The potential of bulk segregant analysis and RAPD technology for identification of molecular markers linked to traits in sugarcane.
The objective of the present study was to investigate the potential use of bulk segregant analysis (Michelmore et al., 1991) as a method to rapidly identify genetic markers linked to traits in sugarcane. Four bulked DNA samples were prepared from progeny of a sugarcane cross, AA157, based on segregation for the fibre trait. The bulks comprised five and ten individuals on either side of the fibre phenotypic extreme. The random amplified polymorphic DNA (RAPD) technique (Williams et ai., 1990) was used to screen for differences between the low and high fibre bulks. A total of 749 fragments were amplified in the bulks, eight of which were polymorphic. The segregation of the bulk specific polymorphism was analysed in 80 progeny of the AA157 cross; and seven were found to reproducibly segregate on a 1: 1 basis. This indicates that they are single dose fragments. A total of 79 polymorphisms were detected between the parents of the cross, indicating 10.5% variation in the genomic region sampled. Twenty two of the parental polymorphisms segregated as single dose fragments in the progeny of the cross AA157. Analyses of variance (ANOVAs), and multiple regression analyses, were used to ascertain linkage of the putative RAPD markers to fibre, and if linked, to determine the fibre variation ascribed respectively. Three RAPD fragments were found linked to the fibre trait. Fragments OPA17438 and OPC16889 (at the 99% significance level), and OPB1l464 (at the 95% significance level). These putative markers ascribed a total of 28.6% fibre variation in the 1993 season. The association of the RAPD markers with fibre in the different seasons (1992, 1993 and 1994) was investigated. Three RAPD markers were found linked to the fibre trait in each season, with a total of 5.5% and 31,4% fibre variation ascribed in the 1992 and 1994 seasons respectively. Marker OPA17438 was found to be linked to the fibre trait in all three seasons investigated, and marker OPC16889, was found linked to the fibre trait in the 1992 and 1993 seasons. Cross validation of the linkages of the RAPD markers to the fibre trait was carried out by a modified form of 'jacknifing' where the sample size was reduced to N-l0, and RAPD marker-fibre trait associations investigated as before. RAPD markers OPA17438 and OPC16889 were still consistent across the seasons, however marker OPA17438 was no longer linked to the fibre trait in the 1992 season. To investigate the genetic behaviour of RAPD based markers in sugarcane and the potential for their application in marker-assisted selection (MAS), two putative RAPD markers were converted to sequence characterised amplified regions (SCARs) (Paran and Michelmore, 1993). The RAPD fragments OPA17438, OPBl1464, and OPC16889 were excised from agarose gels, re-amplified and cloned into the pCR-Script SK (+) phagemid for sequencing. RAPD markers OPA17438 and OPB11 464 were converted to SCARs by using their sequences to design longer specific primers. A third SCAR marker, SAl1640, originally derived from sugarcane cDNA as a potential stem preferential expressed sequence tag, was included in the analysis to increase the sample size. All three SCAR markers segregated in a monomorphic fashion in the parents and progeny of the cross AA157. In addition, monomorphic length variants for markers, OPA17438 and OPB11 464 were detected with the SCAR amplification. All three SCARs segregated in a monomorphic fashion in different commercial varieties and bulks of S. officinarum and S. spontaneum, the progenitors of modern commercial varieties. The segregation analyses of the SCAR markers indicate that the RAPD polymorphism of marker SAl1640 was probably due to a point mutation or mismatch in the priming site. The segregation analyses of SCARs for the markers OPA17438 and OPB11464 indicate that their segregation in the RAPD analyses was due to an insertion mutation in the genetic locus. The combined results of the SCAR and RAPD segregation of markers OPA17438 and OPB11464 are indicative of preferential pairing in the cross AA157. Finally, to investigate the extent of linkage disequilibrium in a modern commercial variety, twenty two single dose RAPD fragments were investigated for their association with four traits in 53 progeny of cross AA157. The four traits investigated were fibre %cane, brix %cane, pol %cane and ers %cane over three seasons (1992, 1993 and 1994), at different ages of harvest (12, 8, and 9 months respectively). Seventeen linkages of RAPD markers to the four traits, over the three seasons, were detected. The phenotypic variation ascribed by the RAPD markers ranged from 7.6% fibre %cane variation explained by one marker in 1992, 29.6% fibre %cane (three markers) in the 1993 season to 10% (three markers) in 1994. A total of 14.1% brix %cane variation was ascribed by two markers in 1992, 9.6% (one marker) in 1993 and 16.3% (two markers) in the 1994 season. A total of 13.5% estimated recoverable sucrose %cane was ascribed by one marker in 1992, 12% (two markers) in 1993 and 15.3% (two markers) in the 1994 season. Two markers explained 17.2% pol %cane variation in 1992 and 25.4% in the 1994 season. Only four markers were detected across different environments, three of which were linked to fibre. These were OPA17438, OPB16618 and OPC16889, each linked to fibre in two seasons. RAPD marker OPB11 464 was linked to estimated recoverable sucrose %cane in two seasons. Two markers were found associated with different traits in a single season. RAPD marker OPB11 464 was found associated with brix %cane and estimated recoverable sucrose %cane in the 1993 season, and RAPD marker OPA17438 was found associated with all four traits in the 1994 season.