|Bell, Alois - Al|
Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/19/2007
Publication Date: 10/1/2007
Citation: Robinson, A.F., Bell, A.A., Dighe, N., Menz, M.A., Stelly, D.M., Nichols, R.L. 2007. Introgression of resistance to nematode Rotylenchulus reniformis into Upland cotton Gossypium hirsutum from G. longicalyx. Crop Science. 47:1865-1877.
Interpretive Summary: There are many kinds of microscopic worms called nematodes that feed on the roots of cotton and other crops, debilitating the plants and markedly decreasing yields. The reniform nematode is one of the most important nematodes on cotton in the U.S. and it is estimated to cause cotton crop losses exceeding $120M every year. All varieties of cotton available to farmers are susceptible to this nematode but there are wild species of cotton that are resistant. Thus, theory says it may be possible to make genetic crosses between resistant wild species and agronomic cotton to develop resistant varieties for farmers. However, it is exceedingly difficult to do this because resistant wild species and agronomic cotton are almost completely incompatible, so that when they are cross pollinated, seed production is rare and any seed produced give rise to plants which in turn are incompatible. To make the goal even more challenging, farmers these days demand that the varieties of cotton they plant have a number of essential agronomic traits and transgenes that seed companies own. Consequently, realistic final development of nematode resistant varieties has to be done by seed companies. This requires that these companies have a compatible source of resistance, make the requisite series of sequential cross pollinations on thousands of plants, and after each cross identify which plants are both resistant and optimal for other essential traits. This means they must either monitor nematode populations in thousands of pots, which is prohibitively expensive for them and incompatible with retention of other essential traits, or else analyze DNA from plant leaves, which is routine these days and can be done quickly and cheaply, but only provided that appropriate and very specific, so-called molecular markers are known. In this study, USDA scientists utilized novel hybrid technology and information from chromosome studies to transfer a very high level of reniform nematode resistance into agronomic cotton from a wild plant from Africa called Gossypium longicalyx. They accomplished this, basically, by creating special hybrid plants, making tens of thousands of attempted crosses with those hybrids to obtain 600 seedlings, eliminating plants that had abnormal chromosomes, and identifying resistant plants by measuring nematode populations. Once normal plants carrying the resistance were in hand, the scientists made thousands of additional sequential crosses, evaluated nematode population development on thousands of additional plants, and conducted DNA studies that resulted, after 6 years of work, in the development of a package of molecular markers and breeding lines that are both resistant and agronomic. Public release of molecular markers and seed generated by this study is expected within 6 months. This accomplishment will make the development and sale of reniform nematode resistant varieties into a highly attractive, straightforward, and cost-effective venture for the cotton seed industry. The anticipated payoff from this research at the U.S. farm gate during the next 10 years is 1 billion dollars.
Technical Abstract: Absence of sources of resistance to the reniform nematode, Rotylenchulus reniformis, is a major impediment to the production of upland cotton (Gossypium hirsutum) in the United States. In this study, two triple hybrids of G. hirsutum, G. longiclalyx, and either G. armourianum or G. herbaceum were utilized as bridges to introgres high resistance to the nematode from G. longicalyx into agronomic G. hirsutum. Introgression was accomplished by recurrent backcrosses to G. hirsutum with cytogenetic analysis of early backcross generations to confirm acquisition of 52 normal chromosomes, selection for nematode resistance at each generation, and examination of self progeny at the first, third, sixth, and seventh backcross to identify and eliminate lineages with undesired recessive traits. Altogether, 689 first backcross generation progeny were generated from the two male-sterile hybrids and 28 progeny families descended from these and carrying nematode resistance, were backcrossed four to seven times to G. hirsutum. The resistance trait segregated (resistant: susceptible) 1:1 in backcross progeny and 3:1 in self progeny. The co-dominant marker BNL 3279 was closely linked (1.7 cM) and marker analysis of 480 self progeny indicated that genotypes segregated 1:2:1 (homozygous resistant: heterozygous resistant: susceptible). There was no obvious diminution of the resistance across seven backcross generations. Hundreds of plants descended from the triple hybrids were indistinguishable from agronomic cotton under greenhouse conditions, and comparisons of 240 homozygous resistant BC6S2 plants with heterozygous, susceptible, and recurrent parent plants in field plantings in 2006 showed normal lint quality and quantity from homozygous resistant plants. The upcoming release of seed from this project is expected to provide the cotton industry with a major new tool for managing the reniform nematode in cotton.