Mitochondrial DNA is a compact, circular, and
double-stranded. For the most part, mtDNA consists of coding regions for
proteins and tRNAs, except for the D-loop region. The D-loop region contains
several promoters and initiation site for H-strand replication. ). mtDNA
replication begins in the D-loop resulting in the formation of a displacement
loop with a newly synthesized heavy, or H, strand of about 700nt known as 7S
DNA (Sbisa et al., 1997). Both strands of the mtDNA are completely transcribed
from the promoters in the D-loop.
A segment called the control region or D-loop region
occurs in the main non-coding area of the mitochondrial DNA molecule (Pereira
et al., 2004). The D-loop of mitochondrial DNA is a major control site for
mtDNA expression and it is important in maternal inheritance. This region in
different population is vary and contains essential transcription and
replication elements and it could be used for detection of mother inheritance,
and human evolution. The location of mitochondrial transcription promoters and
the major control site because it contains the leading-strand for origin of
replication and major promoters for transcription (Miyazono et al., 2002).
In addition to the promoter sequences, there are two
small regions known as the hypervariable regions I and II (HVI and HVII).
Mutation rates in HVI and HVII are especially high on average and there is
evidence that the rates vary within the regions as well (Jazin et al., 1998).
Replication of the H-strand starts at fixed point (oriH) in the D-loop region,
then replicated DNA displaces the non-replicated single strand to form the
D-loop while the L-strand initiates at oriL within a tRNA cluster far from the
D-loop, after the H-strand replication fork has passed through (Yamamoto,
2001). mtDNA analysis has been focused mainly on the HVI and the correlation
between the HVI and HVII regions has not been well established (Salas et al.,
This greater polymorphic rate in the D-loop region has
been reported in humans and polymorphism patterns of this region are variable
in human population of Iran. The purpose of present study was to determine the
polymorphism patterns of D-loop region using PCR-RFLP techniques in Bakhtiarian
population in southwest Iran.
Genetic Variation of Population
Population genetics had built an extensive and
sophisticated theoretical foundation; integrating principles of Mendelian
inheritance with forces affecting changes in allele frequency in populations
that sought to formalize the Darwinian view that biological evolution is a
population process by which genetic variation within species is transformed
into genetic variation between species (Mayr 1963).
Genetic and Fisheries Management
This is a commercial species. It is marketed mainly
fresh and dried salted, but also smoked, canned, and frozen. There are two
major fishing grounds for Longtail Tuna, one off the South China Sea coast of
Thailand and Malaysia and the other off countries bordering the North Arabian
Sea. Longtail tuna is caught mainly by gillnet and in a lesser extent by
artisanal purse seiners. This species is caught in the recreational fishery in
Australia, and is caught as bycatch in trawling. Most of the global catch is
taken in the western Indian Ocean. Catch of this species is increasing in many
areas but landings are frequently confused with Yellowfin Tuna in some regions.
This species grows more slowly and lives longer than
other tuna species of similar size. Coupled with their restricted neritic
distribution, Longtail Tuna may be vulnerable to overexploitation by fisheries.
Worldwide landings have been rapidly increasing, but there is no effort
information or stock assessments. It is listed as Data Deficient. More
information is needed on the status of this species population, including
better catch data and effort information. Management of this species also needs
to be included under a fisheries management organization.
Population genetic is a tool population applied to
marine fisheries in recent decades and has led to improved knowledge of stock
delineation and population dynamic of exploited fishes (Hauser and Seeb, 2008).
There are no known stock assessments for this species. FAO reported worldwide
landings show a gradual increase from 600 tonnes in 1950 to 250,030 tonnes in
2006 (FAO 2009). There is no effort information for this species. In recent
years, the countries attributed with the highest catches of longtail tuna are
Indonesia, Iran, Oman, Yemen and Pakistan (IOTC 2006).
Taxonomy of Thunnus tonggol
It is the second smallest of eight species of Thunnus
and grows to a maximum size of 142 cm total length and 35.9 kg (IGFA, 2008). Maximum
size is about 130 cm fork length (FL), and longevity at least five years. The
smallest mature female in Thailand was 43 cm (FL), although fifty percent of
females in the Gulf of Thailand were mature at 39.6 cm (FL). Fecundity of fish
ranging in size from 43.8–49.1 cm varies from 1.2–1.9 million eggs (Collette
and Nauen 1983, Yesaki 1994, Collette 2010). In Australia, longevity is
estimated to be about 10 years (Wilson 1981), and age at first maturity in
Thailand is estimated to be two years (Boonragsa 1987). This species may live
as long as 18 years in the central Indo-Pacific (Grifiths et al. 2009).
Its native are Australia; Brunei Darussalam; Djibouti;
Eritrea; India; Indonesia; Iran, Islamic Republic of; Japan; Malaysia;
Maldives; Mozambique; Myanmar; Oman; Pakistan; Papua New Guinea; Philippines;
Singapore; Somalia; Sri Lanka; Taiwan, Province of China; Thailand; United Arab
Emirates; Viet Nam; Yemen. While the marine fishing areas stated by FAO is
Indian Ocean – western; Indian Ocean – eastern; Pacific – northwest; Pacific –
As a result of their coastal distribution, longtail tuna
are heavily exploited by small-scales commercial and artisanal fisheries in at
least 17 countries throughout the Indo-Pacific. A detailed description of the
countries and fisheries that exploit longtail tuna is given by Yesaki (1994).
They are mostly targeted by purse-seine, gillnet, and trolling and constitute a
significant portion of multispecies fisheries for small neritic tuna, including
mackerel tuna (Euthynnus affinis) and frigate tuna (Auxis thazard and A.
rochei; Yesaki, 1994). This species is pelagic and oceanodromous. It is a
predominantly neritic species avoiding very turbid waters and areas with
reduced salinity such as estuaries. It may form schools of varying size. It
feeds on a variety of fishes, cephalopods, and crustaceans, particularly
stomatopod larvae and prawns
Indonesia, Malaysia, and Iran contribute most to reported annual landings,
which reached 248 000 t in 2007 (FAO, 2009). As longtail tuna contribute to
important artisanal and subsistence fisheries in many countries, these reported
landings are likely to be underestimates. This Indo-West Pacific species is
found from the Red Sea and East Africa to Papua New Guinea, north to Japan, and
south to Australia. The population does not appear to be continuous.
This species probably spawns more than once a year,
perhaps in two spawning seasons in the Gulf of Thailand. Spawning of this
species is reported to be confined to coastal waters, based on the occurrence
of their larvae which were collected at surface water temperatures of 28°C
(Nishikawa and Ueyanagi 1991). It appears there are two distinct spawning
seasons for this species off the west coast of Thailand: a major spawning
period during the northeast monsoon from January to April and a minor spawning
period during the southwest monsoon in August-September. Spawning is also
apparently seasonal for this species off Papua New Guinea and off New South
Wales, occurring during the austral summer (Yesaki 1994).
This species grows
more slowly and live longer than other tuna species of similar size. Coupled
with their restricted neritic distribution, longtail tuna may be vulnerable to
overexploitation by fisheries, and caution needs to be exercised in managing
the species until more reliable biological and catch data are collected to
assess the status of the population (Griffiths et al. 2009).