The ability to observe allelic variation at isozyme loci has revolutionized research in the fields of biochemical genetics, population genetics, and evolution. This variation, called allozymic polymorphism, has been used in plants to examine genetic processes at every stage of the life cycle and to ascertain genetic diversity in all major crops as well as many other species. Yet the potential for using allozymes as genetic markers was not immediately predicted when isozyme variability was initially described (Hunter and Marker, 1957; Markert and Moller, 1959). In fact, the extent and prevalence of allozyme polymorphism was a rather disconcerting surprise to evolutionary biologists. Classical evolutionary theory had predicted that the most “efficient” form of an enzyme should, over time, become predominant in isolated populations, with an occasional rare allele produced through mutation. The discovery in many populations of relatively high levels of polymorphism at isozyme loci has forced a major reconsideration of evolutionary theory (Kimura and Crow, 1964; Koehn et al., 1983; Kimura, 1983).

Genetics of Plant Isozymes

Linkage maps for the apple cultivars ‘Prima’ and ‘Fiesta’ were constructed using RFLP, RAPD, isozyme, AFLP, SCAR and microsatellite markers in a ‘Prima’×‘Fiesta’ progeny of 152 individuals. Seventeen linkage groups, putatively corresponding to the seventeen haploid apple chromosomes, were obtained for each parent. These maps were aligned using 67 multi-allelic markers that were heterozygous in both parents. A large number of duplicate RFLP loci was observed and, in several instances, linked RFLP markers in one linkage group showed corresponding linkage in another linkage group. Distorted segregation was observed mainly in two regions of the genome, especially in the male parent alleles. Map positions were provided for resistance genes to scab and rosy leaf curling aphid (Vf and Sd
1, respectively) for the fruit acidity gene Ma and for the self-incompatibility locus S. The high marker density and large number of mapped codominant RFLPs and some microsatellite markers make this map an ideal reference map for use in other progenies also and a valuable tool for the mapping of quantitative trait loci.

Aligning male and female linkage maps of apple (Malus pumila Mill.) using multi-allelic markers

The use of polymorphic single genes to facilitate the process of plant breeding was proposed early in this Century (Sax, 1923). The basic principle is that selection for characters with easily detectable phenotypes can simplify the recovery of genes of interest linked to them and more difficult to score. The first marker loci available were those that have an obvious impact on the morphology of the plant. Genes that affect form, coloration, male sterility or disease resistance among others have been genetically analysed in many plant species. In some well characterized crops like maize, tomato, pea, barley or wheat, tens or even hundreds of such genes have been assigned to different chromosomes (O’Brien, 1990).

Marker-assisted selection

Woolly apple aphid (WAA; Eriosoma lanigerum Hausm.) can be a major economic problem to apple growers in most parts of the world, and resistance breeding provides a sustainable means to control this pest. We report molecular markers for three genes conferring WAA resistance and placing them on two linkage groups (LG) on the genetic map of apple. The Er1 and Er2 genes derived from ‘Northern Spy’ and ‘Robusta 5,’ respectively, are the two major genes that breeders have used to date to improve the resistance of apple rootstocks to this pest. The gene Er3, from ‘Aotea 1’ (an accession classified as Malus sieboldii), is a new major gene for WAA resistance. Genetic markers linked to the Er1 and Er3 genes were identified by screening random amplification of polymorphic deoxyribonucleic acid (DNA; RAPD) markers across DNA bulks from resistant and susceptible plants from populations segregating for these genes. The closest RAPD markers were converted into sequence-characterized amplified region markers and the genome location of these two genes was assigned to LG 08 by aligning the maps around the genes with a reference map of ‘Discovery’ using microsatellite markers. The Er2 gene was located on LG 17 of ‘Robusta 5’ using a genetic map developed in a M.9 × ‘Robusta 5’ progeny. Markers for each of the genes were validated for their usefulness for marker-assisted selection in separate populations. The potential use of the genetic markers for these genes in the breeding of apple cultivars with durable resistance to WAA is discussed.

Genome mapping of three major resistance genes to woolly apple aphid ( Eriosoma lanigerum Hausm.)

This paper is adapted from the text of a seminar of the same title presented as part of the Workshop on Sustainability of Vegetable Breeders’ Genetic Resources, 26 July 1993, at the 90th Annual Meeting of the American Society for Horticultural Science. It represents the distillation of a more extensive treatment of genetic markers and germplasm management (Bretting and Widrlechner, 1995). Disciplines Agricultural Science | Agriculture | Agronomy and Crop Sciences | Horticulture | Plant Sciences Comments This article is from HortScience 30, no. 7 (December 1995): 1349–1356. Rights Works produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted. This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/ncrpis_pubs/14

/pdf/genetic-markers-and-horticultural-germplasm-management-1x9rbkcb83.pdf

Genetic markers and horticultural germplasm management

Four zones of enzymatic activity for glutamate oxaloacetate transaminase (GOT) were found in apple tissue. A dimeric gene, GOT-1, determining the fastest migrating zone, was identified. Six alleles were found, including a near null allelle which produced detectable heterodimeric bands but not homodimeric bands. A marked deficit or absence of certain geno-types in all backcrosses and in some crosses between unrelated varieties was attributed to the close linkage (r=0.02±0.005) of GOT-1 with the incompatibility S locus. GOT-1 was also closely linked with the isocitrate dehydrogenase locus IDH-1 (0.03±0.01). Proposed incompatibility genotypes for four cultivars, and the linked GOT-1 alleles are 'Cox': S 1 b/S 2 d, 'Idared': S 3 a/S 4 c, 'Fiesta': S 3 a/S 2 d and 'Kent': S 3 a/S 1 b.

Inheritance and linkage relationships of glutamate oxaloacetate transaminase isoenzymes in apple : 1. The gene GOT-1, a marker for the S incompatibility locus.

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Inheritance and linkage relationships of glutamate oxaloacetate transaminase isoenzymes in apple : 1. The gene GOT-1, a marker for the S incompatibility locus.