Pk. Flook et al., THE SEQUENCE, ORGANIZATION, AND EVOLUTION OF THE LOCUSTA-MIGRATORIA MITOCHONDRIAL GENOME, Journal of molecular evolution, 41(6), 1995, pp. 928-941
The sequencing of the cloned Locusta migratoria mitochondrial genome h
as been completed. The sequence is 15,722 bp in length and contains 75
.3% A+T, the lowest value in any of the five insect mitochondrial sequ
ences so far determined, The protein coding genes have a similar A+T c
ontent (74.1%) but are distinguished by a high cytosine content at the
third codon position, The gene content and organization are the same
as in Drosophila yakuba except for a rearrangement of the two tRNA gen
es tRNA(lys) and tRNA(asp). The A+T-rich region has a lower A+T nucleo
tide content than in other insects, and this is largely due to the pre
sence of two G+C-rich 155-bp repetitive sequences at the 5' end of thi
s section and the beginning of the adjacent small rRNA gene. The sizes
of the large and small rRNA genes are 1,314 and 827 bp, respectively,
and both sequences can be folded to form secondary structures similar
to those previously predicted for Drosophila. The tRNA genes have als
o been modeled and these show a strong resemblance to the dipteran tRN
As, all anticodons apparently being conserved between the two species.
A comparison of the protein coding nucleotide sequences of the locust
DNA with the homologous sequences of five other arthropods (Drosophil
a yakuba, Anopheles quadrimaculatus, Anapheles gambiae, Apis mellifera
, and Artemia franciscana) was performed. The amino acid composition o
f the encoded proteins in Locusta is similar to that of Drosophila, wi
th a Dayhoff distance twice that of the distance between the fruit fly
and the mosquitoes. A phylogenetic analysis revealed the locust genes
to be more similar to those of the Dipterans than to those of the hon
eybee at both the nucleotide and amino acid levels. A comparative anal
ysis of tRNA orders, using crustacean mtDNAs as outgroups, supported t
his. This high level of divergence in the Apis genome has been noted e
lsewhere and is possibly an effect of directional mutation pressure ha
ving resulted in an accelerated pattern of sequence evolution. If the
general assumption that the Holometabola are monophyletic holds, then
these results emphasize the difficulties of reconstructing phylogenies
that include lineages with variable substitution rates and base compo
sition biases. The need to exercise caution in using information about
tRNA gene orders in phylogenetic analysis is also illustrated, Howeve
r, if the honeybee sequence is excluded, the correspondence between th
e other five arthropod sequences supports the findings of previous stu
dies which have endorsed the use of mtDNA sequences for studies of phy
logeny at deep levels of taxonomy when mutation rates are equivalent.