Relatedness of Saccharum species hybrids and wild relatives in eastern South Africa
Gene flow between crops and their cross-compatible wild relatives is undesirable in commercial production systems. The development of genetically modified (GM) sugarcane is set to provide new opportunities to increase yield and grow the global competitiveness of the South African sugar industry. However, the concern when cultivating GM plants is that the transfer of transgenes to related species may enhance their capacity for invasiveness. Therefore, biosafety studies are a legal requirement to evaluate the potential impact of GM crops on the environment before commercial release. The aim of the study was to contribute data to such an initiative by assessing the gene flow potential from sugarcane to its wild relatives in the major sugar production regions of the Mpumalanga and KwaZulu-Natal provinces of South Africa. Three approaches were followed: (1) a systematic literature review was conducted to identify individuals reported to have spontaneously hybridised with sugarcane in the past; (2) two chloroplast (matK and rbcL) and one nucleic (ITS) DNA barcodes of related species were sequenced and phylogenetic analyses were done to investigate relatedness with Saccharum species hybrids; and (3) field assessments were conducted to determine pollen viability of commercial sugarcane varieties using the Iodine Potassium Iodide (IKI) and Triphenyl Tetrazolium Chloride (TTC) staining methods. A total of 36 hybridization incidents were reported in the literature, of which none were spontaneous hybridization with sugarcane. These crosses were mainly made with the genera Saccharum (Syn. Erianthus) and Sorghum. Regardless of breeder’s efforts to cross sugarcane or its progenitors with various species, hybrid generation was rare, and if successful, hybrids were reported to be weak with a slow growth rate. Since members of the Saccharinae and Sorghinae, which have successfully been crossed with sugarcane, are prominent in the study area, phylogenetic studies were conducted to determine the degree of relatedness at the genome level. Generally, sugarcane was found to be closely related to its wild relatives, as the average nucleotide identity (ITS, rbcL and matK) ranged from 78-98%. Based on the three locus barcode clustering, pairwise distances and sequence identity of related species against the sugarcane varieties, Sorghum arundinaceum, Miscanthus ecklonii and Imperata cylindrica are the most closely related species to commercial sugarcane varieties in the cultivation areas. A test for statistical difference revealed that there was no significant difference (p-value= 0.622) between varying percentage pollen viability obtained from the IKI and TTC stains respectively. Pollen viability was found to decrease from the northern regions (85%) to the southern regions (0%) of the study area. The irrigated regions showed the highest pollen viability, which was frequently measured from sugarcane varieties N14 and N36. A significant association between pollen viability with day length, maximum temperature (positive) and soil water content (negative) was observed. However, the study suggested that pollen viability is largely determined by the genetic makeup of the individual and mean maximum temperature. None of the commercial varieties with inserted transgenes showed any pollen viability in this study. Even though sugarcane has the potential to hybridize with the closely related members of the Sorghinae and Saccharinae which are also present in the sugar production regions, gene flow would not occur without the production of viable pollen However, some of the GM developments are with unreleased genotypes, thus, future studies are required to evaluate pollen viability from these genotypes especially when cultivated in the irrigated regions.