Australian Institute of Marine Science, Townsville, Queensland, Australia
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Abstract |
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Introduction |
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The giant tiger prawn, Penaeus monodon, is the major species of shrimp farmed in aquaculture worldwide. The Penaeidae fall within the order Decapoda (class Malacostraca), which contains most commercially important crustacean species. Analyses of penaeid mitochondrial genetics have been carried out using RFLP analysis and single-gene sequence analysis (Palumbi and Benzie 1991
; Bouchon, Souty-Grosset, and Raimond 1994
; Klinbunga et al. 1999
). However, there is no complete mitochondrial genome sequence available for any decapod, or even for any malacostracan crustacean. To date, the only complete crustacean mitochondrial genome sequences available are those of two branchiopods: the brine shrimp Artemia franciscana (Branchiopoda, Anostraca) (Valverde et al. 1994
) and the water flea Daphnia pulex (Branchiopoda, Cladocera) (Crease 1999
). Thus, there is a dearth of mitochondrial sequence data available for phylogenetic comparisons within the Crustacea and related groups.
Recently, partial sequence (8,754 bp) was published for the southern pink shrimp, Penaeus notialis (García-Machado et al. 1999
). This sequence encompassed seven protein- coding genes, and the aligned amino acid sequences of this species, Artemia, and four insects were used to make inferences about phylogeny within the Arthropoda. The resulting analysis supported a paraphyletic Crustacea, with Penaeus being more closely related to insects than to the branchiopod crustacean, Artemia. However, this analysis was based on only partial mitochondrial sequence from P. notialis and a single branchiopod, A. franciscana, as the complete mitochondrial sequence of D. pulex was not then available for inclusion in the analysis. Moreover, the mtDNA of Artemia appears to be evolving at an accelerated rate (Crease 1999
; see fig. 5
), which could affect molecular phylogenetic inference (Hillis, Moritz, and Mable 1996
).
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Materials and Methods |
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Analysis of Codon Bias
Potential amino acid bias due to underlying nucleotide bias was analyzed using the "square plot" analysis of Foster, Jermiin, and Hickey (1997)
. In brief, this analyzes the amino acid composition by looking at the proportion of amino acids with codons that have (1) A or T in both the first and the second positions (2) A or T in either the first or the second position, and (3) only G or C in these two positions. The amino acid leucine was excluded from this analysis, as it can be encoded by codons from two different classes (TTR and CTR) and hence can accommodate changes in underlying base composition without a change in encoded amino acids. The data are presented here as a histogram rather than as square plots to show underlying trends across the Arthropoda.
Phylogenetic Analyses
Protein-coding sequences were individually aligned using Pileup (Genetics Computer Group 1994
) with the gap creation penalty set to 5.0 and the gap extension penalty set to 0.3. Sequences were from 10 of the 11 arthropods for which complete mitochondrial genome sequences were available in GenBank: among the insects (Hexapoda), the dipteran flies Drosophila yakuba (X03240, Hexapoda, Insecta, Diptera), Drosophila melanogaster (U37541, Hexapoda, Insecta, Diptera), Ceratitis capitata (CCA242872, Hexapoda, Insecta, Diptera), Anopheles quadrimaculatus (L04272, Hexapoda, Insecta, Diptera), and Anopheles gambiae (L20934, Hexapoda, Insecta, Diptera) and the orthopteran Locusta migratoria (X80245, Hexapoda, Insecta, Orthoptera); the branchiopod crustaceans A. franciscana (X69067, Crustacea, Branchiopoda, Anostraca) and Daphnia pulex (AF117817, Crustacea, Branchiopoda, Cladocera); and from the Chelicerata, the prostriate tick Ixodes hexagonus (AF081828, Chelicerata, Arachnida, Acari) and the metastriate tick Rhipicephalus sanguineus (AF081829, Chelicerata, Arachnida, Acari). The honeybee, Apis mellifera (L06178, Hexapoda, Insecta, Hymenoptera), was not included in the analysis due to the extreme nucleotide bias which confounds phylogenetic inference based on both nucleotide and protein sequence analyses (Galtier and Gouy 1995
; Foster, Jermiin, and Hickey 1997
). The earthworm Lumbricus terrestris (U24570, Annelida, Oligochaeta) was used as an outgroup. Internal sections of nad2 and nad4 were excluded from the analysis due to severe ambiguities in alignment. In addition, all sequences were trimmed at the N- and C-terminal ends to give an exact and unambiguous alignment. The alignments were then concatenated to give a single multiple sequence alignment containing 3,327 characters (2,335 variable and 1,687 parsimony- informative) for phylogenetic analysis. The alignments generated are available from the corresponding author on request.
The aligned sequences were subjected to distance analysis using MEGA 1.01 (Kumar, Tamura, and Nei 1993
) and PHYLIP 3.572c (Felsenstein 1993
), parsimony analysis using PAUP 3.1.1 (Swofford 1993
), and maximum-likelihood analysis using MOLPHY 2.3b (Adachi and Hasegawa 1996b
). Distances between taxa were estimated in the program MEGA using the number of differences, p-distance, the Poisson correction, and gamma probabilities with gamma set to 1 or 2. Trees were then constructed using neighbor joining. Bootstrap probabilities were estimated with 1,000 replications using pairwise deletion for gaps and missing data. Distance analyses were also carried out with the PHYLIP package using the Dayhoff PAM matrix for amino acid substitutions and neighbor-joining tree building. Bootstrap probabilities were estimated in PHYLIP with 100 replicates. For parsimony analysis, the heuristic search option in PAUP 3.1.1 was used. A simple addition sequence was used and tree bisection-reconnection branch swapping was performed. Bootstrap values were estimated using 1,000 replications. For both distance and parsimony analyses, L. terrestris was defined as the outgroup. For maximum-likelihood analysis, the mtREV-24 model of amino acid substitution in mitochondrial protein-coding genes was used (Adachi and Hasegawa 1996a
), and the data were analyzed using both the star decomposition method and local rearrangement options. For the latter, the 200 best trees using stepwise addition were first generated, and these tree topologies were then subjected to local rearrangements to determine the maximum-likelihood tree.
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Results |
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Gene Content and Order
The gene content was identified primarily by alignment with the known genes from D. yakuba, D. melanogaster, and A. gambiae. In addition, tRNA genes and their anticodons were detected using the program tRNAscan (Lowe and Eddy 1997
). Like all of the other arthropod mtDNAs, the P. monodon mitochondrial genome contains genes for 13 proteins, 22 tRNAs, and two rRNAs. In addition, there is a 991-bp-long A+T-rich region that does not contain any known genes. As the origin of replication of the Drosophila mtDNA maps in the corresponding region in the Drosophila mtDNA genome, this region is presumed to be the equivalent of the vertebrate control region (Wolstenholme 1992
). The order and arrangement of the genes in P. monodon is illustrated in figure 1
. It is identical to that observed in the insects D. melanogaster, D. yakuba, and C. capitata and in the branchiopod D. pulex.
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Coding Sequences
The P. monodon mitochondrial genome appears to use the same genetic code as Drosophila. It is certainly the case that UGA specifies an amino acid rather than a stop codon, since otherwise the majority of proteins would show premature termination. It is also reasonable to presume that the amino acids specified by each codon family are the same as in Drosophila, as use of this code gives rise to the expected open reading frames with a high degree of similarity to the same proteins identified from Drosophila and other arthropods. However, this could be formally proven only by purification and sequencing of P. monodon mitochondrial proteins.
The location and putative initiation and termination codons of the protein-coding sequences are shown in table 1
. Twelve out of the 13 protein coding genes appear to initiate with the codon ATN. However, as is the case with several other arthropod species, the initiation codon of cox1 is unclear. In P. monodon, it appears to start with ACG based on alignment with other species. In Drosophila, Locusta, and Daphnia, there is a tetranucleotide sequence (ATAA or ATTA) immediately preceding the apparent first codon, and it has been suggested that this tetranucleotide could serve as the initiation codon (Clary and Wolstenholme 1983
; de Bruijn 1983
). However, no such sequence is present in P. monodon, and we are not aware of any model indicating how a tetranucleotide might function in this capacity. The absence of such a tetranucleotide has also been observed in other arthropods, e.g., A. gambiae (Beard, Mills, and Collins 1993
), leading to the conclusion that each of the disparate array of first codons in aligned cox1 genes most likely does serve as the initiation codon. An alternative model to explain these atypical start codons is that they are converted to more conventional start codons in the mRNA; for example, in the tomato, the cox1 gene starts with an ACG codon, but this is converted to an AUG in the mRNA by RNA editing (Kadowaki et al. 1995
). The presumed initiation context of cox1 genes from arthropods, for which there is complete mitochondrial sequence, plus that of the penaeid shrimp P. notialis, for which there is partial mitochondrial sequence (GenBank accession numbers X84350X84357), is shown in figure 2
.
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tRNAs
The predicted structures of the P. monodon tRNAs are shown in figure 3
. All tRNAs have the typical cloverleaf structures of other mitochondrial tRNAs except for trnS(ucg), which recognizes the codon AGN. This latter tRNA lacks the dihydrouridine loop, as observed for this tRNA in all other arthropod mitochondrial genomes (Wolstenholme 1992
).
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Base Composition
The A+T content of the P. monodon genome is 70.6% (A, 5,636; C, 2,669; G, 2,028; T, 5,651 on the strand that contains the majority of open reading frames). The 991-bp control region has an AT composition of 81.5%. These values are lower than those for the published insect genomes (75.3%84.9% for total A+T content and 86.0%96.0% for the AT-rich control region), but higher than those for the crustaceans A. franciscana (64.5% total AT content, 68.0% AT in the control region) and D. pulex (62.3% total AT, 67.1% AT in the control region). Among the chelicerates, the horseshoe crab is 67.7% A+T in the 8,459 bp sequenced (Staton, Daehler, and Brown 1997
), and the ticks Ixodes and Rhipicephalus are 72.6% and 77.9% AT-rich, respectively.
There are a total of 3,716 codons in all 13 protein-coding genes, excluding stop codons. The overall AT composition of protein-coding regions is 69.3% but at third positions it is 83.7%. When the nucleotide composition of the third positions of fourfold-degenerate codons is examined, it is 84.9% AT (see table 2 ). This contrasts with much lower values of 68.7% in Artemia and 62.9% in Daphnia. The corresponding value for the partial sequence available for P. notialis (for the protein- coding genes atp6, atp8, cox1, cox2, cox3, nad2, nad3, and nad5) is 74.6%. Table 2 illustrates that these values for the two penaeid shrimp, representing the class Malacostraca, are intermediate between those for the branchiopod crustaceans and the insects. The percentage of A+T in the third positions of fourfold-degenerate codons for the two tick species is similar to that for the penaeids.
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The phylogenetic analysis used data from 11 of the 12 complete arthropod mitochondrial genomes available (10 from the database plus P. monodon). Sequence from the honeybee A. mellifera was omitted as the extreme AT bias in this species makes phylogenetic analyses even of protein-coding regions unreliable (Foster, Jermiin, and Hickey 1997
), as evidenced by the fact that A. mellifera clustered together with the ticks with a bootstrap value of 99100 when included in a similar phylogenetic analysis (Black and Roehrdanz 1998
). The earthworm L. terrestris (Boore and Brown 1995
) was used as the outgroup.
Three different tree structures resulted from application of different phylogenetic algorithms (fig. 5AC ). In all cases, the two chelicerate species form a clade which is quite distinct from a Hexapoda/Crustacea clade. In the first tree structure (fig. 5A ), Artemia and Daphnia form a clade separate from Hexapoda (represented by the class Insecta) and P. monodon; in the second (fig. 5B ), all Crustacea and Hexapoda form a single clade, with Penaeus being the crustacean most closely related to the Hexapoda and Artemia being the most distantly related. In the third tree, Penaeus and Hexapoda form one clade, and the relationship of Daphnia and Artemia to each other and to this clade remains unresolved. The most striking feature held in common by all three trees is the paraphyly of the Crustacea, with Penaeus being more closely related to the insects than to the branchiopods. This relationship of Penaeus to Insecta is supported by bootstrap values of >90% with all tree-building methods.
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Discussion |
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The gene order in the P. monodon mitochondrial genome is identical to that of the three dipteran species D. yakuba, D. melanogaster, and C. capitata, as well as to the branchiopod D. pulex. All other arthropod mtDNAs for which complete mitochondrial sequences are available show distinct differences in the placement of certain genes. The two Anopheles species differ in the placement of two tRNAs, Locusta in the placement of one tRNA, Apis in the placement of eight tRNAs, Artemia in the placement of the block of trnI to trnQ, and Ixodes in the placement of one tRNA, and Rhipicephalus shows major rearrangements. In the latter species, the block of genes from nad1 to trnQ appears to have transposed, and there are two further tRNA rearrangements.
Boore et al. (1995)
, and Boore and Brown (1998)
, and Boore, Lavrov, and Brown (1998)
argue that rearrangements of metazoan mtDNA are highly unlikely to be duplicated by convergences and hence can be used to infer phylogenetic lineages. These authors therefore conclude that the Crustacea and Insecta form a sister group, with the Chelicerata and Myriapoda occupying separate arthropod lineages (Boore, Lavrov, and Brown 1998
). The arrangement of genes in the P. monodon genome is in accord with this hypothesis. This arrangement is identical to that observed in the crustacean D. pulex and appears to be also conserved in the partially characterized genomes of the malacostracan crustaceans Homarus americanus (Boore et al. 1995
) and P. notialis (García-Machado et al. 1999
). Taken together, these data provide further support for the proposed relationship between Crustacea and Insecta (Boore, Lavrov, and Brown 1998
), and the conclusion that the drosophilid mitochondrial gene order represents the ancestral crustacean/insect mitochondrial gene order (Crease 1999
).
Phylogenetic relationships among the major arthropod groups were also investigated by sequence comparisons using concatenated protein-coding sequences. The resulting trees all gave strong support for several conclusions regarding arthropod phylogeny, although it should be noted that these trees are necessarily based on the restricted set of arthropod taxa for which complete mitochondrial sequences are currently available. In particular, the available sequences only represent three of the four extant arthropod subphyla, with no complete mitochondrial sequence from the Myriapoda being available in the database.
The first conclusion is that the Chelicerata form a separate clade from Crustacea/Hexapoda. This appears to be widely supported by molecular evidence on gene sequence (Friedrich and Tautz 1995
; Black and Roehrdanz 1998
), on mitochondrial gene order (Boore, Lavrov, and Brown 1998
), and on the molecular basis of development (Averof and Akam 1995
). It is also a view held by most morphological taxonomists (e.g., Snodgrass 1938
; Kukalová-Peck 1992
; Wheeler, Cartwright, and Hayashi 1993
), although there have been proposals that have placed crustaceans closer to chelicerates (Schram and Emerson 1991
; Briggs, Fortey, and Willis 1992
).
The second conclusion is that Crustacea are a paraphyletic group. This contradicts many phylogenetic trees proposed on the basis of morphological data (Snodgrass 1938
; Wheeler, Cartwright, and Hayashi 1993
). However, the concept of crustacean paraphyly has been proposed previously based on both morphological and molecular characters (Briggs and Fortey 1989
; Averof and Akam 1995
), although the details of the paraphyly may vary (see discussion below). The conclusion that Crustacea is paraphyletic with respect to Hexapoda was also drawn by García-Machado et al. (1999)
in their analysis of partial mitochondrial sequence of P. notialis.
The third conclusion relates to the relationship of the two classes of crustaceans that are now represented by complete mitochondrial genome sequences, the Malacostraca and the Branchiopoda, to each other and to the Hexapoda, represented here by the class Insecta. A corollary of several theories supporting the idea that the Crustacea are paraphyletic is that Hexapoda derive from a crustacean ancestor. If this is the case, then it is an inescapable conclusion that some Crustacea will be more closely related to Hexapoda than to other crustacean groups. This theory is consistent with the trees generated in the present analysis, which indicate that P. monodon is related to the Hexapoda (Insecta), with bootstrap support ranging from 92% to 100%. The exact relationship of the two branchiopods both to each other and to the Penaeus/insect clade is unclear, but in all cases our analysis indicates that the branchiopods are less closely related to insects than is the malacostracan P. monodon. This result was also obtained in the analysis of García-Machado et al. (1999)
, although their analysis included only one branchiopod, A. franciscana, which appears to be subject to accelerated evolution (Crease 1999
; see long branch lengths for Artemia in fig. 5
).
This conclusion on the relative relationships of Malacostraca/Branchiopoda/Insecta contradicts the results of two other molecular studies that included representatives of both crustacean classes. In the first, a combined analysis of the 18S and 28S nuclear ribosomal RNA genes gave strong support for a single crustacean/hexapod clade and weak support for a possible closer relationship of the branchiopod Artemia salina to Hexapoda than the malacostracan Procambarus clarkii (Friedrich and Tautz 1995
). In a more wide-ranging analysis using protein-coding sequence from the nuclear gene encoding EF1
, Regier and Schultz (1997, 1998)
propose that Crustacea is polyphyletic, with Branchiopoda forming a single clade with Hexapoda and Chelicerata being more closely related to this clade than Malacostraca.
The reasons for the disparity in phylogenetic trees obtained with the analysis of different molecules are unclear. One possibility is that the heterogeneity in the evolutionary processes occurring in the different arthropod lineages leads to a misleading result. This was certainly the case in the results of Black and Roehrdanz (1998)
, who carried out a similar analysis using concatenated protein sequences which placed A. mellifera in the same clade as the hard ticks with 99% bootstrap support. The reason for the latter result is the extreme AT bias in the Apis mitochondrial genome, which has a significant influence on the resulting amino acid composition of the encoded proteins (see fig. 4
). However, as figure 4
illustrates, although this bias is evident in the P. monodon mitochondrial genome, it is not as extreme as in the case of Apis or, indeed, any of the other insects. In fact, the degree to which amino acid composition is affected by codon bias appears to be no different for P. monodon than for A. franciscana, and only slightly higher than for D. pulex. Hence, it is unlikely to affect the relative positions of these three species on a phylogenetic tree with respect to each other or to Insecta. In addition, the proposed phylogenetic tree structure is also strongly supported by maximum-likelihood analysis. This method, although still susceptible to producing erroneous phylogenetic trees in cases of extreme bias in nucleotide or amino acid composition, is far less sensitive to small compositional differences than are distance and parsimony analyses (Galtier and Gouy 1995
).
A more likely explanation for the different trees is that each of these molecular phylogenies is based on analysis of only one or two genes or, in the case of the present paper, a set of genetically linked genes. There is always a danger that such analyses may give misleading results due to factors such as insufficient informative characters or convergent evolution.
A more far-fetched, but not impossible, scenario is that some horizontal transfer of either nuclear or mitochondrial sequences has occurred from insects to crustaceans or vice versa. As increasing amounts of data emerge from complete genome sequences, evidence suggests that the evolutionary origins of different groups of genes may differ. For example, Rivera et al. (1998)
recently demonstrated that some groups of genes in eukaryotes are derived from archaebacteria, whereas other groups appear to derive primarily from proteobacteria, leading to the conclusion that there may have been considerable horizontal gene transfer in the evolutionary process.
If the phylogeny suggested by this analysis of mitochondrial sequences is correct, this would have strong implications for the polarity of evolution of morphological characters used in the taxonomy of Crustacea and Hexapoda. In particular, it would imply that characters which are used to distinguish crustaceans from hexapods have been secondarily lost in hexapods, rather than being autapomorphies of Crustacea. However, as the Crustacea are notoriously diverse in body form (Schram 1986
) and the characters that serve to uniquely define them are few, it is possible to develop plausible scenarios by which this may have happened. In fact, evolutionary scenarios which would conform with the proposed phylogeny have been already developed on both morphological and molecular-developmental grounds (Averof and Akam 1995
; Telford and Thomas 1995
).
For example, one character which has been used to define Crustacea is the structure of the head. It has been described as having five segments, two preoral segments bearing the first and second antennae and three postoral segments bearing, respectively, the mandibles and the first and second maxillae (Cisne 1982
; Schram 1986
). Insects, in contrast, have been described as having six segments in the head, three preoral and three postoral (Lawrence, Nielsen, and Mackerras 1991
). However, the nature of the most anterior portion of the crustacean head was never clear (see Averof and Akam 1995
), and recent work indicates that both crustaceans and insects have six cephalic segments, as revealed by looking at the expression of genes that specify segmental identity (Popadic et al. 1998
; Scholtz 1998
). Another commonly cited character defining Crustacea is the presence of a second antenna on the second appendage-bearing segment of the head. However, it is easy to envisage secondary loss of such a structure in Insecta, and, indeed, some insects develop small appendages on this segment during the embryonic stage which could be vestiges on an earlier adult appendage (Tamarelle 1984
).
A third set of characters often deemed to be unique to Crustacea relate to embryology and larval development, namely, the nature of the embryonic fate map and the formation of a nauplius larva (larvae with three sets of appendages used for propulsion, corresponding to the antennules, antennae, and mandibles) or passage through an egg-nauplius phase (Anderson 1982
; Schram 1986
). However, more recent studies have contested Anderson's (1982)
claim that crustacean egg cleavage is spiral (Hertzler and Clark 1992
), and the wisdom of using a character as variable as fate maps in phylogenetic analysis has also been questioned (Averof and Akam 1995
). Moreover, as larval development has clearly undergone considerable diversification within Hexapoda, ranging from ametabolous insects that show no marked metamorphosis at all, to hemimetabolous insects with a gradual larval-to-adult transition, to the holometabolous insects which show complete metamorphosis from larval to adult forms (Chapman 1991
), it is certainly possible to envisage that the nauplius larval stage has been secondarily lost.
Other features which in the past have been used to argue that Crustacea and Hexapoda occupy distinct evolutionary lineages, namely, the origin of the mandibles (gnathobasic vs. whole-limb) and the nature of the limbs (uniramous vs. bi- or polyramous) have already effectively been discounted as being only apparent, rather than real, differences by a number of authors (Kukalová-Peck 1992
; Averof and Akam 1995
; Telford and Thomas 1995
; Scholtz, Mittmann, and Gerberding 1998
).
The proposed phylogeny also has implications for the relative affinities of different crustacean classes to each other and to Insecta. The divergence between the branchiopod clade and the malacostracan/insect clade is consistent with the fossil record. Although their exact affinities may be disputed, fossils with substantial similarities to both Branchiopoda and Malacostraca are known from the Cambrian (Dahl 1983
; Briggs 1983
), and these classes were certainly established as independent lineages well before the appearance of the first fossil insects in the Devonian (Kukalová-Peck 1991
). Moreover, there are certain features held in common between Malacostracan crustaceans and insects that are not shared with "simpler crustaceans," most notably the detailed structures of the compound eye and of the segmental nervous systems (Averof and Akam 1995
; Telford and Thomas 1995
; Osorio, Averof, and Bacon 1995
). The former has been alluded to as "one of the most disconcerting problems of arthropod phylogeny" by Tiegs and Manton (1958)
, who argued that Arthropoda were polyphyletic and hence that the malacostracan and insect eyes had evolved by convergence.
A question that the present paper cannot address is the relative position of the myriapods on the proposed phylogenetic tree, as no complete myriapod mitochondrial gene sequences were publicly available at the time of writing. Traditionally, it has been assumed that Hexapoda evolved from a myriapodlike ancestor and that Crustacea formed a sister group to this "Atelocerate clade" (Kristensen 1991
). However, almost all molecular analyses indicate that Crustacea, rather than Myriapoda, are sister to Insecta (Wheeler, Cartwright, and Hayashi 1993
; Friedrich and Tautz 1995
; Boore, Lavrov, and Brown 1998
), and arguments have also been put forward on morphological grounds, taking into account key features of Myriapoda, Hexapoda, and Crustacea, that make this scenario plausible (Averof and Akam 1995
; Osorio, Averof, and Bacon 1995
; Telford and Thomas 1995
).
One conclusion that is clear from the present paper and the recent completion of the D. pulex mitochondrial genome sequence (Crease 1999
) is that while identity of mitochondrial gene order may be a strong indicator of phylogenetic relatedness due to the low probability of convergence (Boore and Brown 1998
), differences in mitochondrial gene order do not necessarily reflect major phylogenetic differences. This is illustrated in the Arthropoda by the fact that two widely divergent crustaceans and three dipteran flies all share the same mitochondrial gene order, while those of other dipterans and hexapods differ in the placement of one or more tRNAs. Most strikingly, analysis of the mitochondrial genome of two hard ticks, which lie within the same chelicerate family, has revealed major mitochondrial rearrangements in four out of five subfamilies within that group (Black and Roehrdanz 1998
; Campbell and Barker 1998
). Thus, the various tRNA rearrangements present in the mitochondrial genomes of Anopheles, Locusta, Apis, and Artemia all appear to be characters that were derived after the initial radiation of hexapods and crustaceans.
In summary, the complete sequence of the malacostracan crustacean P. monodon gives further weight to the conclusion that the mtDNA gene order observed in D. yakuba is the ancestral crustacean/hexapod arrangement. Phylogenetic analysis comparing available arthropod mitochondrial genomes indicates a sister group relationship between Malacostraca and Insecta and hence throws further fat on the controversial fire of arthropod phylogeny. However, a more in-depth understanding of arthropod phylogeny will require the completion both of the sequence of a greater variety of arthropod mitochondrial genomes, most notably with the inclusion of some myriapod genomes, as well as a better understanding of the processes of mitochondrial evolution.
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Acknowledgements |
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Footnotes |
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1 Keywords: Malacostraca
complete mitochondrial genome sequence
arthropod phylogeny
crustacean/insect relationships
penaeid shrimp
2 Address for correspondence and reprints: Kate Wilson, Australian Institute of Marine Science, PMB 3, Townsville MC, Queensland 4810, Australia. E-mail: k.wilson{at}aims.gov.au
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