The complete mitochondrial genome of the Antarctic springtail Cryptopygus antarcticus (Hexapoda: Collembola)

Abstract

Background: Mitogenomics data, i.e. complete mitochondrial genome sequences, are popular molecular markers used for phylogenetic, phylogeographic and ecological studies in different animal lineages. Their comparative analysis has been used to shed light on the evolutionary history of given taxa and on the molecular processes that regulate the evolution of the mitochondrial genome. A considerable literature is available in the fields of invertebrate biochemical and ecophysiological adaptation to extreme environmental conditions, exemplified by those of the Antarctic. Nevertheless, limited molecular data are available from terrestrial Antarctic species, and this study represents the first attempt towards the description of a mitochondrial genome from one of the most widespread and common collembolan species of Antarctica.

Results: In this study we describe the mitochondrial genome of the Antarctic collembolan Cryptopygus antarcticus Willem, 1901. The genome contains the standard set of 37 genes usually present in animal mtDNAs and a large non-coding fragment putatively corresponding to the region (A+T-rich) responsible for the control of replication and transcription. All genes are arranged in the gene order typical of Pancrustacea. Three additional short non-coding regions are present at gene junctions. Two of these are located in positions of abrupt shift of the coding polarity of genes oriented on opposite strands suggesting a role in the attenuation of the polycistronic mRNA transcription(s). In addition, remnants of an additional copy of trnL(uag) are present between trnS(uga) and nad1. Nucleotide composition is biased towards a high A% and T% (A+T = 70.9%), as typically found in hexapod mtDNAs. There is also a significant strand asymmetry, with the J-strand being more abundant in A and C. Within the A+T-rich region, some short sequence fragments appear to be similar (in position and primary sequence) to those involved in the origin of the N-strand replication of the Drosophila mtDNA.

Conclusion: The mitochondrial genome of C. antarcticus shares several features with other pancrustacean genomes, although the presence of unusual non-coding regions is also suggestive of molecular rearrangements that probably occurred before the differentiation of major collembolan families. Closer examination of gene boundaries also confirms previous observations on the presence of unusual start and stop codons, and suggests a role for tRNA secondary structures as potential cleavage signals involved in the maturation of the primary transcript. Sequences potentially involved in the regulation of replication/transcription are present both in the A+T-rich region and in other areas of the genome. Their position is similar to that observed in a limited number of insect species, suggesting unique replication/transcription mechanisms for basal and derived hexapod lineages. This initial description and characterization of the mitochondrial genome of C. antarcticus will constitute the essential foundation prerequisite for investigations of the evolutionary history of one of the most speciose collembolan genera present in Antarctica and other localities of the Southern Hemisphere.

Figure 1

The mitochondrial genome organization of Cryptopygus antarcticus. Genes for proteins and rRNAs are indicated with standard abbreviations, whereas those for tRNAs are designated by a single letter for the corresponding amino acid. Arrows indicate direction of coding regions. Black color is used for the genes oriented on the J-strand, red for those with opposite polarity.

Figure 2

Putative secondary structures of tRNAs present in the mitochondrial genome of C. antarcticus.

Figure 3

Graphical representation of the percentage of As (red) and Ts (black) across the whole mtDNA segment calculated using the program MacVector^® 7.2.3 (Accelrys). Y-axis values represent nucleotide %, calculated with a 100-bp sliding window; x-axis values represent the nucleotide positions corresponding to the linearized genome. Below the graphs, position and orientation of each gene.

Figure 4

Across-strand (N & J) comparison of frequencies of codons ending with each nucleotide. Values on the y-axis represent the sum of RSCU values of codons ending with the same nucleotide across all codon families.