Applications
of DNA markers in aquaculture genetics
What are the high-tech tool in genetics that
are applicable to guppy aquaculture?
Is the guppy aquaculture industry ready to
make a quantum leap by incorporating hight-tech methods for
the mass production of top-quality healthy stock? Given the
high the level of effort in scientific research directly related
to ornamental species, and guppies in particular, the advances
being made are significant. Take for example all the recent
research using modern genetic tools previously reserved for
the food fish aquaculture industry.
Recent
Scientific Research About Guppy Genetics
Shikano and Taniguchi (2002) examined the relationship
between heterozygosity at four microsatellite loci and salinity
tolerance using 17 populations with various genetic backgrounds.
They did this to explore the usefulness of DNA markers as
an indicator for the level of inbreeding depression. Salinity
tolerance was used to quantify the level of inbreeding depression.
Salinity tolerance is strongly sensitive to inbreeding, and
shows a linear decrease with an increase in inbreeding coefficient.
Shikano and Taniguchi (2002) observed a significant positive
correlation between mean heterozygosity (measured using microsatellites)
and salinity tolerance. This result indicated that microsatellites
may be useful for predicting inbreeding depression.
Shikano and Taniguchi (2003) later reported
on the use of specific DNA markers for estimation of inbreeding
depression and heterosis in the guppy. According to Shikano
and Taniguchi (2003), when inbreeding depression has occurred
for some quantitative traits, a cross between genetically
different individuals is way to recover the traits. We already
knew that out-breeding often result in re-gaining vigor in
an inbred and depressed line, but now there are way to measure
these effects and use tools to guide us and measure progress
in terms of genetic improvements towards our goals.
Because the amount of heterosis depends on
genetic differences among the individuals used for such a
cross, the level of genetic diversity measured by microsatellite
and RAPD (random-amplified polymorphic DNA) markers is useful
for predicting the amount of heterosis resulting from various
(strain) combinations. In other words, not all outcrosses
result in the same "recovery". If valuable traits can decrease
through inbreeding in guppies, inbreeding must be avoided
through the use of a large effective population size as broodstock
(like large breeding groups - see our article in the Jan 2004 issue). When inbreeding
depression has occurred for some important traits, crosses
between populations that are genetically distant can recover
the traits. DNA markers are now available to estimate inbreeding
depression in guppies and to predict the level of heterosis
that could result from specific crosses.
But
what about the effects of such "oout-crossing" on
the genetic make up of the future breeding groups? Are there
ways to assess the "purity" of potential strains
before out-crossing at such a scale?
Degani (2004) suggested that RAPD-PCR (random-amplified
polymorphic DNA-polymerase chain reaction) is a method that
can be used to determine a strain's genotype in order tomaintain
the purity of a guppy strain under production indefinitely
(with respect to the phenotype of interest), as well to define
the differences between new strains or lines created by cross-breeding
(hybridization ), and the original more inbred lines. Degani
(2004) also found DNA variations in various laboratory guppy
strains by comparing their fragments of the cytochrome b gene
(mitochondrial DNA). However, according to Degani (2004),
although this particular method might be useful as a genetic
marker to differentiate between these laboratory strains,
a more detailed study is needed to determine whether this
marker can of use in making distinctions between guppy strains.
Quantitative trait loci (QTL) mapping is also
a high-tech genetics application that may be very useful to
aquaculture (Liu and Cordes, 2004). Watanabe et al. (2003)
reported on the isolation and characterization of 43 microsatellite
DNA markers for guppy. These microsatellite DNA markers are
now available for the analysis of QTLs in guppy breeding programs,
for measuring differences between guppy strains, as well as
for use in assessing genetic variation in inbreeding mating
systems.
Genetic linkage and QTL mapping in aquaculture
species are not as advanced as they are in other production
species such as tomato, soybean, cattle, and pig. Information
is available for salmon, rainbow trout, catfish, tilapia,
oysters, and shrimp and further research of selected genome
regions is under way (reference).
Some of the QTL that have been mapped and characterized
in aquaculture species are: QTL for upper thermal tolerance,
spawning time, and embryonic development rate (rainbow trout),
QTL controlling body color and sex determination and QTL controlling
a number of biochemical parameters related to innate immunity
response to stress have been recently identified in tilapia,
several markers have been identified in catfish that are linked
to feed conversion efficiency, disease resistance, and two
putative QTL have been identified to be associated with resistance/susceptibility
to infectious pancreatic necrosis virus (IPNV) in rainbow
trout. With the availability of resource families and DNA
markers, it is expected that greater successes will be achieved
in the near future in QTL mapping in aquaculture species,
which will eventually lead to marker-assisted selection (MAS).
MAS refers to a selection process in which
future breeders are chosen based on genotypes using molecular
markers. To implement MAS, researchers need to produce high-resolution
linkage maps, understand the number of QTL affecting a given
performance or production trait and their mode of inheritance
and relative contribution, determine the linkage and potential
interactions of different QTL for the trait and for other
traits, and estimate the economic importance of each trait.
Selection of one trait may be made at the expense of another,
and a well-planned MAS program should take all economically
important traits into consideration. A selection index may
be useful in achieving a balanced approach in cases where
contradictory decisions are called for regarding different
traits.
Individual
farmers are not likely to take advantage of all these techniques
on their own. Some sort of centralized support may be needed
in order to implement some of these techniques in the field.
The implementation of such programs in the field would eliminate
much of the guess work, or trial-and-error practices. However,
it requires a high capital investment and specialized personnel.
An
Example of an Operation With Potential For The Application
of High-Tech Protocols...
| If
you are a commercial grower, you will need a lot of
capital investment in order to take full advantage of
some of the technologies described in this article,
unless you have heavy government subsidies available.
Israel is another country that has achieved significant
advances in aquaculture and, from what we can see, significant
effort is being made to incorporate these advances in
the ornamental aquaculture industry. Guppy is one of
the ideal candidates.
Again,
commercial operations in Israel, even of such size,
most likely could not afford the capital investment
required to take full advantage of the tools that are
becoming increasingly available to the guppy growers.
But these techniques will be within the reach of commercial
operations soon. |
|
| |
| If
it can be done with tilapia or carp in Israel, it can
be done with guppies. I suspect that bags of product like
these will soon be of the highest quality possible. There
is no reason (that I can think of) of why commercial growers
will not be able to select for the traits that make a
show-quality guppy, while ensuring the maintenance of
the genetic attributes that make a robust strain. Add
to that advances in husbandry, engineering and health
management, and the future for high-tech guppy production
looks bright! |
|
How
Does This Benefit The Hobbyist?
The aquarium
hobby has several tiers. The most abundant of these steers,
the hobbyists that purchases their fish from the local fish
stores, is already benefiting from improvements in guppy aquaculture.
A good selection of fancy strains are available to the public
at large. But the quality of the product available is not
at a level to satisfy the needs of collectors and high-end
hobbyists.
A significant
portion of the potential for guppy aquaculture has yet to
materialize. We say this because much of the ground work needed
specifically for guppy culture is now being laid down. There
is no reason to doubt that some day soon, these specific (for
guppies) protocols will be well developed and the most adept
component of the international guppy farming establishment
is going to incorporate them into their culture practices.
When this happens, it is possible that the higher tiers of
the hobby, the collectors and the show-bench winners, are
also going to benefit in some fashion.
