The mitochondrial genome of the 'twisted-wing parasite' Mengenilla australiensis (Insecta, Strepsiptera): a comparative study
© McMahon et al; licensee BioMed Central Ltd. 2009
Received: 15 July 2009
Accepted: 14 December 2009
Published: 14 December 2009
Strepsiptera are an unusual group of sexually dimorphic, entomophagous parasitoids whose evolutionary origins remain elusive. The lineage leading to Mengenilla australiensis (Family Mengenillidae) is the sister group to all remaining extant strepsipterans. It is unique in that members of this family have retained a less derived condition, where females are free-living from pupation onwards, and are structurally much less simplified. We sequenced almost the entire mitochondrial genome of M. australiensis as an important comparative data point to the already available genome of its distant relative Xenos vesparum (Family Xenidae). This study represents the first in-depth comparative mitochondrial genomic analysis of Strepsiptera.
The partial genome of M. australiensis is presented as a 13421 bp fragment, across which all 13 protein-coding genes (PCGs), 2 ribosomal RNA (rRNA) genes and 18 transfer RNA (tRNA) sequences are identified. Two tRNA translocations disrupt an otherwise ancestral insect mitochondrial genome order. A+T content is measured at 84.3%, C-content is also very skewed. Compared with M. australiensis, codon bias in X. vesparum is more balanced. Interestingly, the size of the protein coding genome is truncated in both strepsipterans, especially in X. vesparum which, uniquely, has 4.3% fewer amino acids than the average holometabolan complement. A revised assessment of mitochondrial rRNA secondary structure based on comparative structural considerations is presented for M. australiensis and X. vesparum.
The mitochondrial genome of X. vesparum has undergone a series of alterations which are probably related to an extremely derived lifestyle. Although M. australiensis shares some of these attributes; it has retained greater signal from the hypothetical most recent common ancestor (MRCA) of Strepsiptera, inviting the possibility that a shift in the mitochondrial selective environment might be related to the specialization accompanying the evolution of a small, morphologically simplified completely host-dependent lifestyle. These results provide useful insights into the nature of the evolutionary transitions that accompanied the emergence of Strepsiptera, but we emphasize the need for adequate sampling across the order in future investigations concerning the extraordinary developmental and evolutionary origins of this group.
Strepsiptera are an unusual group of obligate endoparasitoid insects . They occur as a small (approx. 600 spp.) monophyletic insect order with uncertain evolutionary origins, inclusive of any clear understanding over the group's nearest extant relative. Strepsiptera parasitize 7 orders of insects, including silverfish (Thysanura); cockroaches (Blattaria); mantids (Mantodea); crickets and grasshoppers (Orthoptera); bugs (Hemiptera); wasps, ants and bees (Hymenoptera) and flies (Diptera). Understandably, the very uniqueness of Strepsiptera makes their placement within insects using morphological taxonomic methods a contentious task. Strepsiptera have been placed as the sister-group to myriad different insect groups, from beetles to true flies, even being placed outside of Holometabola: each hypothesis being founded on one or two 'key' characteristics that at one time or another have come under question [2–15]. Confident assertions of classification have especially been restricted because intermediate forms have largely gone extinct and are unrecorded in the fossil record (see  for a notable exception). Simplification of gross morphology during strepsipteran evolutionary specialization can also be viewed as a significant component of this problem: observable morphological variation is low and unevenly distributed between extremely dimorphic sexes.
Females are especially strongly simplified (lacking in most typical adult characters such as wings, legs or mouthparts), and in most strepsipteran species, they remain in the living host until the end of the reproductive cycle. Conversely, males metamorphose in a typical holometabolan fashion; developing wings and a usual suite of adult insect characters (such as antennae, mouthparts and compound eyes) only to leave the host immediately in search of a mate; usually in the form of a female-containing host [17–19]. Males deliver sperm through a brood canal opening in the cephalothorax (the modified "head") of the female, who as a reproductive adult is only partially exposed to the environment, as an extrusion between the tergites or sternites of living hosts. After fertilization, females are capable of producing many hundreds of thousands of active 1st instar larvae who emerge from the cephalothorax.
Members of the strepsipteran lineage, Mengenillidae, are the sister-group to Stylopidia, a clade that includes all other extant Strepsiptera [, unpublished data]. This family represents a transitional phase in the evolutionary specialization of Strepsiptera, whereby both sexes leave the host before pupation, and females do not reproduce or release progeny in the unusual fashion outlined above. The level of simplification in free living mengenillid females is much less extreme than in Stylopidia (e.g. X. vesparum); legs, mouthparts and compound eyes are all still present, although strongly reduced. The invention of a completely endoparasitic female was probably one of the most important novelties leading to the radiation of this unique group . Evolutionary studies attempting to unravel the developmental and ecological phenomena that make Strepsiptera so biologically interesting therefore require that species from before this important transition occupy a central role in research.
Insect mitochondrial genomes provide a useful medium to deepen and connect comprehension of microevolutionary forces of populations, like neutral drift and selective sweeps, to macroevolutionary events affecting species and/or deeper levels of divergence. The abundance of mitochondria in most metazoan cell and tissue types makes mtDNA an easily obtainable, universally plentiful marker, where a lack of introns or duplicate genes and non-coding variable spacer regions make amplification of mtDNA relatively uncomplicated  (although pseudogenes co-opted by the nucleus from the mitochondrial genome can create separate difficulties [23–26]). The near-complete sequence of the mt genome of Xenos vesparum is available, and as a member of the more specialized clade Stylopidia, it is important to place it into context. The mitochondrial genome of Mengenilla australiensis (Family Mengenillidae) was therefore chosen for sequencing. Genome arrangement, nucleotide content, codon usage and the secondary structure of ribosomal RNA genes are each comparatively assessed between two distant Strepsiptera, and more widely across Holometabola (a major division of insects defined by the process of complete metamorphosis, including the 4 very species-rich orders; Lepidoptera, Diptera, Hymenoptera, Coleoptera and 6 smaller orders).
Results and discussion
Analyses to date do not reveal general patterns of mitochondrial genome evolution across holometabolan parasitic lineages [37, 38], but these have concentrated largely on the comparative analysis of genome organization (gene order/orientation) itself. Within orders like Hymenoptera, although a correlation between the extent of mt genome modification and parasitism appears to be lacking [39, 40] (with an additional implication that the position of mt genes might largely be neutral ), the truncation of PCGs as presented in this report, probably represents a separate issue. Determining the extent to which PCG truncation occurs within Strepsiptera must be a target for future investigations. Unfortunately, direct measures of insect mitochondrial effective population size have not yet been investigated.
These results are consistent with prevailing neutral mutational theories positing that genomic G+C content is the most significant factor in determining codon bias between organisms [44–47]. This may explain why X. vesparum has significantly relaxed ENC-MILC values, whose global G+C content is 5% higher than M. australiensis. For the thysanuran and beetle genomes, although G+C content in the 3rd codon position of individual PCGs is not necessarily elevated (Figure 6B), it is global G+C% content that appears to matter: it has been documented that codon bias can be accurately predicted from intergenic regions , and it is unsurprising that these genomes should also have the highest genome wide G+C content across the 68 Holometabola dataset (Additional file 2). It is noticeable that the coleopteran T. bruchi has a very A-skewed genome (Figure 4A), which could also partly explain some of the discrepancies appearing between the different methods.
Secondary structure of ribosomal and transfer RNAs
The mitochondrial genome of X. vesparum displays a number of characteristics that are not shared by its distant relative M. australiensis. In the latter, only 2 tRNA translocations disrupt an otherwise ancestral insect genome organization, whereas in X. vesparum, 3 tRNA translocations and at least 2 duplications have occurred (Figure 2). Codon bias in X. vesparum is also significantly relaxed: M. australiensis occupies a much more typical region of ENC/MILC space (Figures 5 and 6). Further, rRNA secondary structural model predictions in M. australiensis are much more consistent with current insect consensus structures (Figures 7, 8, 9 and 10). In X. vesparum, increased variability at the primary sequence level precludes the assembly of well corroborated models in several parts of the rRNA genes. Both strepsipteran genomes are unusually C-skewed and A+T% rich. Interestingly, PCGs are noticeably truncated in both strepsipteran taxa, but especially so in X. vesparum which has lost nearly twice as many codons (154) as M. australiensis (81), equating to 4.3% and 2.2% fewer codons than the average holometabolan genome respectively.
These results provide useful insight into the evolution of strepsipteran mitochondria spanning the shift from a largely free-living insect into an extremely unique and simplified endoparasitoid, since the emergence of Strepsiptera over 100 million years ago . The modifications to the mitochondrial genomes of X. vesparum and M. australiensis appear consistent with organisms that have evolved extremely derived lifestyles. M. australiensis represents a transitional phase in the evolutionarily specialization of Strepsiptera, and the composition, architecture and structure of its genome reflects this. These observations raise important questions about the changing selective environment in (mitochondrial) genomes belonging to small, and almost entirely host-dependent endoparasitoids. Future investigations into the evolutionary and developmental origins of this unique biological system must ensure that strepsipteran species from Mengenillidae are adequately sampled: wide sampling across this group is crucial if the underlying ancestral signal is to be maximized.
DNA was used from a small set of free-flying males collected in light traps, leaving little possibility for contamination through thysanuran host DNA. Nuclear and mitochondrial gene alignments from a widely sampled Strepsiptera taxon set (unpublished) confirm this. We also include 3 thysanuran genomes in Figure 8 to emphasize the genome composition differences between M. australiensis and its target host group. Specimens were collected on the 16th March 2006 in Australia, Queensland, Blackdown Tablelands NP, South Mimosa Creek, 50 m dstr road, 794 mao. Coordinates as follows: S23 47.687'E 149 04. 195'. Light trap loc 34 (Collectors were N. Jönsson, T. Malm & D. Williams). Complete species name: Mengenilla australiensis Kifune & Hirashima 1983.
Genome isolation and sequencing
M. australiensis specimens were extracted for PCR either by macerating tissue and digesting overnight at 50°C with Chelex 100 grade resin (Bio-Rad Cat. no 142-1253, Hemel Hampstead, UK) and enzyme Proteinase K (BIOLINE, Cat. no BIO-37037, London, UK) or by employing a QIAGEN blood and tissue column extraction protocol. Amplification of the M. australiensis mitochondrion was carried out by amplifying three ~3 kb fragments, with intervening sections that did not overlap being amplified by standard PCR. Long PCR mixes containing 1 μl BIO-X-ACT long DNA polymerase (BIOLINE, Cat. no. BIO-21049), 1 μl dNTPs, 25 μl 2× Polymate additive (BIOLINE, Cat. no. BIO-37041) for A+T rich sequences, 5 μl 10× opti-buffer, 4 μl MgCl2 solution, 3 μl forward/reverse primers and 5-10 μl of genomic DNA were made up to 50 μl reactions with ultrapure (NANOpure Diamond™) water. Long PCR cycling parameters were as follows: 92°C for 2 minutes; 10 cycles of: 92°C for 10 seconds, 53°C for 30 seconds and 68°C for 13 minute; 28 cycles of: 92°C for 15 seconds, 53°C for 30 seconds and 68°C for 14 minute; with an extension step of 7 minutes at 68°C. Long PCR fragments were gel extracted (QIAGEN Cat no. 28704, Hemel Hampstead, UK) and used as templates in subsequent sequencing reactions. Standard PCR mixes contained 0.5 μl Taq polymerase, 1 μl dNTPs, 5 μl MgCl2 free buffer, 4 μl MgCl2solution, 3 μl forward/reverse primers and 2 μl of genomic DNA, made up to 50 μl reactions with ultrapure (NANOpure Diamond™) water. Standard PCR cycling parameters were as follows: 94°C for 2 minutes, then 35 cycles of 94°C for 1 minute, T°C for 45 seconds (T varied with primer pair) and 72°C for 1 minute, with an extension step of 5 minutes at 72°C. For difficult regions, Phusion high fidelity Taq was utilized, following the manufacturer's instructions. Standard PCR products were cleaned using 2 μl of Shrimp Alkaline Phosphatase (SAP) and 3 μl of Exonuclease I (in a 1:10 SAP buffer dilution) per 50 μl PCR reaction, or fragments were extracted from agarose gels and sequenced directly. Problematic fragments that did not sequence directly were inserted into pGEM-T Easy plasmid vectors (Promega, Southampton, UK), and multiple clones were sequenced for each fragment. Sequencing reactions were performed using the BigDye® Terminator v3.1 cycle sequencing kit with the following modifications: 1 μl of BigDye®, 1.5 μl 5× Buffer, 1 μl primer (3.2 pmol), 1-3 μl genomic DNA made up to 10 μl reactions with ultrapure (NANOpure Diamond™) water. Sequence reads were generated using an Applied Biosystems 3730xl DNA Analyzer. Sequences were imported into GridinSoft Notepad (Lite Edition; http://notepad.gridinsoft.com) and BioEdit (version 126.96.36.199; ) for analysis. Primer pairs employed are given in Additional file 3. The annotated genome fragment can be found in Genbank under the accession number GU188852. Voucher specimens and extractions are deposited in the Hope Entomological Collections, Oxford University Museum of Natural History, Oxford, UK, and the Swedish Museum of Natural History, Stockholm, Sweden.
PCGs and their boundaries were diagnosed via sequence comparison across alignments derived from the 68 holometabolan genome dataset. Start codons found to be in-frame and not overlapping with upstream genes on the same strand were usually identified as 5' gene boundaries. Stop codons were typically identified as TAA, TAG, TA, or T. A complete annotation of the M. australiensis genome is given in Additional file 4. One non-coding fragment, 24 bp in length, was identified between the stop codon of nad1 and the beginning of tRNA-Ser2. Secondary structural predictions for ribosomal and transfer RNAs were ascertained by implementing a comparative structural framework [51–55] (for an online tutorial see the jRNA web site: http://hymenoptera.tamu.edu/rna/), in conjunction with a thermodynamic-based RNA folding algorithm (mfold; ). mfold gave initial raw estimates of structure: a tool that is especially useful for variable domains with highly divergent primary sequence. In this approach, structures with the lowest thermodynamic stability values were most highly considered (although structures with only marginal thermodynamic differences were also evaluated) and robust comparative evidence where available, took priority. The following ribosomal RNA structural templates from the Comparative RNA Web (CRW) site database  were used: Escherichia coli, Drosophila melanogaster, Drosophila virilis, Chorthippus parallelus and Apis mellifera . For tRNA genes, the following models were used as comparative data points: X. vesparum (Strepsiptera: ), O. lunifer (Lepidoptera: ), V. eucnemidarum (Hymenoptera: ) and Cryptopygus antarcticus (Collembola: ), structures were verified for accuracy in tRNA-Scan . Gene order was assessed across Holometabola using the inferred ancestral insect arrangement as a comparative standard [29, 30]. As a measure of gene length fluctuation, total and individual gene length was counted across strepsipteran PCGs, and compared across the dataset of 68 holometabolan genomes. To include nad2 gene, the 5' portion (approx. 100 amino acids) was excluded from analysis so that M. australiensis could be assessed alongside the entire dataset.
The 68 holometabola mitochondrial genome dataset was compiled from previously published data across the 6 major holometabolan divisions: Diptera [60–74], Lepidoptera [33, 75–81], Hymenoptera [39, 82–87], Coleoptera [88–93], Neuropterida [34, 94, 95] and Strepsiptera .
Nucleotide content and Codon usage
A+T% and G+C% values were calculated for the α strand of the M. australiensis genome, which runs clockwise in the direction of transcription from nad2 (Figure 1). Genomic A- and G-skew measures were calculated using total nucleotide % values in the following manner: [A-T]/[A+T] and [G-C]/[G+C] following Perna & Kocher . A complete list of nucleotide content values for the 68 holometabolan dataset is given in Additional file 3. Codon usage was investigated by importing the complete cDNA datasets of 68 available holometabolan mitochondrial genomes, including X. vesparum and the sequenced genome of M. australiensis into INCA v2.1 . Two independent measures of codon bias: ENC (Effective Number of Codons used) and MILC (Measure Independent of Length and Composition) are implemented, whose statistical justification and behaviour under simulation differ markedly . These two independent measures were combined as a singe vector statistic as follows: √(ENC2 + MILC2). The shortest gene, atp8 was also excluded from analyses due to artificially increased ENC/MILC bias in genes shorter than around 100 amino acids. nad4L was of borderline length (approx. 90-105 amino acids) and was retained, although nad4L from Drosophila sechellia and Melipona bicolor produced arbitrary ENC 'cut-off' values of 61, because ENC behaves less stably at borderline gene lengths; these data points were removed. Readers are referred to  for a detailed explanation.
- A+T region:
the putative control region
- α strand:
leading strand of transcription
Effective Number of Codons used
Measure Independent of Length and Composition
- rrnL and rrnS:
large (16S) and small (12S) subunit ribosomal RNA (rRNA) gene
genes are denoted as single letter amino acid IUPAC-IUB abbreviations
- atp6 and atp8:
ATP synthase subunits 6 and 8
- cob :
cytochrome c oxidase subunits 1-3
- nad1-6 and nad4L:
NADH dehydrogenase subunits 1-6 and 4L
Most Recent Common Ancestor.
We owe gratitude to Niklas Jönsson, Tobias Malm and Dawn Williams for collection of material, to Clive Cook Director of the Queensland Parks and Wildlife Service - Northern Region for the Collecting Permit (# WITK03408905), and to B. Viklund (Swedish Museum of Natural History, Stockholm, Sweden) for donation of material. Thanks must also be extended to Peter Holland for useful discussions, the Leverhulme Trust (grant no. F/08 502G) for funding to JK, and the Elizabeth Hannah Jenkinson fund (University of Oxford) for an award to DPM.
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