Gene pools of pigeonpea conquered
Introgression of useful traits from wild relatives of pigeonpea
It is not easy to cross legumes from different gene pools to produce usable hybrids, but scientists at ICRISAT recently made a breakthrough when they successfully crossed wild relatives of pigeonpea with a cultivated one to produce fertile hybrids. The fact that the hybrids are fertile (ie, no further human intervention is necessary to produce new generations), is highly prized. This significant breakthrough opens up avenues to not only broaden the genetic base of pigeonpea with distant (in terms of gene pools) genetic material, but to introduce both nutritional and therapeutic traits into the plants for the future.
ICRISAT mandate legumes, like many other leguminous crops, have a narrow genetic base, ie, there are not too many choices from which to draw useful genes. In such instances, wild relatives gain prominence. Wild relatives of crop plants, conserved in genebanks for present and future use, are priceless components of germplasm collections as they are sources of genetic variation much needed to improve crop plants.
One of ICRISAT’s mandate legumes, pigeonpea [Cajanus cajan (L.) Millspaugh], is a major source of protein for vegetarians in Asia, Africa and the Caribbean, but levels of resistance to some insect pests, diseases and abiotic stresses in this cultivated gene pool are low. Exploiting wild relatives from different gene pools (see Box A) has broadened the genetic base of pigeonpea and made it possible to introduce useful traits to create new improved varieties. Different techniques were used to make inter-specific crosses.
A gene pool is defined as a pool of species that can inter-breed. Among plants, a primary gene pool comprises cultivated species and landraces within a species that can readily cross-breed with each other. The secondary gene pool comprises species that can be cross-bred with cultivated species, and the tertiary gene pool comprises those species that can be cross-bred only by using advanced techniques (such as embryo rescue). A quaternary gene pool comprises related genera, but these are not reported to cross with each other.
Gene pools of the genus Cajanus.
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Pigeonpea, Cajanus cajan, originated in India, and its progenitor Cajanus cajanifolius is separated from it by only 5 to 6 traits, and hence the narrow genetic base. So, using traits from several wild relatives in the secondary gene pool, scientists were able to broaden pigeonpea’s genetic base. Progenies with resistance to pod borers, eg, Helicoverpa armigera (Hubner), were developed utilizing Cajanus acutifolius, a wild relative from the secondary gene pool. Some of the resultant progenies were also resistant to insect pests such as pod fly (Melanagromyza obtusa) and bruchids (Callosobruchus maculatus). In addition, many of these progenies also had a higher 100-seed weight than normal, and a desirable beige color seed coat. At present there are 70 such lines available at ICRISAT. A source of cytoplasmic male sterility (CMS), named the A5 CMS system, was also derived from C. acutifolius. Another source of CMS and bruchid resistance was Cajanus lanceolatus from the secondary gene pool, in addition to a few other Cajanus species that offered useful traits for introgression into new lines.
Little or no pollen on the anthers showing male sterility in the progeny from the cross
C. platycarpus x C. cajan
E15: Flower, anther bundle and single anther, E 4: Flower, anther bundle and anther,
85010: Flower, anther bundle and single anther
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The tertiary gene pool also holds many wild relatives with useful traits. So far, ICRISAT is the only institute to have successfully utilized wild relatives from this gene pool. C. platycarpus, a wild species from the tertiary gene pool, was used to develop advanced generation lines with multiple disease and insect resistance. At present there are 140 stable lines derived from C. platycarpus. Another CMS system, called A7, was developed utilizing traits from this species. Yet another wild species from the tertiary gene pool, Cajanus volubilis, was utlilized to develop high yielding, extra-short duration and dwarf progeny lines.
Tapping useful genetic variation from C. platycarpus
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Rhynchosia bracteata branch with pods.
Venturing into the realm of the quaternary gene pool of pigeonpea, we find 11 related genera. The one currently evoking much interest is genus Rhynchosia, which has more than 200 species.
Many of the Rhynchosia species are sources of nutritional and therapeutic properties highly desirable in traditional medicine, and used in several countries. The list of the phytochemicals they contain is impressive – alkaloids, glycosides, anthraquinones, carotenoids, coumarins, dihydrochalcones, fatty acids, flavonoids, steroids and triterpenoids – these are used for medicinal treatment of both humans and animals. Some tribal communities in India soak the seeds, boil them in water, decant several times and eat them for nutritional benefits and to help with the healing of wounds. Apart from this, many of the Rhynchosia species are known to exhibit antitumor, and thus curative, properties.
ICRISAT has successfully cross-bred two Rhynchosia species with cultivated species of pigeonpea, thus leading the way for the development of many more pigeonpea hybrids with useful traits from this wild relative.
Lead picture: Dr Nalini Mallikarjuna, Principal Scientist (Cell Biology).
For more information contact N.Mallikarjuna@cgiar.org or firstname.lastname@example.org or C.Gowda@cgiar.org
Find more information about genetic research on other ICRISAT crops at http://witblog.icrisat.org/?p=1306.
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