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Heterosis is an important component of hybrid yield performance. Identifying high yielding hybrids is expensive and involves testing large numbers of hybrid combinations in multi-environment trials. Molecular marker diversity has been proposed as a more efficient method of selecting superior combinations. The aim of this study was to investigate the value of molecular marker-based distance information to identify high yielding grain sorghum hybrids in Australia. Data from 48 trials were used to produce hybrid performance-estimates for four traits (yield, height, maturity and stay green) for 162 hybrid combinations derived from 70 inbred parent lines. Each line was screened with 113 mapped RFLP markers. The Rogers distances between the parents of each hybrid were calculated from the marker information on a genome basis and individually for each of the ten linkage groups of sorghum. Some of the inbred parents were related so the hybrids were classified into 75 groups with each group containing individual hybrids that showed similar patterns of Rogers distances across linkage groups. Correlations between hybrid-group performance and hybrid-group Rogers distances were calculated. A significant correlation was observed between whole genome-based Rogers distance and yield ( r = 0.42). This association is too weak to be of value for identifying superior hybrid combinations. One reason for the generally poor association between parental genetic diversity and yield may be that important QTLs influencing heterosis are located in particular chromosome regions and not distributed evenly over the genome. Variation in the sign and magnitude of correlations between Rogers distance and hybrid-group performance for particular linkage groups observed in this study support this hypothesis. The concept of using diversity on individual linkage groups to predict performance was explored. Using data from just two linkage groups 38% of the variation in hybrid performance for grain yield could be explained. A model combining phenotypic trait data and parental diversity on particular linkage groups explained 71% of the variation in grain yield and has potential for use in the selection of heterotic hybrids.

Microsatellites or simple sequence repeats (SSRs) are ubiquitous in eukaryotic genomes. Single-locus SSR markers have been developed for a number of species, although there is a major bottleneck in developing SSR markers whereby flanking sequences must be known to design 5'-anchors for polymerase chain reaction (PCR) primers. Inter SSR (ISSR) fingerprinting was developed such that no sequence knowledge was required. Primers based on a repeat sequence, such as (CA)n, can be made with a degenerate 3'-anchor, such as (CA)8RG or (AGC)6TY. The resultant PCR reaction amplifies the sequence between two SSRs, yielding a multilocus marker system useful for fingerprinting, diversity analysis and genome mapping. PCR products are radiolabelled with 32P or 33P via end-labelling or PCR incorporation, and separated on a polyacrylamide sequencing gel prior to autoradiographic visualisation. A typical reaction yields 20-100 bands per lane depending on the species and primer. We have used ISSR fingerprinting in a number of plant species, and report here some results on two important tropical species, sorghum and banana. Previous investigators have demonstrated that ISSR analysis usually detects a higher level of polymorphism than that detected with restriction fragment length polymorphism (RFLP) or random amplified polymorphic DNA (RAPD) analyses. Our data indicate that this is not a result of greater polymorphism genetically, but rather technical reasons related to the detection methodology used for ISSR analysis.

Finger millet (Eleusine coracana), an allotetraploid cereal, is widely cultivated in the arid and semiarid regions of the world. Three DNA marker techniques, restriction fragment length polymorphism (RFLP), randomly amplified polymorphic DNA (RAPD), and inter simple sequence repeat amplification (ISSR), were employed to analyze 22 accessions belonging to 5 species of Eleusine. An 8 probe--3 enzyme RFLP combination, 18 RAPD primers, and 6 ISSR primers, respectively, revealed 14, 10, and 26% polymorphism in 17 accessions of E. coracana from Africa and Asia. These results indicated a very low level of DNA sequence variability in the finger millets but did allow each line to be distinguished. The different Eleusine species could be easily identified by DNA marker technology and the 16% intraspecific polymorphism exhibited by the two analyzed accessions of E. floccifolia suggested a much higher level of diversity in this species than in E. coracana. Between species, E. coracana and E. indica shared the most markers, while E. indica and E. tristachya shared a considerable number of markers, indicating that these three species form a close genetic assemblage within the Eleusine. Eleusine floccifolia and E. compressa were found to be the most divergent among the species examined. Comparison of RFLP, RAPD, and ISSR technologies, in terms of the quantity and quality of data output, indicated that ISSRs are particularly promising for the analysis of plant genome diversity.

Genetic segregation experiments with plant species are commonly used for understanding the inheritance of traits. A basic assumption in these experiments is that each gamete developed from megasporogenesis has an equal chance of fusing with a gamete developed from microsporogenesis, and every zygote formed has an equal chance of survival. If gametic and/or zygotic selection occurs whereby certain gametes or zygotic combinations have a reduced chance of survival, progeny distributions are skewed and are said to exhibit segregation distortion. In this study, inheritance data are presented for the trait seed testa color segregating in large populations (more than 200 individuals) derived from closely related mungbean (Vigna radiata L. Wilcek) taxa. Segregation ratios suggested complex inheritance, including dominant and recessive epistasis. However, this genetic model was rejected in favor of a single-gene model based on evidence of segregation distortion provided by molecular marker data. The segregation distortion occurred after each generation of self-pollination from F1 thru F7 resulting in F7 phenotypic frequencies of 151:56 instead of the expected 103.5:103.5. This study highlights the value of molecular markers for understanding the inheritance of a simply inherited trait influenced by segregation distortion. [ABSTRACT FROM AUTHOR]

A linkage map of Lablab purpureus consisting of 127 RFLP and 91 RAPD loci was constructed in an F2 population of 119 individuals. This population was derived from a cross between ’Rongai’ (an annual cultivar) and CPI 24973 (a perennial wild accession). The map comprises 17 linkage groups and covers 1610 centiMorgans (cM) with an average distance of 7 cM between markers. Severe segregation distortions were observed, with the very extreme situation where no paternal type was recovered from the mapping population. These results strongly suggest the presence of a gene conferring preferential transmission from the maternal parent ’Rongai’. It was also clear that, while the majority of RAPD markers are valuable when used together with RFLP or other stringent marker systems, they could be problematic when used solely in mapping exercises. [ABSTRACT FROM AUTHOR]