Recent morphological and molecular evidence has changed interpretations of arthropod phylogeny and evolution. Here we compare complete mitochondrial genomes to show that Collembola, a wingless group traditionally considered as basal to all insects, appears instead to constitute a separate evolutionary lineage that branched much earlier than the separation of many crustaceans and insects and independently adapted to life on land. Therefore, the taxon Hexapoda, as commonly defined to include all six-legged arthropods, is not monophyletic.

To understand the processes of invasions by alien insects is a pre-requisite for improving management. The whitefly Bemisia tabaci is a cryptic species complex that contains some of the most invasive pests worldwide. However, extensive field data to show the geographic distribution of the members of this species complex as well as the invasion by some of its members are scarce. We used field surveys and published data to assess the current diversity and distribution of B. tabaci cryptic species in China and relate the indigenous members to other Asian and Australian members of the complex. The survey covered the 16 provinces where indigenous B. tabaci occur and extends this with published data for the whole of China. We used molecular markers to identify cryptic species. The evolutionary relationships between the different Asian B. tabaci were reconstructed using Bayesian methods. We show that whereas in the past the exotic invader Middle East-Asia Minor 1 was predominant across China, another newer invader Mediterranean is now the dominant species in the Yangtze River Valley and eastern coastal areas, and Middle East-Asia Minor 1 is now predominant only in the south and south eastern coastal areas. Based on mtCO1 we identified four new cryptic species, and in total we have recorded 13 indigenous and two invasive species from China. Diversity was highest in the southern and southeastern provinces and declined to north and west. Only the two invasive species were found in the northern part of the country where they occur primarily in protected cropping. By 2009, indigenous species were mainly found in remote mountainous areas and were mostly absent from extensive agricultural areas. Invasions by some members of the whitefly B. tabaci species complex can be rapid and widespread, and indigenous species closely related to the invaders are replaced.

We present the complete 15,455-nt mitochondrial DNA sequence of the springtail Tetrodontophora bielanensis (Arthropoda, Hexapoda, Collembola). The gene content is typical of most metazoans, with 13 protein-coding genes (PCGs), 2 genes encoding for ribosomal RNA subunits, and 22 tRNA genes. The nucleotide sequence shows the well-known A+T bias typical of insect mtDNA; its A+T content is lower (72.7%) than that observed in other insect species, but still higher than that in other arthropodan taxa. The bias appears to be uniform across the whole molecule, unlike other insect taxa, which show increased A+T content in the so-called A+T-rich region. However, the bias is slightly higher in the third codon positions of the PCGs (81.4%). Anomalous initiation codons have been observed in the nad2 and the cox1 genes. In the latter, the ATTTAA hexanucleotide is suggested to be involved in the initiation signaling. All tRNAs could be folded into the typical cloverleaf secondary structure, but the tRNA for cysteine appears to be missing the DHU arm. Long tandemly repeated regions (193 nt) were found in the A+T-rich region, which in turn was shown to have the possibility of forming a complex array of secondary structures. One of these structures encompassed the junction between the repeats. The A+T-rich region was also interesting in that it showed heteroplasmy in the number of repeats. Three haplotypes were found, possessing 2, 3, and 4 identical repeats, respectively. The order of protein coding and rRNA genes in the molecule was determined and was identical to that of all insects studied so far. However, two tRNA translocations were found which were unprecedented among Arthropoda. These involved the trnQ, which was found between the rrnS and the A+T-rich region, and the trnS(ucn), which was located between trnM and trnI. A preliminary phylogenetic analysis based on the amino acid sequence of the PCGs failed to find support for the monophyly of Hexapoda.

Background: The phylogeny of Arthropoda is still a matter of harsh debate among systematists, and significant disagreement exists between morphological and molecular studies. In particular, while the taxon joining hexapods and crustaceans (the Pancrustacea) is now widely accepted among zoologists, the relationships among its basal lineages, and particularly the supposed reciprocal paraphyly of Crustacea and Hexapoda, continues to represent a challenge. Several genes, as well as different molecular markers, have been used to tackle this problem in molecular phylogenetic studies, with the mitochondrial DNA being one of the molecules of choice. In this study, we have assembled the largest data set available so far for Pancrustacea, consisting of 100 complete (or almost complete) sequences of mitochondrial genomes. After removal of unalignable sequence regions and highly rearranged genomes, we used nucleotide and inferred amino acid sequences of the 13 protein coding genes to reconstruct the phylogenetic relationships among major lineages of Pancrustacea. The analysis was performed with Bayesian inference, and for the amino acid sequences a new, Pancrustacea-specific, matrix of amino acid replacement was developed and used in this study.Results: Two largely congruent trees were obtained from the analysis of nucleotide and amino acid datasets. In particular, the best tree obtained based on the new matrix of amino acid replacement (MtPan) was preferred over those obtained using previously available matrices (MtArt and MtRev) because of its higher likelihood score. The most remarkable result is the reciprocal paraphyly of Hexapoda and Crustacea, with some lineages of crustaceans (namely the Malacostraca, Cephalocarida and, possibly, the Branchiopoda) being more closely related to the Insectas.s. (Ectognatha) than two orders of basal hexapods, Collembola and Diplura. Our results confirm that the mitochondrial genome, unlike analyses based on morphological data or nuclear genes, consistently supports the non monophyly of Hexapoda.Conclusion: The finding of the reciprocal paraphyly of Hexapoda and Crustacea suggests an evolutionary scenario in which the acquisition of the hexapod condition may have occurred several times independently in lineages descending from different crustacean-like ancestors, possibly as a consequence of the process of terrestrialization. If this hypothesis was confirmed, we should therefore re-think our interpretation of the evolution of the Arthropoda, where terrestrialization may have led to the acquisition of similar anatomical features by convergence. At the same time, the disagreement between reconstructions based on morphological, nuclear and mitochondrial data sets seems to remain, despite the use of larger data sets and more powerful analytical methods.