Integrating Molecular Techniques to Maximise the
Genetic Potential of Forage Legumes
Derek R. Woodfield and E. Charles Brummer
AgResearch Grasslands, Private Bag 11008, Palmerston North, New Zealand, and
Department of Agronomy, Iowa State University, Ames IA50011, USA
Key words: breeding methods, genetic gains, heterosis, backcrossing, marker-assisted
selection, interspecific hybridisation, genomics, transgenics
Abstract: Genetic gains from forage legume breeding have exceede d 1% per year for white
clover but are more variable for alfalfa and red clover depending on the trait. A change from
synthetic to hybrid varieties would provide better control of economic traits in most out-
crossing forage species. New breeding strategies that capture heterosis are essential in forage
legumes with historically slow yield improvement and where producing hybrids is difficult.
More emphasis is needed on characterisation and evaluation of genetic resources, particularly
to identify heterotic groups, using both classical and molecular methods. Application of
molecular marker- and genomics-based techniques will advance our understanding of the
genetic control underpinning phenotypic traits. Markers will be used to understand and
capture heterosis, to identify quantitative trait loci, to develop detailed genetic linkage maps,
to introgress unique genetic variation from conventional and transgenic sources, to conduct
marker-assisted selection, and to determine the factors involved in genotype by environment
interactions. In conjunction with international genomics efforts in higher plants, economically
important phenotypes will ultimately be linked through their underlying physiology and
metabolism to the genes responsible for them. Combining molecular genetic innovations with
strong population improvement programs will enable the maximum genetic potential of
forage legumes to be realised.
In most temperate regions, and particularly in Australasia, grazed pastures
are the preferred nutritive source for animal production because of their low
cost and competitive trade advantage. Forage legumes provide nitrogen and
improve nutritive value, quality and intake rates of grazed forage. Forage
breeding has traditionally focused on increasing animal productivity by
providing high yielding, persistent forages resistant to a range of pests and
2 Woodfield and Brummer
diseases. While these remain important, breeding objectives have diversified
to improve feeding value, and to manipulate compounds that affect animal
and human health, animal welfare, reproductive fertility, animal product
quality, flavour and texture.
Phenotypic recurrent selection has been the predominant breeding method
for forage legumes; however, in most forage species and particularly those
with slower genetic improvement there is a clear need for new breeding
methods that provide reliable improvement (Brummer 1999). We extend
previous reviews of breeding methods used for herbaceous, out-crossing,
perennial forage legumes (Taylor 1987; Bowley 1997) by providing
information on methods to develop transgenic varieties, methods to capture
heterosis, applications of molecular markers and the potential of genomics.
2. POPULATION IMPROVEMENT
Population improvement strategies focus on identification of elite genotypes
with good general combining ability. By crossing these genotypes, breeders
aim to accumulate favourable genes in subsequent generations while
maximising heterozygosity. Open-pollinated populations are a hetero-
geneous mixture of highly heterozygous individuals, with high levels of
genetic variation both within and between populations. This variation has
made good selection responses possible with simple breeding methods.
However, traits with lower heritability require other recurrent selection
schemes involving progeny testing.
Intra-population recurrent selection methods, including mass selection,
phenotypic, half-sib, full-sib and inbred progeny (S 1 or S 2) recurrent
selection schemes, are much more widely used than inter-population
methods due to their low cost and simplicity.
Mass selection has been the driving force behind the array of natural
ecotype populations that have provided unique genetic variation in many
forage legumes (Williams 1987; Taylor 1987). Bulked seed from the
population of interest is grown in the target environment, and after natural
selection has occurred, seed is harvested, bulked, and the process repeated.
Phenotypic recurrent selection differs from mass selection in that
breeders have pollen control over one and commonly both parents. A good
example is its use to develop resistance to clover cyst nematode (Heterodera
trifolii) and clover root-knot nematode ( eloidogyne trifoliophila) in white
clover. Five cycles of phenotypic selection for clover cyst nematode
resistance and seven cycles for clover root-knot nematode were conducted
with approximately 1000 plants screened in each cycle. For clover cyst
nematode resistance, an average reduction of 122 cysts per gram of root
Maximising the Genetic Potential of Forage Legumes 3
dryweight was achieved per cycle. This equates to a genetic response of
7.3±2.0% per cycle of selection. Over 7 cycles of selection for clover root-
knot nematode resistance, the mean decrease per cycle was 164 galls per
gram of root dry-weight, which equates to a genetic response of 2.3±0.2%
per cycle of selection (Mercer et al. 2000).
Recurrent selection methods involving half-sib, full-sib or inbred progeny
testing provide better information on the relative importance of additive and
non-additive genetic effects. Half-sib and full-sib progeny tests are being
used extensively but with many modifications to suit the crop and trait.
Bowley (1997) presented a combined among- and within -HS progeny test
method that was more effective than half-sib selection for improving
birdsfoot trefoil seed yield.
Inter-population recurrent selection schemes such as reciprocal recurrent
selection (RRS) can exploit all types of gene action responsible for heterosis
(Brummer 1999). In RRS, each population serves as the tester for the other
population, and improvement is carried out within each population
separately but based upon test-cross performance. In alfalfa, Rowe and Hill
(1981) results suggested that inter-population hybrid performance could be
improved by RRS or top-cross progeny testing. No published information is
available on the effectiveness of RRS relative to other breeding methods in
forage legumes; however, RRS has been partially integrated into some
commercial alfalfa breeding programmes (Moutray pers comm.).
Backcrossing has been under-utilised in most out-crossing species due to
perceptions that performance gains in backcross-derived populations may
not justify the time and resource involved. Concerns about inbreeding
depression occurring during backcrossing have also been expressed, but this
can generally be avoided by changing the recurrent parents for each
backcross generation (Bingham 1990). Backcrossing is frequently used for
introgressing traits from wild relatives such as potato leaf hopper resistance
transferred from Medicago sativa subsp. glandulosa to cultivated alfalfa (M.
sativa subsp. sativa) (Shade and Kitch 1986; McCaslin 1998). Similarly,
tetraploid M. sativa subsp. falcata germplasm, WISFAL, was developed by
backcrossing between ploidy levels and sub-species (Bingham 1990).
Backcrossing is particularly attractive for autotetraploid species such as
alfalfa as it works with equal efficiency for diploid, allotetraploid (disomic)
or autotetraploid (tetrasomic) inheritance (Micallef and Bingham 1995). The
number of backcross generations required depends on the relative
performance of the donor parent. The better the performance of the donor
parent the fewer backcrosses that are required.
Field Evaluation: Evaluating forages as spaced plants in the absence of
companion species has decreas ed. Spaced plant evaluations provide useful
morphological information and are essential for PVR descriptions but the
4 Woodfield and Brummer
agronomic information is frequently a poor indicator of performance in on-farm
situations. An example is the poor correlation between performance of white
clover genotypes under cutting and their performance under intensive grazing
(Evans et al. 1992). Cutting in white clover favours large-leaved genotypes,
while small- and medium -leaved genotypes are more persistent and productive
under intensive grazing. Similarly in alfalfa, evaluating material under grazing
has enabled breeders to identify superior genotypes and populations. The first
true dual-purpose alfalfa cultivar, Alfagraze, was developed by selecting
surviving plants after two cycles of intensive grazing by beef cattle (Bouton
et al. 1991). Alfagraze had superior grazing tolerance but yielded slightly
less than the top entries under cutting and had low resistance to phytophthora
root rot. Newer varieties developed by integrating the protocol developed
with Alfagraze (Bouton and Smith 1998) into commercial breeding
programs have yields and disease resistance profiles equal to or better than
the best commercially available varieties (Brummer and Moore 2000).
Genetic Gains: While there are few published reports, genetic gains have
been achieved in the major temperate forage legumes. Genetic gain for white
clover yield under various management systems, stolon density and nitrogen
fixation has exceeded 1% but genetic gains have been more variable in red
clover and alfalfa (Table 1). With the exception of McKersie (1997), genetic
improvement in alfalfa has consistently been less than 0.35% yr -1 (Table 1).
Part of the reason for lower rates of genetic improvement is tetrasomic
inheritance, which increases the number of possible allelic combinations at a
single heterozygous locus (Hill et al. 1988). However, other reasons may
have been more important, such as an emphasis on disease and insect
resistance breeding and a near total disregard for developing populations that
provide heterotic yield increases (see below).
Improvements in animal performance were lower than the increases in
forage yield and quality (Table 1). Achieving animal production responses
relies upon the ability to utilise additional forage produced, and this often
requires higher stock rates and better grazing management. Poor
management can erase all the gains obtained from breeding. Other on-farm
benefits can also be observed when animal health is improved through better
nutrition or elimination of an anti-quality factor. An example of this is the
strong selection response for reducing formononetin content of red clover
(Table 1), that subsequently provided higher ovulation rates in ewes grazing
these low-formononetin red clovers (Rumball et al. 1997).
Accumulation of favourable alleles, with both additive and dominant
effects, in linkage blocks along with a decreased frequency of deleterious
recessive alleles has provided population improvements in alfalfa and is
consistent with explanations for improvements in various self - and cross-
pollinated crops (Bingham 1998).
Table 1. Estimates of rates of genetic gains in temperate forage legume species .
Species/Trait Benchmark Variety Trials/Varieties Genetic gain Reference
Alfalfa (%. yr )
Forage Yield Vernal 14 trials (150 var) 0.26 Hill et al. 1988
Forage Yield Cossack, Ladak 2 trials (12 var) 0.18 Holland & Bingham 1994
Forage Yield Saranac Multiple trials (80 var) 0.18 -1.07 McKersie 1997
Forage Yield Wairau 3 trials (5 NZ var) 0.35 Woodfield 1999
Forage Yield G. Turoa G.Hamua, Pawera 0.43 Anderson 1973
Forage Yield G. Hamua (2x) G. Colenso (2x) 0.21 Claydon et al. 1993
Forage Yield G. Pawera (4x) G27 (4x) 1.39 Rumball et al. 1997
Formonenetin G. Pawera (4x) G27 (4x) 2.83 Rumball et al. 1997
Forage Yield -Sheep grazing G. Huia 8 trials (10 var) 1.49 Woodfield 1999
Forage yield -Cattle grazing G. Huia 5 trials (9 var) 1.21 Woodfield 1999
Stolon density G. Huia 5 trials (9 var) 1.09 Woodfield 1999
Clover content G. Huia 110 var 0.60 Woodfield & Caradus 1994
Nitrogen fixation G. Huia 3 trials (7 var) 1.19 Woodfield 1999
Lamb growth G. Huia G. Demand 0.33 Ryan & Widdup 1997
Lamb growth G. Huia G. Tahora 0.48 Chapman et al. 1993
3. CAPTURING HETEROSIS IN FORAGES
Severe inbreeding depression and heterosis have been reported for most
cross-pollinated forage legumes (Jones and Bingham 1995; Brummer 1999).
In diploids and allotetraploids heterosis is maximised in the single-cross
generation, while autotetraploids exhibit progressive heterosis with double-
cross performance exceeding that of single-crosses from inbred parents
(Bingham et al. 1994). Heterosis results from alleles or linkage blocks with
partial to complete dominance and with different frequencies between
populations (Bingham 1998; Brummer 1999). The presence of heterosis in
forage legumes has led to considerable discussion on developing hybrid
varieties. Currently all commercially available varieties of white clover, red
clover, birdsfoot trefoil and alfalfa are synthetics, developed by intermating
selected parents for 4 to 6 generations. Hybrid varieties have potential
advantages for reducing the number of generations of seed multiplication,
capitalising on both additive and non-additive gene action, and providing
higher yield and better environmental stability. Hybrids would also be
simpler for deploying and pyramiding transgenes.
Pollination control mechanisms are required to develop hybrid varieties.
Male-sterility systems have been identified in alfalfa, rose clover and birdsfoot
trefoil (Viands et al. 1988; Molina-Freaner and Jain 1992; Negri and Rosselini
1996), while he gametophytic self-incompatibility system of white and red
clover also provides excellent pollination control. Breeding schemes for
producing hybrids using self-incompatibility have been described by
Williams (1987) and Taylor (1987).
Despite suitable pollination control mechanisms, development of hybrid
alfalfa varieties has not been viable due to reduced pollination of male-sterile
rows by bees (Viands et al. 1988). Such difficulties suggest new breeding
strategies to capture hybrid vigour are essential in forage legumes such as
alfalfa. Concurrently, more emphasis is required on characterisation and
evaluation of genetic resources, particularly identifying heterotic groups,
using both classical and molecular methods.
Brummer (1999) proposed a semihybrid breeding scheme that uses
population improvement within separate heterotic populations. This scheme
relies on the identification of heterotic groups, which unfortunately has
received almost no attention in forage legumes. In alfalfa, Kidwell et al.
(1994a) demonstrated that M. sativa subsp. falcata and, to a lesser degree,
Peruvian germplasm were genetically distinct from other subsp. sativa
germplasm pools. Riday and Brummer (1999) have confirmed that sativa x
falcata interpopulation hybrids performed better than both intra-population
Maximising the Genetic Potential of Forage Legumes 7
crosses, while Ray et al. (1999) reported good heterosis between non-
dormant and dormant alfalfa germplasm pools. Identifying and using similar
heterotic patterns in other forage legumes is urgently required. The
semihybrid scheme also relies on independent population improvement
continuing within the respective populations (Brummer 1999). This is
consistent with reciprocal recurrent selection approaches used for maize.
Developing more than two heterotic groups may be required for crops or
breeding programs that cover a wide geographical or environmental range.
Current breeding strategies that essentially bulk all germplasm into one large
gene pool need urgent re-evaluation, or identification and development of
heterotic groups will become almost impossible.
In order to identify heterotic groups, crosses among germplasm sources
need to be made. Numerous diallel analyses in other forage legumes have
partitioned additive and dominance genetic variance into general (GCA) and
specific (SCA) combining ability. However, this information has not been
used to identify heterotic groups. Two populations whose hybrids have
higher average SCA than crosses within either population are assigned to
different heterotic groups. Thus, assessing germplasm in order to develop
heterotic groups is a non-trivial task.
The development of conventional F hybrids is still of interest to forage
breeders. Inbreeding programs have been initiated in most forage legumes
(Jones and Bingham 1995; Taylor 1987) but whether these will result in
successful hybrid varieties remain s uncertain. Regardless of its utility for
hybrid production, inbreeding remains an important tool for reducing the
genetic load within cross-pollinated species.
4. INTRODUCING NEW GENETIC VARIATION
Recurrent selection and backcross methods rely primarily o additive genetic
variation present in the primary gene pool. When the required genetic
variation is absent, or has very low heritability, interspecific hybridisation
and transformation have been used to generate new genetic variation.
Mutagenesis has been largely unsuccessful in forages (Bowley 1997).
Interspecific hybridisation : Gene transfer from related species in the
secondary gene pool has proven difficult in most forage legumes due to
strong hybridisation barriers. Embryo rescue has frequently been required to
obtain hybrids following failure of the endosperm to develop, and many of
the resulting hybrids have either been sterile or have had meiotic instability.
Interspecific hybridisation has been more successful in white clover than
in red clover. Red clover has been crossed with four other species but none
has resulted in transfer of useful variation (Taylor 1987). White clover has
8 Woodfield and Brummer
been successfully hybridised with Trifolium nigrescens, to transfer clover
cyst nematode resistance and increased seed production (Hussain et al. 1997;
Marshall et al. 1998). Similarly, several research groups have obtained
fertile hybrids between Caucasian (kura) clover (T. ambiguum M. Bieb) and
white clover, with virus resistance and the ability to form both rhizomes and
stolons present in the hybrids. Hussain and Willams (1997) developed a
ploidy series of hybrids from 4x to 8x between these two species by
developing a fertile bridge cross. The initial hybrids required embryo rescue
and chromosome doubling to provide a fertile octoploid (8x H-435),
however, an array of congruity backcross progenies were subsequently
generated from this genotype. Recent crosses have produced the desired
recombinants with both rhizomes and stolons (Hussain pers comm.).
Similarly, Abberton et al. (1998) used conventional backcrossing of the
initial interspecific hybrid to transfer rhizomes into white clover.
Interestingly, Abberton et al. (1998) predominantly used tetraploid hybrids,
while Hussain and Williams (1997) have achieved their successful
recombinants at the hexaploid level.
Interspecific hybridisation has successfully reduced the coumarin levels
in yellow sweetclover (Melilotus officinalis L.) and white sweetclover
(Melilotus alba Desr.) by transferring a recessive alle le (cu) from a wild
relative (M. dentata Walst et Kit.)(reviewed by Bowley 1997). Similarly,
interspecific hybridisation of serredella (Ornithopus ssp.) resulted in a
commercial variety (cv. G. Spectre) that combined the prostrate growth habit
and hard seed of yellow serredella (O. compressus Mill.) with the higher
forage yield of pink serredella (O. sativus Brot.) (Williams et al. 1987).
Transgenics: The insertion of foreign genes to provide unique genetic
variation has received considerable interest for forage legume improvement
(this volume). Transformation and regeneration protocols have been
developed for all the major forage legumes and a range of cloned genes have
been introduced. Si gle-gene transformants in alfalfa (Austin and Bingham
1997) and white clover (White et al. 2000) have been field tested; however,
no transgenic forage legumes have been commercially released yet.
Developing synthetic transgenic varieties in cross-pollinating species is
more difficult than in species where clonal, inbred or hybrid varieties are
used (Conner and Christey 1994). The development of transgenic varieties
relies on adequate expression and stable inheritance of the introduced
gene(s), and the ability to cost-effectively conduct field and laboratory
testing prior to commercial release. Screening primary transformants for
transgene expression levels is standard practice, however, few attempts have
been made to modify expression levels through conventional selection.
Instead most effort has gone into modifying gene constructs to increase
expression. Identifying genetic backgrounds that enhance the function of the
Maximising the Genetic Potential of Forage Legumes 9
transgene(s) and selecting for enhanced transgene expression is feasible.
Scott et al. (1998) found that both genetic background and selection
influenced expression of a GUS (CaMV35S promoter-uidA) gene.
Range of genotypes Single genotype homozygous
from elite population for inserted transgene
F 1 families
(h e t e r o z y g o u s f o r t r a n s g e n e i n a h e t e r o g e n e o u s g e n e t i c b a c k g r o u n d )
Intercross within or among F 1 families
F 2 families N o n -t r a n s g e n i c
( 0 to 2 copies of transgene) X tester
Further generations of seed multiplication
(h o m o z y g o u s f o r transgene in a heterogeneous background)
Figure 1. A modified synthetic breeding scheme for transgenic varieties in outcrossing diploid
and allotetraploid species.
While it is probably unnecessary to have the transgene expressed in all
progeny of a synthetic variety, breeding schemes must have the flexibility to
produce varieties with predictable transgene(s) frequencies. For transgenic
breeding strategies to succeed they must also avoid causing inbreeding
depression. Two breeding strategies, a modified synthetic scheme (Fig. 1)
and a backcross scheme, that provide predictable transgene inheritance in
outcrossing disomic and tetrasomic populations have been successfully
tested (Scott et al. 1998; Micallef and Bingham 1995). Scott et al. (1998)
reported stable inheritance of a GUS gene through three generations of
crossing to non-transgenic plants, while Micallef and Bingham (1995)
reported stable inheritance of GUS and a recovery of agronomic
performance through 3 generations of backcrossing. These results indicate
transgenic plants containing a single transgene insert could be used as the
basis of a breeding program. Backcrossing maintains the transgene in a
heterozygous form in diploids and allotetraploids, and in a simplex condition
in autotetraploids (Micallef and Bingham 1995). Approximately 50% of the
10 Woodfield and Brummer
backcross progeny have the transgene and transgenic progeny must be
identified in each generation for subsequent backcrossing. The main
advantage of backcrossing is that a single transgene can be rapidly
transferred into multiple genetic backgrounds.
Strategies that minimize the generations of intermating required for
variety development would also be beneficial and therefore direct
transformation of inbred lines is an attractive longer-term option. Direct
transformation of inbred lines (S2 or S3) would allow stabilization of the
transgene during inbreeding to S5 or beyond. Inbreds with optimal transgene
expression and from backgrounds with know n heterotic response could be
combined during hybrid production to pyramid multiple transgenic traits in
the final variety. I nbreeding approaches, like backcrossing, can give rise to
inbreeding depression, but also offer the potential to capitalise on hybrid
vigour. Once the transgene is fixed in a homozygous condition, transgenic
varieties could be produced using a single- or double-cross hybrid system
Transgene dosage can be manipulated using these methods to provide
varieties with predictable genetic composition and reliable expression of the
target trait. However, further erosion of potential heterotic groups is a
concern with transgenics as a single genotype or chromosome block could be
introgressed into a wide range of germplasm. More consideration of this
potential problem is needed.
5. INTEGRATING MOLECULAR TECHNIQUES
The application of molecular marker- and genomics-based techniques will
advance our understanding of the genetic control of phenotypic traits. The
development of various marker systems and detailed genetic linkage maps
will enable molecular markers to be used to understand and capture
heterosis, to identify quantitative trait loci, to introgress unique genetic
variation from conventional and transgenic sources, to conduct marker-
assisted selection, and to determine the factors involved in genotype by
Genetic Diversity and Parental Selection: Molecular markers have
proven useful in detecting genetic diversity between germplasm sources
(Brummer et al. 1991; Kidwell et al. 1994a; Ghérardi et al. 1998) and their
use could be extended to identifying heterotic groups and parents (Brummer
The relationship between molecular marker heterozygosity and heterosis
has been studied in alfalfa and white clover (Kidwell et al. 1994b; Woodfield
et al. 1999). In alfalfa, genetic dissimilarities based on 244 restriction
Maximising the Genetic Potential of Forage Legumes 11
fragments were poorly correlated with diploid progeny performance but
highly correlated with the performance of tetraploid progeny (Kidwell et al.
1994b). Genetic dissimilarity estimates based on 861 restriction fragments
were significantly correlated with single-cross performance for two white
clover synthetics in year 1 but the correlations decreased to nearly zero by
year 3 (Woodfield et al. 1999). Kidwell et al. (1994b) suggested that
genotypes selected for maximum genetic dissimilarity could be used to
improve forage yield in cross-pollinated species. However, the yield of
selections based on the genetic similarity and dissimilarity of synthetic
parents was inconsistent (Kidwell et al. 1999). Correlations between genetic
distance and hybrid or synthetic performance may be improved by only
using markers linked to genes or QTL for yield in the computation
(Bernardo 1992), but another factor that could be influencing the relationship
is linkage equilibrium in the populations under study (Kidwell et al. 1999).
Genome Mapping and Marker-Assisted Selection: Among the forage
legumes, alfalfa genome mapping is most advanced. Several independent
maps were developed from a variety of diploid crosses (Brummer et al.
2000a). The most complete linkage map is based on an F2 population derived
from a cross between M. sativa subsp. falcata and subsp. coerulea that
includes more than 900 RFLP, RAPD, isozyme, seed protein, and
morphological markers (Kaló et al. 2000). Several genes or quantitative trait
loci have been mapped, including unifoliolate leaves, aluminium tolerance,
somatic embryogenesis, flower colour, dwarfness, yield, and winterhardiness
(summarised in Brummer et al. 2000a). Genetic mapping of tetraploids has
been more difficult, but two maps have been or are being developed
(Brouwer and Osborn 1999; Brummer et al. 2000b). Mapping efforts are
underway in other forage legumes but are less advanced (White et al. 2000).
The advent of DNA markers such as RFLPs, AFLPs, RAPDs and micro-
satellites has increased the scope and precision with which marker assisted
selection (MAS) can be pursued. MAS relies on identifying markers at or
closely linked t genes of interest that can be used to indirectly select for
traits that are difficult or expensive to evaluate. Combinations of markers
could theoretically be used to select for multiple traits simultaneously,
without time-consuming evaluations. Although MAS is being practised in
many crops (Young 1999), the true benefits of the technology may be most
successfully realised in perennial forage crops, where each cycle of selection
takes several years. For example, selection for winter hardiness in alfalfa
takes three to five years, because winter severity is variable. A MAS
approach could decrease cycle time significantly if markers linked to winter
survival could be identified. Perhaps the most important requirement for
using MAS is the development of transportable maps based on microsatellite
12 Woodfield and Brummer
and single dose restriction fragments that will facilitate moving mapping
information across populations.
Impact of genomics: International genomics efforts are underway in
several forage legumes, and two of these, Medicago truncatula and Lotus
japonicus, are being extensively studied as model species (Cook et al. 1997).
Sequence information from these model species can be compared with those
from other higher plants such as Arabidopsis and rice, for which nucleotide
sequences are nearly complete, to assign a putative function for many genes.
Complete sequencing of many forage legumes is impractical in the short-
term due to prohibitively large genome size or limited perceived economic
value, so alternative approaches such as random sequencing of expressed
sequence tag libraries from various plant tissues are being pursued. These
approaches may be less effective for identifying low copy number genes.
Genomics has the potential to ultimately link economically important
phenotypes through their underlying physiology and metabolism to the genes
responsible for them (DeRisi et al. 1997; Somerville and Somerville 1999).
This offers real opportunities for forage breeders to systematically
accumulate the genes responsible for control of quantitative traits. Since
complete sequences will be possible for these genes, markers can be
developed to accurately transfer them into appropriate genetic backgrounds.
The precision that genome sequencing provides should also enable the
underlying genetic control of quantitative traits to be determined. In
particular the role of epistasis in plants and the importance of linked genes
on chromosome segments can be examined (Bingham 1998).
We caution that the power of these molecular genetic innovations cannot
be effectively used without strong population improvement programmes.
However, used in concert, they will enable the maximum genetic potential of
forage legumes to be achieved.
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