*Biosciences Research Laboratory, Red River Valley Agricultural Research Center, Agricultural Research Service, United States Department of Agriculture, Fargo;
Department of Microbiology, Colorado State University
![]() |
Abstract |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The order of mitochondrial genes, especially the positions of tRNA genes, has been used to support a phylogenetic tree of the major arthropod groups. A unique location of tRNALeu(UUR)(L2) between the COI and COII genes is shared by the crustaceans and the insects and supports their sister taxa status. The ancestral position for tRNA(L2), between 16SrRNA and ND1, is found in chelicerates and myriapods and also some annelids, mollusks, and chordates (Boore et al. 1995
; Boore, Lavrov, and Brown 1998
; Boore 1999
). If rearrangements in mtDNAs are relatively rare in evolutionary history, these events can be useful synapomorphies. Convergent or parallel translocations and reversions in nonrelated lineages would be expected to be uncommon. Because more taxa are examined, new rearrangements are being discovered that have the potential to add detail to our view of arthropod phylogeny. Major rearrangements of mitochondrial genes have been observed in a subgroup of chelicerates, the metastriate ticks (Black and Roehrdanz 1998
; Campbell and Barker 1998, 1999
), in a crustacean, the hermit crab Pagurus (Hickerson and Cunningham 2000
), and in several hemipteroid insects, including the wallaby louse, thrips, and some psocopterans (Shao, Campbell, and Barker 2001; Shao et al. 2001
). Lesser rearrangements, involving the reshuffling of tRNA genes, have been documented in honey bees (Crozier and Crozier 1993
), other hymenopterans (Dowton 1999
; Dowton and Austin 1999
), mosquitoes (Beard, Hamm, and Collins 1993
; Mitchell, Cockburn, and Seawright 1993
), collembolans (Nardi et al. 2001
), and an anostracan crustacean (Valverde et al. 1994
).
We report the finding of three new mtDNA arrangements in the arthropods. The approach used long-polymerase chain reaction (PCR) and conserved insect-based primers in a manner described previously (Roehrdanz 1995
; Roehrdanz and Degrugillier 1998
). The rearrangements are detected as amplicons of altered size when compared with a standard insect mtDNA sequence. The validity of using this approach to detect major rearrangements is supported by sequence data that are consistent with the PCR results (Black and Roehrdanz 1998
). This technique provides an avenue for sampling a large number of taxa to find those with novel gene orders that would be prime candidates for a more detailed examination.
![]() |
Materials and Methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
The following mitochondrial primers were usedMET (TM-N-193) 5'-TGGGGTATGAACCCAGTAGCTT-3'; C1 (C1-J-2195) 5'-TTGATTTTTTGGTCATCCAGAAGT-3'; C2 (C2-N-3662) 5'-CCACAAATTTCTGAACATTGACC-3'; C2R (C2-J-3684) 5'-GGTCAATGTTCAGAAATTTGTGG-3'; N4-87 (N4-N-8487) 5'-TCAGCTAATATAGCAGCTCC-3'; N4 (N4-J-8944) 5'-GGAGCTTCAACATGAGCTTT-3'; CB2H (CB-N-10920) 5'-TCCTCAAAATGATATTTGTCCTCA-3'; N1 (N1-N-12595) 5'-GTAGCATTTTTAACTTTATTAGAACG-3'; 16S (LR-N-12868) 5'-TTACATGATCTGAGTTCAAACC-3'; 16SR (LR-J-12883) 5'-CTCCGGTTTGAACTCAGATC-3'; 16S2 (LR-N-12945) 5'-GCGACCTCGATGTTGGATTAA-3'; 16SA (LR-J-13417) 5'-ATGTTTTTGATAAACAGGCG-3'; 12SF (SR-J-14241) 5'-CGGGCGATGTGTACATAATT-3'; 12S (SR-N-14588) 5'-AAACTAGGATTAGATACCCTATTAT-3'; 12SR (SR-J-14612) 5'-AGGGTATCTAATCCTAGTTT-3'; ISO (TI-N-24) 5'-ATTTACCCTATCAAGGTAA-3'; ISOR (TI-J-42) 5'-TTACCTTGATAGGGTAAAT-3'; N1B (N1-J-12248) 5'-AAGCTAATCTAACTTCATAAG-3'; TL2 (TL2-N-3014) 5'-TCCAATGCACTAATCTGCCATATTA-3'.
The primer positions are defined by their 3' end in the Drosophila yakuba (Dya) mtDNA sequence (Clary and Wolstenholme 1985
). All the primers, except C2R which is a direct complement of C2, are described and referenced in the Appendix of Simon et al. (1994)
. Primers were synthesized by NBI (Plymouth, Minn.). Amplification products are described with the abbreviations of their flanking primers.
The mtDNA of Dya serves as a reference because many of the primers were derived from its sequence. Its arrangement of the large coding regions is the same as the arrangement of the presumed ancestral arthropod (also found in Limulus, Ixodes, Artemia, and Penaeus). Mitochondrial genomes with the same arrangement of major coding regions would be expected to return amplicons of approximately the same size as Dya. Because the primer combinations often span several genes, major rearrangements of the mtDNA relative to the standard insect gene order would show up as amplified products, with significant size differences relative to Dya. In addition to the size variation of the amplified products, two other outcomes of the amplification can be informative. Rearrangements that reorient primers so that they point in the same direction would fail to amplify a product. Conversely, primers that are repositioned so that they are now aligned in opposite directions can allow the amplification of a new product. The former can only be used after the fact to support the case for a rearrangement, but the latter can be diagnostic for a certain rearrangement.
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
|
|
The other arachnid group, comprising the metastriate hard ticks, is significantly different (figs. 1C and 2
). The 12S-C2R, C2-N4, and MET-N4 amplicons are substantially smaller in each of the three genera of metastriate ticks than they are in Dya. In addition, the MET-12SR segment is much larger in the three tick genera. The inferred arrangement was confirmed in the previously published complete sequence of one of these six species, Rsa (Black and Roehrdanz 1998
).
In the centipede, ND1 has moved to the opposite side of ND4, leaving Cytb adjacent to 16SrRNA (figs. 1A and 2 ). There is not enough room between ND4 and Cytb for ND6, so it must also have moved, although the data do not assign it a position. The tRNA(M) has been inserted between ND1 and ND4. None of the four genes (ND1, tRNA(M), ND4, Cytb) was inverted in the process. The space between COII and ND1 can accommodate the Atp8/6-COIII-ND3-ND5 gene block, and there is adequate space to the right of 16SrRNA for the 12SrRNA-control region-ND2 block, so there is no requirement for additional rearrangements; however, they cannot be ruled out.
Fewer amplified products were obtained from the millipede, but the results suggest another reshuffling of the arthropod mtDNA gene organization (figs. 1B and 2 ). The 16S-ND4 distance is similar to the basal arthropod, whereas the small MET-16SA segment is consistent with tRNA(M) being relocated to a new position between 12SrRNA and 16SrRNA. Cytb is now to the left of ND4 on the map. The ISO-C2 and ISOR-12SF amplicons indicate that tRNA(I) has a reverse orientation relative to most arthropods. The size of ISO-C2 suggests that ND2 is not located between tRNA(I) and COI.
The amplicons spanning the recovered portion of the isopod mtDNA total only about 8.6 kb and do not contain any large stretches devoid of defined genes (figs. 1D and 2 ). ND1 and possibly Cytb move close to COII. The 12SrRNA and 16SrRNA genes are inverted relative to ND1. As in the millipede, tRNA(M) is inserted between 12SrRNA and 16SrRNA. ND4 has moved closer to 16SrRNA and retains its orientation relative to that gene, which means it is inverted relative to the COI-ND1 block. No amplification products were recovered that span the long region from ND4 to COI.
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The primers were chosen because they had been successfully used with insects. However, even in insects it was impossible to predict whether a particular primer pair would be successful from one insect species to the next (Roehrdanz and Degrugillier 1998
). This situation is exacerbated when insect primers are applied to a broad range of arthropods.
The long-PCR method described here was used to obtain the material for sequencing tick mtDNA. The PCR results, from the beginning, indicated that ND4 and the 12S-16SrRNA genes had exchanged places relative to the rest of the mtDNA in the metastriate ticks (figs. 1C and 2
). This did not match the known patterns of mtDNA organization in arthropods. The complete sequence of the metastriate tick, Rsa, mtDNA confirmed our interpretation based on long-PCR data and added some additional details, e.g., tRNA shuffles and duplicated control region (Black and Roehrdanz 1998
).
The centipede included a translocation of tRNA(M) and translocations that result in new neighbors for at least four other genes, ND4, ND6, Cytb, and ND1. Although the position of ND6 was not fixed, there does not seem to be enough room for it to be retained between ND4 and Cytb. At minimum, it seems two more translocation events would be required to arrive at the final arrangement. As an example, translocation of ND1 to a position immediately to the left of ND4 would give the order ND1-ND4-ND6-Cytb-16SrRNA. A translocation of ND6 to another site would leave the ND1-ND4-Cytb-16SrRNA that was observed. Other scenarios that either move different genes or move them in a different sequence but arrive at the same final arrangement of genes also involve at least two additional translocations.
The millipede has two identified translocations of tRNAs. As in the centipede, tRNA(M) has moved. Here, it has inserted between the 12S- and 16SrRNAs. It retains the basal arthropod orientation with respect to the rRNA genes, which means that it would be transcribed from the opposite DNA strand as the 12S- and 16SrRNA genes. tRNA(I) has moved closer to COI and has switched strands in the process. A very similar position for tRNA(I) has been reported for the crustacean, Artemia franciscana, where both tRNA(I) and tRNA(Q) have moved to the opposite side of ND2, with tRNA(I) being inverted in the process (Valverde et al. 1994
). The position of tRNA(I) relative to ND2 was not determined here. Also, in the millipede, Cytb has been relocated to the other side of ND4. Because the distance between ND4 and 16SrRNA is relatively unchanged, there must have been a corresponding movement of a similar sized block of DNA in the opposite direction. The double switch means that at least two translocations are required, combined with the two tRNA translocations; a minimum of four translocations must have occurred to arrive at the current gene arrangement.
The isopod rearrangement is both complicated and the least well defined. The small TL2-C1 amplicon indicates that this isopod has tRNA(L2) between COI and COII that has been found in nearly all insects and crustaceans. The isopod has tRNA(M) inserted between 12SrRNA and 16SrRNA, the same position observed for the millipede. An approximation of the isopod order for this half of the mt genome can be obtained with a translocation and two inversions, in addition to the tRNA(M) translocation. Some information from the other half of the mtDNA is needed for a better understanding of how this arrangement might have originated.
The results here indicate two new positions for tRNA(M). In most arthropods tRNA(M) is one of a trio of tRNAs found between the control region and ND2 with the arrangement, control-tRNA(I)-tRNA(Q)-tRNA(M)-ND2. Rare exceptions have occurred by the addition or subtraction of tRNAs other than tRNA(M), by the repositioning of tRNA(I)-tRNA(M)-ND2 between ND3 and ATP6 in the hermit crab, and the complete reassortment of these genes in the wallaby louse. In the centipede we have found tRNA(M) situated between ND1 and ND4, a position not yet found in any other arthropod. We also found tRNA(M) between the 12S and 16SrRNA genes in two diverse groups, the diplopod millipede and the crustacean isopod (fig. 3
). This second arrangement raises the possibility of convergent evolution, that the same translocation occurred more than once in arthropod history. The orientation of tRNA(M) relative to 12S and 16SrRNA is the same for both events. However, the PCR results do not prove that the two translocations are identical. All other arthropods, except the wallaby louse, have the order 16SrRNA-tRNA(V)-12SrRNA. The tRNA(M) could have inserted on either side of tRNA(V), or tRNA(V) could have moved elsewhere when tRNA(M) arrived, making the two translocations nonidentical. Duplication of part or all of tRNA(M) or the reassignment of tRNAs also should be considered. Precedent for duplication exists with identification of two slightly different copies of tRNA(M) in the mollusc, Mytilus (Hoffmann, Boore, and Brown 1992
). In a PCR assay, only the 20+ bases of the primer binding site need to be present to favor amplification of shorter products.
|
The isopods are a diverse group that have marine, freshwater, and terrestrial forms. The example used here is one of the terrestrial isopods (Oniscidea). RFLP results from another terrestrial isopod species have indicated that the mt gene array exists in two physical forms. One form is linear, and the other is a circular dimer composed of two monomers linked in opposite directions (i.e., head-to-head and tail-to-tail). Both forms apparently coexist in cells (Raimond et al. 1999
). These conformations could hinder PCR amplification across a long stretch of the isopod DNA and offer one explanation for our failure to amplify a large block of the isopod mtDNA. Additional isopods need to be examined to determine if our novel gene arrangement of isopod mtDNA is restricted to the oniscideans or is representative of a broader array of isopods or malacostracans.
Mitochondrial gene order may be more labile than previously believed which calls for caution about putting too much phylogenetic weight on any one arrangement. Figure 3
shows that the major rearrangements discovered to date are nearly all out at the tips of the tree branches. Some of this is an artifact of limited sampling. However, only the tRNA(L2) translocation is verifiably on an internal branch of the arthropod tree. If the tRNA(M) translocations of the diplopod and isopod prove to be identical, they would most likely represent convergent events in the tree. The new arrangements described here remain out at the ends of branches until proven otherwise. What is striking is that such divergent taxa as the prostriate tick, Ixodes, and the horseshoe crab, Limulus, are identical, whereas major rearrangements appear in the metastriate ticks. Similarly, the gene array for the malacostracan decapod shrimp, Penaeus, matches the base insect-crustacean pattern and is identical to an insect, Dya (Wilson et al. 2000
), yet eight rearrangements are found in another decapod hermit crab, Pagurus (Hickerson and Cunningham 2000
), seven completely different rearrangements have been found in a group of braconid wasps (Dowton 1999
; Dowton and Austin 1999
), and all the tRNA genes and nine protein coding genes have been reordered in the wallaby louse, Heterodoxus (Shao, Campbell, and Barker 2001; Shao et al. 2001
). Heterodoxus also exhibits an autapomorphy as the first example of an insect-crustacean, where tRNA(L2) is no longer adjacent to either COI or COII. It appears as if rearrangements are much more common in recent evolutionary history than they were eons ago when the major arthropod lineages diverged. This could reflect recent increases in the rate of gene rearrangement events, such as translocations or partial duplications followed by partial deletions. Alternately, it could be testimony to the rapid divergence of basal arthropod lineages. Although the focus here is on the putative new mt gene arrays, it should not be forgotten that representatives from two previously unexamined chelicerate taxa, soft ticks and opiliones, yielded PCR products that are consistent with the ancestral arthropod order.
Long-PCR can be used to give a quick indication of gross mt gene rearrangements by surveying a large number of taxa. Detailed sequencing should then be performed on selected taxa to confirm the details of the inferred novel arrangements. Our sequencing of the tick mtDNAs is an example where this was done. In another instance we reported some PCR anomalies, including a larger than expected C1-C2 amplicon in Thrips sp. (Thysanoptera) that suggested a possible rearrangement in that insect (Roehrdanz and Degrugillier 1998
). Shao et al. (2001)
sequenced this region from Thrips and found ND3 inserted between COI and COII, an arrangement consistent with our larger PCR product. These follow-ups are essential because there is always the possibility that a PCR reaction can amplify a nonspecific target or nuclear resident mt gene copies. Although the insect conserved primers used here are widely available and thus convenient to use, the efficiency of the PCR could probably be improved by designing new primers closer to the taxon of interest. Confirmed sequences could serve as the basis for new primers to detect the new array in related taxonomic groups without extensive sequencing. In addition to describing new gene arrangements, large amplicons have been used for nucleotide sequencing (Black and Roehrdanz 1998
; Hickerson and Cunningham 2000
) and are ideal for RFLP analysis at the lower taxonomic and population levels (Szalanski et al. 1999
; Roehrdanz 2001
).
![]() |
Acknowledgements |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
Footnotes |
---|
Keywords: mtDNA
long-PCR
gene order
arthropod phylogeny
Address for correspondence and reprints: Richard L. Roehrdanz, Biosciences Research Laboratory, 1605 Albrecht Blvd., Fargo, North Dakota 58105. roehrdar{at}fargo.ars.usda.gov
.
![]() |
References |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Beard C. B., D. M. Hamm, F. H. Collins, 1993 The mitochondrial genome of the mosquito Anopheles gambiae, DNA sequence, genome organization and comparisons with mitochondrial sequences of other insects Insect Mol. Biol 2:103-124[Medline]
Black W. C. IV,, J. S. H. Klompen, J. E. Keirans, 1997 A phylogeny of hard and soft tick taxa based on mitochondrial 16S ribosomal DNA sequences Mol. Phylo. Evol 7:129-144[ISI][Medline]
Black W. C. IV,, R. L. Roehrdanz, 1998 Mitochondrial gene order is not conserved in arthropods: prostriate and metastriate tick mitochondrial genomes Mol. Biol. Evol 15:1772-1785
Boore J. L., 1999 Animal mitochondrial genomes Nucleic Acids. Res. 27:17671780 [this review is updated at the web site: http://biology.lsa.umich.edu/jboore/]
Boore J. L., T. M. Collins, D. Stanton, L. L. Daehler, W. M. Brown, 1995 Deducing the pattern of arthropod phylogeny from mitochondrial DNA rearrangements Nature 376:163-165[ISI][Medline]
Boore J. L., D. V. Lavrov, W. M. Brown, 1998 Gene translocation links insects and crustaceans Nature 392:667-668[ISI][Medline]
Campbell N. J. H., S. C. Barker, 1998 An unprecedented major rearrangement in an arthropod mitochondrial genome Mol. Biol. Evol 15:1786-1787
. 1999 The novel mitochondrial gene arrangement of the cattle tick, Boophilus micropuls: fivefold tandem repetition of a coding region Mol. Biol. Evol 16:732-740[Abstract]
Cheung W. Y., N. Hubert, B. Landry, 1993 A simple and rapid DNA microextraction method for plant, animal, and insect suitable for RAPD and other PCR analysis PCR Methods Appl 3:69-70[ISI][Medline]
Clary D. O., D. R. Wolstenholme, 1985 The mitochondrial DNA molecule of Drosophila yakuba: nucleotide sequence, gene organization, and genetic code J. Mol. Evol 22:252-271[ISI][Medline]
Crease T. J., 1999 The complete sequence of the mitochondrial genome of Daphnia pulex (Cladocera: Crustacea) Gene 233:89-99[ISI][Medline]
Crozier R. H., Y. C. Crozier, 1993 The mitochondrial genome of the honeybee Apis mellifera: complete sequence and genome organization Genetics 133:97-117
Curole J. P., T. D. Kocher, 1999 Mitogenomics: digging deeper with complete mitochondrial genomes Trends Ecol. Evol 14:394-398[ISI][Medline]
Dowton M., 1999 Relationships among the cylclostome braconid (Hymenoptera: Braconidae) subfamlies inferred from a mitochondrial tRNA gene rearrangement Mol. Phylogenet. Evol 11:283-287[ISI][Medline]
Dowton M., A. D. Austin, 1999 Evolutionary dynamics of a mitochondrial rearrangement "hot spot" in Hymenoptera Mol. Biol. Evol 16:298-309[Abstract]
Flook P. K., C. H. F. Rowell, G. Gellissen, 1995 The sequence, organization, and evolution of the Locusta migratoria mitochondrial genome J. Mol. Evol 41:928-941[ISI][Medline]
Garcia-Machado E., N. Dennebouy, M. O. Suarez, J. C. Mounolou, M. Monnerot, 1996 Partial sequence of the shrimp Penaeus notialis mitochondrial genome C. R. Acad. Sci. Paris 319:473-486[Medline]
Garcia-Machado E., M. Pempera, N. Dennebuoy, M. Oliva-Suaarez, J. C. Mounolou, M. Monnerot, 1999 Mitochondrial genes collectively suggest the paraphyly of Crustacea with respect to Insecta J. Mol. Evol 49:142-149[ISI][Medline]
Giribet G., S. Carranza, M. Riutort, J. Baguna, C. Ribera, 1999 Internal phylogeny of the Chilopoda (Myriapoda, Arthropoda) using complete 18S rDNA and partial 28S rDNA sequences Philos. Trans. R. Soc. Lond. B. Biol. Sci 29:215-222
Hickerson M. J., C. W. Cunningham, 2000 Dramatic mitochondrial gene rearrangements in the hermit crab Pagurus longicarpus (Crustacea, Anomura) Mol. Biol. Evol 17:639-644
Hoffmann R. J., J. L. Boore, W. M. Brown, 1992 A novel mitochondrial genome organization for the blue mussel, Mytilus edulis Genetics 131:397-412
Lavrov D. V., J. L. Boore, W. M. Brown, 2000 The complete mitochondrial DNA sequence of the horseshoe crab Limulus polyphemus Mol. Biol. Evol 17:813-824
Lessinger A. C., A. C. M. Junqueira, T. A. Lemos, E. L. Kemper, F. R. Da Silva, A. L. Vettore, P. Arruda, A. M. L. Azerado-Espin, 2000 The mitochondrial genome of the primary screwworm fly Cochliomyia hominivorax (Dipter: Calliphoridae) Insect Mol. Biol 9:521-529[ISI][Medline]
Lewis D. L., C. L. Farr, L. S. Kaguni, 1995 Drosophila melanogaster mitochondrial DNA: completion of the nucleotide sequence and evolutionary comparisons Insect Mol. Biol 4:263-278[ISI][Medline]
Mitchell S. E., A. F. Cockburn, J. A. Seawright, 1993 The mitochondrial genome of Anopheles quadrimaculatus species A: complete nucleotide sequence and gene organization Genome 36:1058-1073[ISI][Medline]
Nardi F., A. Carapelli, P. P. Fanciulli, R. Dallai, F. Frati, 2001 The complete mitochondrial DNA sequence of the basal hexapod Tetrodontophora bielanensis: evidence for heteroplasmy and tRNA translocations Mol. Biol. Evol 18:1293-1304
Norris D. E., J. S. H. Klompen, W. C. Black IV., 1999 Comparison of the mitochondrial 12S and 16S ribosomal DNA genes in resolving phylogenetic relationships among hard ticks (Acari: Ixodidae) Ann. Entomol. Soc. Am 92:117-129[ISI]
Raimond R., I. Marcade, D. Bouchon, T. Rigaud, J.-P. Bossy, C. Souty-Grosset, 1999 Organization of the large mitochondrial genome in the isopod Armadillidium vulgare Genetics 151:203-210
Roehrdanz R. L., 1995 Amplification of complete insect mitochondrial genome in two easy pieces Insect Mol. Biol 4:169-172[ISI][Medline]
. 2001 Genetic differentiation of southeastern boll weevil and thurberia weevil populations of Anthonomus grandis (Coleoptera: Curculionidae) using mitochondrial DNA Ann. Entomol. Soc. Am 94:928-935[ISI]
Roehrdanz R. L., M. E. Degrugillier, 1998 Long sections of mitochondrial DNA amplified from fourteen orders of insects using conserved polymerase chain reaction primers Ann. Entomol. Soc. Am 91:771-778[ISI]
Shao R., N. J. H. Campbell, S. C. Barker, 2001 Numerous gene rearrangements in the mitochondrial genome of the wallaby louse, Heterodoxus macropus (Phthiraptera) Mol. Biol. Evol 18:858-865
Shao R., N. J. H. Campbell, E. R. Schmidt, S. C. Barker, 2001 Increased rate of gene rearrangement in the mitochondrial genomes of three orders of hemipterid insects Mol. Biol. Evol 18:1828-1832
Simon C., F. Frati, A. Beckenbach, B. Crespi, H. Liu, P. Flook, 1994 Evolution, weighting, and phylogenetic utility of mitochondrial gene sequences and a compilation of conserved polymerase chain reaction primers Ann. Entomol. Soc. Am 87:651-701[ISI]
Spanos L., G. Koutroumbas, M. Kostyfakis, C. Louis, 2000 The mitochondrial genome of the Mediterranean fruit fly, Ceratitis capitata Insect Mol. Biol 9:139-144[ISI][Medline]
Staton J. L., L. L. Daehler, W. M. Brown, 1997 Mitochondrial gene arrangement of the horseshoe crab Limulus polyphemus L.: conservation of major features among arthropod classes Mol. Biol. Evol 14:867-874[Abstract]
Szalanski A. L., R. L. Roehrdanz, D. B. Taylor, L. D. Chandler, 1999 Genetic variation in geographical populations of Western and Mexican Corn Rootworm Insect Mol. Biol 8:519-526[ISI][Medline]
Valverde J. R., B. Batuecas, C. Moratilla, R. Marco, R. Garesse, 1994 The complete mitochondrial DNA sequence of the crustacean Artemia franciscana J. Mol. Evol 39:400-408[ISI][Medline]
Van Raay T. J., T. J. Crease, 1994 Partial mitochondrial DNA sequence of the crustacaean Daphnia pulex Curr. Genet 25:66-72[ISI][Medline]
Wilson K., V. Cahill, E. Ballment, J. Benzie, 2000 The complete sequence of the mitochondrial genome of the crustacean Penaeus monodon: are malacostracan crustaceans more closely related to insects than to branchiopods? Mol. Biol. Evol 17:863-874