. , 2014; Kergoat et al. , 2014a; Mckennaet al. , 2019). Zopheridae was a sister to Tetratomidae in earlier phylogenetic studies based on mitochondrial and nuclear genes (Kergoatet al. , 2014a), which suggests that the position and monophyletic status of Zopheridae and Pyrochroidae need be further determined. The families Salpingidae and Scraptiidae seem monophyletic, and are sister groups each other in the present study. The monophyly of Salpingidae and Scraptiidae was also proposed in the earlier phylogenetic studies based on mitochondrial and nuclear genes (Bocak et al. , 2014; Zhanget al. , 2018; Mckenna et al. , 2019), whereas Scraptiidae was proposed a paraphyletic group in other molecular phylogenetic studies (Hunt et al ., 2007; Kergoat et al. , 2014a; Mckennaet al. , 2015). The sister relationship of Salpingidae and Scraptiidae remains unclear due to the limited inclusion of only two or three species. Therefore, the position and monophyletic status of Salpingidae and Scraptiidae need be further determined with more species included.