16
FISHERIES SCIENCE
K Tabata and N Taniguchi
similarly found such a difference in detection ability
between direct sequencing and restriction analysis. Ac-
cordingly, when data are compared, it is important that
all the data were obtained by the same method.
Japanese red sea bream, Taniguchi et al. concluded that
isozyme genes had a low diversification, a level equiva-
lent to a subspecies status.6
The difference in the bump index between the red sea
bream and the snapper was significant, although no sig-
nificant differences were found when comparing other
external morphological characteristics. The latter results
are in agreement with Akazaki4 and Paulin.5 The differ-
ence in the bump between the red sea bream and the
snapper could be presented in a quantitative manner.
Even though this difference may not be clear in younger
fish, it becomes progressively clearer in older fishes since
the bump of the snapper increases with fish growth.29,30
To summarize our results, genetic differences at the
DNA level were distinguishable between the red sea
bream and the snapper. Judging from the RFLP and
sequencing analysis, however, these genetic differences
were low when considered at an interspecific level, but
were high at an interpopulational level. Moreover, sig-
nificant differences were observed in the bump phase,
although no other morphological differences could be
established.
According to Ovenden, the intraspecific mtDNA
sequence divergence by restriction analysis in marine
fishes ranges from 0.2 to 2.2%.22 Ovenden concluded
that it is more common for marine than for terrestrial
species to have a low divergence (i.e. <1.0%), although
two menhaden species, Brevoortia tyrannus and Brevoor-
tia partronus have high diversities. Gold and Richardson
documented that the intraspecific mtDNA sequence
divergence by restriction analysis of seven marine spe-
cies, including estuarine-dependent, reef-associated,
and pelagic species, was in the range 0.06–0.57%.23 The
restriction analysis of the D-loop region yielded the fol-
lowing data: 1.87% in the armorhead Pseudopentaceros
wheeleri9 and 0.47% in Evynis japonica (Tabata K, unpubl.
data). In contrast, sequencing of the D-loop region has
given the following results: 4.33% in Japanese flounder
Paralichthys olivaceus,13 3.8% in swordfish Xiphias gladius,24
and approximately 3% in ayu Plecoglossus altivelis,25 an
amphidromous species. The intraspecific mtDNA
sequence divergences of the red sea bream and the
snapper analyzed from restriction analysis in this study
were 0.9–1.1% and 0.4–1.0%, respectively, and 2.7–2.8%
and 2.3–3.1%, respectively, from sequencing. These
values might be average values.
The interspecific sequence divergences of mtDNA by
restriction analysis in marine, estuarine, and catadro-
mous species were in the range 3.7–13%,26 and the values
for salmonids in the range 2.7–7.5%.27 It should be noted
that these are not net values. The interspecific diver-
gences (1.3–1.6% as the value contained intraspecific
value, 0.57–0.67% as a net value) between the red sea
bream and the snapper obtained from restriction analy-
sis are considerably lower than the interspecific values
in some other marine species and in salmonids. For D-
loop sequencing, the interspecific sequence divergences
between salmonids are documented as 3.8–10.0% by
Shedlock et al.28 We should note, however, that the
sample number per species of this data28 is only 1 or 2.
Only when there are low intraspecific diversity, will these
data be effective. We think that comparison with the net
value will be appropriate when comparing the red sea
bream data having a high intraspecific diversity. The
interpopulational sequence divergence from the D-loop
sequencing between clades of swordfish is 2.7%,24 and
the values between amphidromous ayu and landlocked
ayu are 0.24–0.53%, all as net values.25 The interspecific
divergences (3.0–4.0% as net) between the red sea bream
and the snapper obtained from sequencing were less
than the lowest level of salmonid interspecific values
and slightly higher than the interpopulational values of
swordfish but much higher than the interpopulational
values in ayu. In addition, as regards the isozyme data
between the Australian/New Zealand snapper and the
Substitution rates and division time
Estimates of mtDNA substitution rates contained in the
gene-coded region range from 0.3 to 0.7% per million
years in ectotherms,31,32 to 1–2% per million years in ter-
restrial animals.33,34 However, estimates of control region
substitution rates ranged from 0.5% per million years in
cetaceans,35,36 to 21% per million years in birds.37 Thus,
the variation in the control region is large and depends
on the species and other factors such as metabolic
rates.31,38 Since the nucleotide substitution between the
red sea bream and the snapper obtained from the control
region was approximately twice that obtained from the
cytochrome b region (Tabata K, unpubl. data), the sub-
stitution rate from the control region may be assumed to
be 0.6–1.4% per million years. Accordingly, the division
time between the red sea bream and the snapper
would be assumed to be before 2–6 million years
(3.48%/0.6–1.4%/million years). However, if we take
into account that red sea bream is a warm-water species,
the substitution rate may be slightly higher.
Akazaki presented the following theory for the
historical dispersion of the Sparidae:39 the Sparidae
diverged from an ancestral of Perciformes several ten
million years between the Cretaceous to the Paleocene,
and, during this period, six subfamilies which included
the Pagrinae evolved. The center of distribution of
the Sparidae is considered to be the Old Mediterranean
Sea. The global dispersion of coastal fish generally
occurs associated with continental drift. The dispersion
of Pagrinae to the western Pacific Ocean occurred over
a very long period of time because global continental
drift did not occur in this area. The genus Pagrus that