DEVELOPMENT OF MOLECULAR MARKERS FOR MARKER-ASSISTED SELECTION OF DIEBACK DISEASE RESISTANCE IN LETTUCE (LACTUCA SATIVA)

The Molecular Determination of Hybridity and Homozygosity Estimates in Breeding Populations of Lettuce (Lactuca sativa L.)

The development of new varieties of horticultural crops benefits from the integration of conventional and molecular marker-assisted breeding schemes in order to combine phenotyping and genotyping information. In this study, a selected panel of 16 microsatellite markers were used in different steps of a breeding programme of lettuce (Lactuca sativa L., 2 n = 18). Molecular markers were first used to genotype 71 putative parental lines and to plan 89 controlled crosses designed to maximise recombination potentials. The resulting 871 progeny plants were then molecularly screened, and their marker allele profiles were compared with the profiles expected based on the parental lines. The average cross-pollination success rate was 68 ± 33%, so 602 F1 hybrids were completely identified. Unexpected genotypes were detected in 5% of cases, consistent with this species' spontaneous out-pollination rate. Finally, in a later step of the breeding programme, 47 different F3 progenies, selected by phenotyping for a number of morphological descriptors, were characterised in terms of their observed homozygosity and within-population genetic uniformity and stability. Ten of these populations had a median homozygosity above 90% and a median genetic similarity above 95% and are, therefore, particularly suitable for pre-commercial trials. In conclusion, this study shows the synergistic effects and advantages of conventional and molecular methods of selection applied in different steps of a breeding programme aimed at developing new varieties of lettuce.

High-Resolution DNA Melting Analysis in Plant Research

Genetic and genomic studies provide valuable insight into the inheritance, structure, organization, and function of genes. The knowledge gained from the analysis of plant genes is beneficial to all aspects of plant research, including crop improvement. New methods and tools are continually being developed to facilitate rapid and accurate mapping, sequencing, and analyzing of genes. Here, I review the recent progress in the application of high-resolution melting (HRM) analysis of DNA, a method that allows detecting polymorphism in double-stranded DNA by comparing profiles of melting curves. Use of HRM has expanded considerably in the past few years as the method was successfully applied for high-throughput genotyping, mapping genes, testing food products and seeds, and other areas of plant research.

Complete genome sequence and biological characterization of Moroccan pepper virus (MPV) and reclassification of Lettuce necrotic stunt virus as MPV

Moroccan pepper virus (MPV) and Lettuce necrotic stunt virus (LNSV) have been steadily increasing in prevalence in central Asia and western North America, respectively, over the past decade. Recent sequence analysis of LNSV demonstrated a close relationship between the coat proteins of LNSV and MPV. To determine the full extent of the relationship between LNSV and MPV, the genomes of three MPV isolates were sequenced and compared with that of LNSV. Sequence analysis demonstrated that genomic nucleotide sequences as well as virus-encoded proteins of the three MPV isolates and LNSV shared 97% or greater identity. A full-length clone of a California LNSV isolate was developed and virus derived from infectious transcripts was used to evaluate host plant reactions under controlled conditions. Symptoms of LNSV matched those described previously for MPV on most of a select series of host plants, although some differences were observed. Collectively, these molecular and biological results demonstrate that LNSV should be classified as MPV within the family Tombusviridae, genus Tombusvirus, and confirm the presence of MPV in North America.