A Diploid, Interspecific, Fertile Hybrid from Cultivated Sorghum, Sorghum Bicolor, and the Common Johnsongrass Weed Sorghum Halepense

Abstract

Valuable agronomic traits are often present but inaccessible in the wild relatives of cultivated crop species. Utilization of wild germplasm depends on the production of fertile interspecific hybrids. Several unsuccessful attempts have been made to hybridize cultivated sorghum with its wild relatives to broaden its genetic base and enhance agronomic value. The successful approach used in this study employed the nuclear male sterility gene ms3 to generate a diploid fertile hybrid between the diploid cultivated sorghum (Sorghum bicolor (L) Pers.) and its weedy tetraploid wild relative Johnsongrass (Sorghum halepense (L.) Pers.). Eight sorghum plants were selected from a Nebraska stiff stalk collection that contains the male sterility gene ms3 and were used as the female parent. About 36,000 florets of male sterile sorghum were pollinated with Johnsongrass pollen to produce an average of one well-developed and 180 severely shriveled seed/18,000 crosses. The well-developed seed gave rise to a self-fertile diploid, while none of the shriveled seed were able to germinate. The F1 hybrid was confirmed by using cultivated sorghum SSR markers and was selfed to produce an F2 population. A sub-sample of 96 segregating F2 plants was examined with 36 sorghum polymorphic SSR markers. Thirty-four markers showed a normal 1:2:1 segregation ratio, evidence of normal recombination across the genome. Preliminary results showed that several desirable traits from Johnsongrass, including resistance to greenbug and chinch bug and adaptability to cold temperatures, were expressed in the resulting progenies. These observations suggest that speciation within the genus Sorghum, giving rise to widely divergent phenotypes, is effected largely by ploidy-maintained crossing barriers but apparently not by extensive genomic divergence.