Agriculture Reference
In-Depth Information
of germplasm representing the breadth of their collection, but with as little duplication
as possible, are identifi ed for fi rst pass evaluation by scientists or breeders who can then
return for more targeted sampling. Global information systems (GIS) and computer
models are also being used to help identify accessions of particular value for specifi c
purposes.
There are many examples of where wild relatives have provided signifi cant com-
mercial gains to cropping industries. Bioversity International (2008) has listed a few as
follows: 'The desirable traits of wild sunfl owers (
Helianthus
spp.) are worth an estimated
US$267 million to US$384 million annually to the sunfl ower industry in the United
States; one wild tomato variety has contributed to a 2.4 percent increase in solids content
worth US$250 million; and three wild peanuts have provided resistance to the root knot
nematode, which cost peanut growers around the world US$100 million each year.'
In wheat wild relatives have provided a key source of variation for resistance to stem
rust (
Puccinia graminis
) and leaf rust (
P. triticina
), two diseases whose control helps
to maintain food supplies to a substantial proportion of the world population. Initially,
cultivated tetraploid wheats were used as donors of stem rust resistance for bread wheats
(Hayes
et al
., 1920; McFadden, 1930) and then subsequently wild tetraploid and diploid
species were used by others to introduce a range of rust and powdery mildew (
Blumeria
graminis
f. sp.
tritici
) resistance genes and other non-biotic stress traits. Possibly the
most important such introduction came from deployment of the
Sr
2 gene derived from
Triticum dicoccum
(McFadden, 1930). This gene has subsequently been shown to pro-
vide durable partial resistance to stem rust and is being widely used in wheat breeding
programs in the twenty-fi rst century to counter this disease.
In rice, resistance to bacterial blight caused by
Xanthomonas oryzae
pv
. oryzae
was
identifi ed by researchers in India in 1977 from
Oryza longistaminata
, a wild species
of rice from Mali, and transferred into cultivated varieties. The resistance gene, known
as
Xa
21, has been transferred and pyramided along with other genes with resistance to
bacterial blight into elite breeding lines in India, China, the Philippines and Korea, with
some being released for cultivation in China.
6.3.2
Distantly related species
Where closely related species do not provide suffi cient variation, then other more distantly
related species might be used, providing some means is available for transferring the
resistance where straight crosses do not produce viable progeny. This has involved the use
of cytogenetic technologies that have facilitated the intercrossing of species that would
otherwise remain genetically isolated. In the case of wheat, this has included the use of
a 'bridging' species, an example of which was the transfer of rust and eyespot (
Tapesia
yallundae
) resistance from
Aegilops ventricosa
. This wild tetraploid species 'V' was
crossed to another tetraploid wheat
Triticum persicum '
P' and the resulting hybrid was
then crossed to the French hexaploid wheat variety Marne 'M'. Progeny generated from
the hybridisation became known as 'VPM' lines (Doussinault
et al.
1983) and have been
used in the successful development of numerous wheat varieties with resistance to eyes-
pot and rust in the UK, France, United States and Australia (Friebe
et al.
1996).
A problem with obtaining genes from more distantly related species has been a lack
of recombination occurring between the chromosomes of the different species. This
