The Hs1(pro-1) locus confers resistance to the beet cyst nematode (Heterodera schachtii Schmidt), a major pest in the cultivation of sugar beet (Beta vulgaris L.). The Hs1(pro-1) gene was cloned with the use of genome-specific satellite markers and chromosomal break-point analysis. Expression of the corresponding complementary DNA in a susceptible sugar beet conferred resistance to infection with the beet cyst nematode. The native Hs1(pro-1) gene, expressed in roots, encodes a 282-amino acid protein with imperfect leucine-rich repeats and a putative membrane-spanning segment, features similar to those of disease resistance genes previously cloned from higher plants.

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Rhizomania is a serious disease of sugar beet, caused by beet necrotic yellow vein virus (BNYVV). The disease can only be controlled by the use of resistant cultivars. The accession Holly contains a single dominant gene for resistance, called Rz. The identification of a locus for resistance that differs from Rz would provide possibilities to produce cultivars with multiple resistance to BNYVV. Inheritance of resistance to BNYVV was studied by screening progenies of crosses between resistant plants of the accessions Beta vulgaris subsp. maritima WB42 and B. vulgaris subsp. vulgaris Holly-1-4 or R104. Observed and expected segregation ratios were compared to elucidate whether the resistance genes in the three accessions are alleles or situated on different loci. STS markers, linked to the genes for resistance, were used to study the segregation in more detail. The results demonstrated that the genes for resistance to BNYVV inHolly-1-4 and WB42 are closely linked. The gene for resistance in R104 is at the same locus as in Holly-1-4, and also closely linked to the gene in WB42. As the Holly resistance gene has been named Rz, the name Rz2 is proposed to refer to the resistance gene in WB42. Consequently, the gene Rz should be referred to as Rz1.

Tomato lines from diverse breeding programs were evaluated in the field for resistance to a natural infestation of the potato aphid, Macrosiphum euphorbiae, in Davis, CA. It was noted that all lines that carried the nematode-resistance gene, Mi, displayed aphid resistance. A greenhouse assay for aphid resistance was developed to investigate this relationship. Association of nematode and aphid resistances in near-isogenic lines suggested that these traits are tightly linked. Analysis of an F2 population segregating for nematode resistance indicated that aphid resistance segregated as a single major locus genetically linked to Mi. The name Meul is proposed for this locus. It is likely that Meul was introduced into tomato along with Mi from the wild species Lycopersicon peruvianum. The presence of aphid resistance in the line Motelle, which contains a very small region of introgressed DNA, and the lack of recombinants suggest that Meul is tightly linked to Mi or possibly is the same gene. The map-based strategy currently being used to clone Mi should be applicable to cloning Meul. The potato aphid, Macrosiphum eluphorbiae (Thomas), is capable of infesting many plant species. This pest can build up to large numbers on tomato and cause severe damage to some varieties (1-3). Damage includes yield losses caused by stunted growth; shoot dieback; malformation of the leaves; and terminal growth, chlorosis, and necrosis of leaves. Aphids are also efficient virus vectors for many plant viruses. M. euphorbiae is becoming a significant problem in the Sacramento valley area of California, and pesticides are currently being used to control the insect. Because of environmental concern over the use of pesticides in agriculture, alternate means of pest control are being sought. Host-plant resistance is a method of choice for plant-pest suppression. Resistance to potato aphid has been reported in Lycopersicon pennellii Corr (D'Arcy), a wild relative of cultivated tomato Lycopersicon escitlentucm Mill (4).Incorporation of the resistance into tomato has not yet been successful because of the complex inheritance of resistance (5). The resistance is attributed to the sugar esters present in the glandular exudates of type IV trichomes (6).Field observation of a diverse group of tomato lines for resistance to natural infestations of insects revealed a correlation between potato aphid resistance and nematode resistance. It was noted that tomato lines carrying the nematoderesistance gene, Mi, had relatively low numbers of aphids. Mi is a single dominant gene conferring resistance to three species of root-knot nematodes (7,8). The gene was introduced to cultivated tomato from its wild relative Lycopersicon pernvianuim (9). Isozyme and DNA markers tightly linked to Mi have been identified, and a detailed genetic map of the Mi region has been obtained (10-12). A PCR-based marker, REX-1, tightly linked to Mi, has been developed to easily follow segregation of Mi. No recombinants between REX-1 and Mi have been reported (13). In this...

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From crosses between diploid and autotetraploid cytotypes of Hordeum vulgare L. (cultivated barley) and H. bulbosum L. (bulbous barley grass) diploid, triploid and tetraploid interspecific hybrids were produced. Both directly and after vegetative segregation crosses in either direction also gave rise to haploids and dihaploids resembling H. vulgare. The use of embryo culture was necessary. Plant morphology of the hybrids was much like that of H. bulbosum, although the hybrid plants were less vigorous. Meiosis in the hybrids was more or less disturbed, and this seemed to be the main cause of the high level of sterility.