[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
[SANET-MG] marker ssisted selection
http://www.i-sis.org.uk/MAS.phpISIS Press Release 08/09/05
Marker Assisted Selective Breeding
Prof. Joe Cummins gives the current state of play in how molecular
genetic analysis can aid in selective breeding without genetic modification
The fully referenced version of this article is posted on ISIS members’
website. Details here
Quantitative traits not determined by single genes
Genetically modified (GM) crops are based on inserting synthetic foreign
genes mainly from bacteria, to impart herbicide tolerance or insect
resistance into the genomes of crop plants. This technology has so far
provided little if any increase in yield, stress tolerance or long-term
resistance to microbes or nematodes. Traditional breeding of crops and
animals has been based on the use of genetic markers that are inherited.
The main agricultural traits governing yield (or size), stress
resistance or long-term disease protection are quantitative trait loci
(QTL, ‘loci’ is another word for genes). One of the founders of the
study of population genetics, Ronald A. Fisher, described QTL as many
independent loci that added together to determine traits such as size
[1]. QTL are seldom tightly linked on a chromosome and the loci are
dispersed over many chromosomes in the genome. Selection of QTL traits
has been inherently slow and meticulous, but has resulted in major
improvements to crops and livestock.
While Fisher believed that QTL were made up of very many genes each
adding small increments to a trait, recent findings indicate that some
QTL may be made up of a relatively small number, say twenty or so,
genetic markers that could be easily selectedprovided they could be
identified. Currently, it appears that many QTL may have relatively few
loci but some important QTL may be closer to the very large number of
genes envisioned by Fisher, in which case, identifying and selecting
such traits by the molecular markers are unlikely to be cost-effective.
Molecular markers can be used to aid selective breeding
There is a growing arsenal of molecular markers (polymorphisms) that aid
in identifying QTL and selecting them for crop and animal enhancement.
The process ofusing such markers is called marker-assisted selection
(MAS), which differs from genetic modification because the genes being
selected for crop or animal improvement are not altered in any way. The
molecular markers used in selection are probed using sequences from a
gene bank and identified. The markers used to probe the progeny of a
cross are not the QTL genes themselves but they are close to the QTL on
the genetic map. Of course the markers can be used to determine the
molecular identity of the QTL, but the molecular marker is used even
when the QTL is identified because the marker is cheaper and quicker to
use to identify a large number of progeny. Recombination may separate
the marker from a QTL, but the closer the marker is to the QTL, the more
remote is the chance of separation by recombination. The more
polymorphic markers available for a breeding programme, the more
effective it will be.
There are several types of mk but requires a relatively large amount of
DNA and is rather expensive in a large screening program [2]. RAPD
utilizes low stringency polymerase chain reaction (PCR) amplification
with single primers of arbitrary sequence to generate strain-specific
arrays of anonymous DNA fragments [3]. The method requiresolecular
markers used in MAS; these include restriction fragment length
polymorphism (RFLP), random amplification of polymorphic DNA (RAPD),
amplified restriction fragment length polymorphism (AFLP), single
sequence repeats (SSR) and single nucleotide polymorphisms SNPs [2].
RFLP involves the use of restriction enzymes to cut chromosomal DNA at
specific short restriction sites, polymorphisms result from duplications
or deletions between the sites or mutations at the restriction sites.
RFLP provided the basis for most early wor tiny DNA samples and analyses
a large number of polymorphic loci [2]. AFLP requires digestion of
cellular DNA with a restriction enzyme, then using PCR and selective
nucleotides in the primers to amplify specific fragments [4]. The method
measures up to 100 polymorphic loci and requires a relatively small DNA
sample for each test [4]. SSR analysis is based on DNA micro-satellites
(short-repeat) sequences that are widely dispersed throughout the genome
of eukaryotes, which are selectively amplified to detect variations in
simple sequence repeat [5]. SSR analysis requires tiny DNA samples, and
has a low cost per analysis [2]. SNPs are detected using PCR extension
assays that efficiently pick up point mutations [6]. The procedure
requires little DNA per sample and costs little per sample once the
method is established [2]. One or two methods are used in a typical MAS
breeding programme.
MAS has been employed in breeding cereals, and extensively so in maize
breeding. Corporations including Monsanto and Syngenta have invested
heavily in the programme. SNP appear to be the dominant marker for
selection. Wheat has seen less progress in MAS than maize, but there is
good success in the area of quantitative disease resistance. Rice has
also seen extensive activity in MAS centering on pyramiding disease
resistance genes. SNPs appear to be identified for all the major cereals
[7]. MAS is being used to improve forage crops through QTL for nitrogen
use efficiency, and there was a strong response [8]. The pome fruits,
apple and pear, have extensive MAS programmes, mainly based on RFLP,
RAPD, SSR and AFLP. The traits being selected include fruit production,
storage and disease resistance [9]. A global strategy using MAS for
livestock genetic improvement in the developing world was proposed. QTL
mapping would be used in genetic improvement and to bring together
desirable traits from around the world [10]. It has been proposed that
assessment of genetic markers will greatly enhance the conservation of
genetic diversity in wild crop relatives [11], and the information from
wild crop relatives could be directly employed in MAS of the crop plant.
Does MAS actually work?
A recent review by William Hill of Edinburgh University focused on the
QTLs for oil production in maize and for body size in chickens. In
neither case could individual QTL with substantive quality be detected.
Instead, identified QTLs created small additive increments that could be
selected, but only with patience [12]. Hill’s report suggested that
Fisher’s view of QTLs prevailed and that the use of MAS might not be
cost-effective. It may be that MAS is effective in traits such as
disease resistance and certain agronomic performance but that important
traits such as oil production in maize or body size in chickens are most
effectively bred using traditional selection methods.
Farmers in developing countries and even some farmers in the developed
world face the growing control of seed production by a few multinational
corporations. One solution has been to help the farmer breed varieties
tuned to the local environment and free of the greedy demands of seed
corporations. It is highly unlikely that indigenous farmers will take to
MAS and molecular genomics. However, those scientists working with
indigenous farmers would recognize markers linked to valuable agronomic
traits and pass on that knowledge to the indigenous plant breeders to
assist them in making selections that are beneficial.
In the long run it seems likely that MAS will play an important role in
plant breeding, even though it may not be as large as has been claimed
by advocates. MAS should not affect organic certification because
transgenes are not introduced into the crop. Molecular genetics is used
only in analyzing the crosses. Nevertheless, MAS has far more to offer
in crop and animal improvement than genetic modification.
The Institute of Science in Society, PO Box 32097, London NW1 OXR
telephone: [44 20 8452 2729] [44 20 7272 5636]
General Enquiries sam@i-sis.org.uk - Website/Mailing List
press-release@i-sis.org.uk - ISIS Director m.w.ho@i-sis.org.uk
MATERIAL ON THIS SITE MAY BE REPRODUCED IN ANY FORM WITHOUT PERMISSION,
ON CONDITION THAT IT IS ACCREDITED ACCORDINGLY AND CONTAINS A LINK TO
http://www.i-sis.org.uk/
********************************************************
To unsubscribe from SANET-MG:
1- Visit http://lists.sare.org/archives/sanet-mg.html to unsubscribe or;
2- Send a message to <listserv@sare.org> from the address subscribed to the list. Type "unsubscribe sanet-mg" in the body of the message.
Visit the SANET-MG archives at: http://lists.sare.org/archives/sanet-mg.html
For more information on grants and other resources available through the SARE program, please visit http://www.sare.org.