Paper Presentation
Agriculture
Yogesh PARMESSUR
Email:
7677802/4541061 Ext 3258
Genic Mapping Of Sugarcane Variety M 134/75 Using EST-SSR Marker Systems
Parmessur Y, Aljanabi S and Dookun-Saumtally A
Mauritius Sugar Industry Research Institute
Abstract
Yellow spot is a major disease of sugarcane and can account for up to 35% yield loss in susceptible varieties cultivated in the superhumid zone of Mauritius. The disease caused by the fungus Mycovellosiella koepkei. The selection for resistant varieties in our current breeding program is costly, time-consuming and its accuracy is dependent on a number of factors such as relative humidity and pathogen inoculum. In an attempt to select more precisely for resistant varieties, a project was initiated to identify of molecular markers linked to yellow spot disease resistance quantitative trait loci (QTL) for their eventual use in marker assisted selection. In a previous study, a population of 227 progenies derived from a bi-parental cross between a yellow spot disease resistant parent (M 134/75) and a susceptible parent (R 570) was screened with AFLP and genomic sugarcane microsatellite markers (SSR). A genetic linkage map was constructed for variety M 134/75 containing 566 single dose markers and distributed across 95 linkage groups (LGs). Field disease resistance data of the mapping population scored over two ratoon crops coupled with molecular marker data enabled identification of a major QTL responsible for 23.8 % field phenotypic variation for field resistance to yellow spot. This QTL is flanked by an AFLP marker and an SSR marker at 14 cM and 18 cM respectively.
In order to fine map this QTL, sorghum EST-SSR markers were used to further saturate the genetic linkage map of variety M 134/75. A subset of 104 primers from a total of 600 sorghum EST-SSR available was screened on the mapping parents by PCR to test for their cross-species transferability to sugarcane. PCR products run by electrophoresis on agarose gels showed that only two sorghum EST-SSR primers failed to amplify, suggesting a high level of gene sequence conservation between sorghum and sugarcane. However, amplicon sizes were not comparable between sugarcane and sorghum probably due to variation in the number of SSR repeats or indels within the flanking primers. Mapping parents were screened in duplicate by 434 radioactively labeled EST-SSR primers among which 283 were found polymorphic for the resistant parent M 134/75. A total of 582 polymorphic markers were revealed with an average of 2.05 per primer, a figure comparable to genomic sugarcane SSR.
The mapping population of 227 progenies was screened with 21 EST-SSR primers and 68 markers were scored. This data was compiled with the existing AFLP and SSR marker data and a new genetic linkage map was contructed using gMendel software with a recombination threshold of 0.25 and an LOD threshold of 3.0. A genetic map of 102 linkage group was obtained containing 19 mapped sorghum markers. Marker order was visualized using the Mapchart software. The average map distance between mapped genic sorghum EST-SSR markers was smaller than average map distance of non-genic markers suggesting their clustering into gene rich regions. Furthermore, most sorghum EST-SSR markers were mapped into the extremity of the LGs, a phenomenon also observed in sorghum. In some cases, alleles of varying size derived from the same primer were found located on the same LG. A number of gene duplication events during sugarcane evolution from a common ancestor of sorghum could have lead to the formation of multiple copies of the same gene, differences in sizes arising most probably from non-coding regions.
The exploitation of sorghum genic marker systems in sugarcane is justified by its high level of gene collinearity with sugarcane. The initial step involves the identification sorghum markers located on the sugarcane LG harboring the major yellow spot disease resistance QTL. The corresponding region in sorghum containing these markers could eventually be identified from available extensive sorghum linkage maps. Thereafter, additional sorghum markers in this vicinity would be mapped back onto sugarcane, in an attempt to fine-map this QTL.
Keywords: sugarcane, sorghum, EST-SSR, linkage map and yellow spot disease.