Cytogenetics and fertility of an induced tetraploid Sorghum bicolor x S. propinquum hybrid

Authors: PORTER, NICK - Texas A&M University; Burson, Byron; Washburn, Jacob; Klein, Robert - Bob; JESSUP, RUSSELL - Texas A&M University

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/9/2021
Publication Date: 5/1/2021
Publication URL: https://handle.nal.usda.gov/10113/7709466
Citation: Porter, N.T., Burson, B.L., Washburn, J.D., Klein, R.R., Jessup, R.W. 2021. Cytogenetics and fertility of an induced tetraploid Sorghum bicolor x S. propinquum hybrid. Crop Science. 61(3):1881-1889. https://doi.org/10.1002/csc2.20482.
DOI: https://doi.org/10.1002/csc2.20482

Interpretive Summary: Sorghum is grown throughout much of the world primarily to produce grain for human and livestock consumption; however, some types are grown for forage and sugar production. Grain sorghum is the most widely grown, but it grows as an annual and must be replanted every year. Growing perennial sorghums rather than annuals can provide both economic and environmental benefits to farmers and society. Perennial sorghum species have underground stems called rhizomes, and this trait needs to be transferred into annual sorghum for it to be a perennial. Johnsongrass cannot be used because it has vigorous spreading rhizomes that make it a very invasive weed. A perennial sorghum species that grows in southeast Asia, known only as Sorghum propinquum, produces short, non-spreading rhizomes, is a better candidate for transferring rhizomes into annual sorghum to produce a perennial forage or grain crop. Grain sorghum was crossed with this species and promising F1 hybrids were recovered. The chromosomes of one hybrid were doubled (increased from 20 [diploid] to 40 [tetraploid]) with a chemical. Chromosome pairing behavior in the developing pollen of all these plants were microscopically examined. The following was determined: grain sorghum and this Asian species are genetically closely related; the F1 hybrids and the tetraploid hybrid are fertile enough to be propagated by seed; and they are perennials with short rhizomes that do not pose a weediness threat. Plans are to advance these plants in a selection and breeding program.

Technical Abstract: Developing perennial alternatives to annual crops can alleviate some of agriculture's current challenges. Perennial sorghum has potential, being a low maintenance, drought tolerant, high yielding forage or bioenergy crop. Cytoplasmic male sterile Sorghum bicolor (L.) Moench (ATx623) was crossed with S. propinquum (Kunth) Hitchc. accession, and the F1 hybrids were treated with colchicine to produce novel tetraploids. One tetraploid plant was recovered and is the first induced tetraploid S. bicolor x S. propinquum hybrid reported. The fertility and cytology of this novel S. bicolor x S. propinquum tetraploid hybrid, the diploid F1 S. bicolor x S. propinquum hybrids, and both parents were studied to learn more about the relationship between S. bicolor and S. propinquum. Pollen stainability ranged from 85 to 90% in S. bicolor (BTx623), S. propinquum, and the male fertile F1 hybrids, but it was lower (57%) in the induced tetraploid hybrid. Mean seed set of both parents was 65-67% but it was lower (43%) for the tetraploid hybrid. As expected, meiotic chromosome pairing in both diploid (2n=2x=20) parents consisted primarily as 10 bivalents. The diploid F1 hybrids also consisted primarily of 10 bivalents at metaphase I, but a higher frequency of univalents than observed in the parents indicating small chromosomal differences. Meiotic chromosome pairing in the tetraploid hybrid (2n=4x=40) included 0.45 univalents 17.77 bivalents, 0.02 trivalents, and 0.98 quadrivalents per cell. These cytological findings support previous reports that S. bicolor and S. propinquum have a homologous genome but identified meiotic differences between the tetraploid hybrid and S. halepense (L.) Pers. This brings into question whether or not S. propinquum is a progenitor of S. halepense.