697 
Preliminary study of albacore ( Thunnus alalunga ) 
stock differentiation inferred from 
microsatellite DNA analysis 
Motohiro Takagi 
Tetsuro Okamura 
Department of Cultural Fisheries 
Faculty of Agriculture 
Kochi University, Monobe 
Nankoku, Kochi 783-8502, Japan 
Seinen Chow 
National Research Institute of Far Seas Fisheries 
5-7-1 Orido 
Shimizu 424-8633, Japan 
Nobuhiko Taniguchi 
Department of Cultural Fisheries 
Faculty of Agriculture 
Kochi University, Monobe 
Nankoku, Kochi 783-8502, Japan 
Present address (for N Taniguchi, contact author): Graduate school of Agriculture 
Tohoku University 
1-1 Tsutsumidori, Amamiyamachi 
1-2 Sendai 981-8555, Japan 
E-mail address (for N. Taniguchi, contact author): nobuhiko@bios.tohoku.ac.|p 
The albacore (Thunnus alalunga ) is a 
highly migratory large pelagic tuna, 
common from tropical to temperate 
areas of all oceans, including the 
Mediterranean Sea. Although alba- 
core populations of each ocean or each 
hemisphere have been managed sepa- 
rately, relationships between albacore 
populations of northern and southern 
hemispheres within an ocean are con- 
troversial. Differences in morphome- 
try, movement, and catch statistics of 
albacore between northern and south- 
ern hemispheres within Atlantic and 
Pacific Oceans have been reported 
(Kurogane and Hiyama, 1958; Ishii, 
1965; Nakamura, 1969). Examining 
mtDNA variation, Chow and Ushiama 
(1995) detected very little genetic dif- 
ference between samples from north- 
ern and southern hemispheres. They 
proposed that minor gene flow may 
have occurred between albacore popu- 
lations of northern and southern hemi- 
spheres — enough to prevent genetic 
differentiation. However, it is also 
possible that insufficient time has 
elapsed since population subdivision 
for mtDNA genotypic rearrangement. 
Because all tuna species of the genus 
Thunnus are thought to be phylogenet- 
ically new (Chow and Kishino, 1995), 
use of highly variable genetic markers 
may be necessary to investigate genetic 
differentiation between stocks. 
Recently, Takagi et al. ( 1999) isolated 
four microsatellite loci from Pacific 
northern bluefin tuna (Thunnus thyn- 
nus oriental is) and demonstrated suc- 
cessful cross-species amplification of ho- 
mologous microsatellites in other tuna 
species. In our study, we used these mi- 
crosatellite primers to evaluate genetic 
variation within and between albacore 
samples from the Atlantic and the Pacif- 
ic Oceans and present genetic evidence 
of population structuring between and 
within ocean samples of albacore. 
Materials and methods 
Five albacore samples were drawn 
from archival stock materials in the 
National Research Institute of Far 
Seas Fisheries laboratory. One sample 
each came from the Northwest Pacific 
(NW Pacific; Japan), Southwest Pacific 
(SW Pacific; Australia), Southeast 
Pacific (SE Pacific; Chile and Peru), 
and two came from the Northeast 
Atlantic (NE Atlantic; Biscay Bay) and 
the Southwest Atlantic (SW Atlantic; 
Brazil). These samples were derived 
from the same sample lots used by 
Chow and Ushiama (1995). Nucleo- 
tide sequences of the four primer sets, 
PCR amplification conditions needed 
to amplify the four microsatellite 
loci ( Ttho-1 *, -4*, -6* and -7*) devel- 
oped for Pacific northern bluefin tuna 
(T. thynnus orientalist, and electro- 
phoresis procedures can be found in 
Takagi et al. (1999). Differentiation of 
allele frequencies between and among 
samples was estimated by a fixation 
index (F ST ) with Arlequin version 1.1 
(Schneider et al., 1997). 
Results 
Alleles observed in each locus were as 
follows: 9 in Ttho-1 *, 29 in Ttho-4 *, 31 
in Ttho-6*, and 18 in Ttho-7* (Table 1). 
All four sets of PCR primers success- 
fully amplified scorable microsatellite 
loci for all samples. Observed hetero- 
zygosity (H 0 ) ranged from 0.391 to 
0.914 at Ttho-1 * , from 0.688 to 0.886 
at Ttho-4 . from 0.548 to 0.884 at 
Ttho-6 and from 0.857 to 1 at 77/m- 7 . 
We found no substantial discrepancy 
between observed and expected number 
of genotypes for any locus. 
All samples except NE Atlantic 
shared the most common alleles for all 
loci, whereas the NE Atlantic sample 
shared the most common alleles only 
within Ttho-1 ' . The NE Atlantic sam- 
ple also did not share the second 
most common allele with the other 
samples for all loci. F ST among all five 
Manuscript accepted 11 April 2001. 
Fish. Bull. 99:697-701 (2001). 
