The use of crop wild relatives (CWR) genes to improve crop performance is well established with important examples dating back more than 60 years. In this paper, we review available information on the presence of genes from CWR in released cultivars of 16 mandate crops of the CGIAR institutes, and some selected additional crops, focusing on the past 20 years—the period since a comprehensive review by Robert and Christine Prescott-Allen in 1986. It appears that there has been a steady increase in the rate of release of cultivars containing genes from CWR. While there continues to be a strong emphasis on using pest and disease resistance genes, a wider range of characteristics are being introduced than in the past. Those crops whose wild relatives have traditionally been used as sources of useful traits (e.g., wheat, tomato) continue to be most likely to include new genes from their wild relatives. CWR are continually gaining in importance and prevalence, but, we argue, their contributions to the development of new cultivars remain less than might have been expected given improved procedures for intercrossing species from different gene pools, advances in molecular methods for managing backcrossing programes, increased numbers of wild species accessions in gene banks, and the substantial literature on beneficial traits associated with wild relatives.

The use of wild relatives in crop improvement: a survey of developments over the last 20 years

Resistance against the aphid Macrosiphum euphorbiae previously was observed in tomato and attributed to a novel gene, designated Meu-1, tightly linked to the nematode resistance gene, Mi. Recent cloning of Mi allowed us to determine whether Meu-1 and Mi are the same gene. We show that Mi is expressed in leaves, that aphid resistance is isolate-specific, and that susceptible tomato transformed with Mi is resistant to the same aphid isolates as the original resistant lines. We conclude that Mi and Meu-1 are the same gene and that Mi mediates resistance against both aphids and nematodes, organisms belonging to different phyla. Mi is the first example of a plant resistance gene active against two such distantly related organisms. Furthermore, it is the first isolate-specific insect resistance gene to be cloned and belongs to the nucleotide-binding, leucine-rich repeat family of resistance genes.

https://www.pnas.org/content/pnas/95/17/9750.full.pdf

The nematode resistance gene Mi of tomato confers resistance against the potato aphid

In mammals, cadmium is widely considered as a non-genotoxic carcinogen acting through a methylation-dependent epigenetic mechanism. Here, the effects of Cd treatment on the DNA methylation patten are examined together with its effect on chromatin reconfiguration in Posidonia oceanica. DNA methylation level and pattern were analysed in actively growing organs, under short- (6 h) and long- (2 d or 4 d) term and low (10 mM) and high (50 mM) doses of Cd, through a Methylation-Sensitive Amplification Polymorphism technique and an immunocytological approach, respectively. The expression of one member of the CHROMOMETHYLASE (CMT) family, a DNA methyltransferase, was also assessed by qRT-PCR. Nuclear chromatin ultrastructure was investigated by transmission electron microscopy. Cd treatment induced a DNA hypermethylation, as well as an up-regulation of CMT, indicating that de novo methylation did indeed occur. Moreover, a high dose of Cd led to a progressive heterochromatinization of interphase nuclei and apoptotic figures were also observed after long-term treatment. The data demonstrate that Cd perturbs the DNA methylation status through the involvement of a specific methyltransferase. Such changes are linked to nuclear chromatin reconfiguration likely to establish a new balance of expressed/repressed chromatin. Overall, the data show an epigenetic basis to the mechanism underlying Cd toxicity in plants.

/pdf/in-posidonia-oceanica-cadmium-induces-changes-in-dna-3cu7uj83mr.pdf

In Posidonia oceanica cadmium induces changes in DNA methylation and chromatin patterning

It has been suggested that genetic modification could lead to crops with enhanced invasiveness and persistence. These new genotypes could invade natural ecosystems and cause undesirable change, either through spread of the crops themselves or through hybridization with wild relatives. We review the progress made in the genetic modification of the major UK crops, and identify those crops and traits in which genetic modification is most advanced. Data on the potential for the spread of transgenes through pollen movement and the relative performance of modified and unmodified crops are examined. It is concluded that the spread of modified crops and their hybrids with wild relatives can be modelled in the same manner as for unmodified crops (...)

Genetically modified crops and hybridization with wild relatives: a UK perspective

Electrical penetration graphs (EPG's) of Nasonovia ribisnigri (Mosley) (Homoptera, Aphididae) on resistant and susceptible lettuce (Lactuca sativa, Compositae) showed a large reduction in the duration of the food uptake pattern (E2) on the resistant line. No differences in EPG's were observed before the phloem was reached. Therefore, resistance is believed to be located in the phloem vessel. Both mechanical blocking of the sieve element after puncturing and a difference in composition of the phloem sap are possible resistance factors. However, a chemical factor seems more likely because of the specificity of the resistance against N. ribisnigri.

Tissue localisation of lettuce resistance to the aphid Nasonovia ribisnigri using electrical penetration graphs

Interspecific crosses were carried out between the three Lactuca species L. virosa, L. serriola and L. sativa to transfer resistance to the leaf aphid Nasonovia ribis nigri from L. virosa to the cultivated lettuce. L. sativa, L. serriola was used as an intermediate parent between the other two species. Many irregularities were observed in the interspecific hybrids, ranging from premature dying of F1 plants resulting from a kind of bastard necrosis till complete male and female sterility and deviating microsporogenesis. Using in vitro culture and after several backcrosses male and female fertile plants were obtained with a L. sativa habit and with resistance to the leaf aphid.

Resistance of lettuce (Lactuca) to the leaf aphid Nasonovia ribis nigri. 1. Transfer of resistance from L. virosa to L. sativa by interspecific crosses

The inheritance of resistance to Nasonovia ribis nigri in L. sativa was investigated. Parents and F1 and F2 populations from crosses between the susceptible cultivar Ravel and two resistant breeding lines were tested. In both breeding lines one dominant gene appeared responsible for resistance.

Resistance of lettuce (Lactuca) to the leaf aphid Nasonovia ribis nigri. 2. Inheritance of the resistance

Six crosses between six lettuce cultivars with partial resistance to Myzus persicae were evaluated in the F3 generation. In one cross, after F3 line selection, lines were obtained witha significantly reduced aphid reproduction. Compared to modern lettuce cultivars the most resistant lines have shown a very valuable level of resistance. Introduction of this resistance into modern cultivars could reduce the aphid problem in lettuce growing.

Selection of lines of lettuce ( Lactuca sativa L.) with a high level of partial resistance to Myzus persicae (Sulzer)

A reliable impression of the resistance of lettuce plants to the leaf aphid Myzus persicae can be obtained via aphid honeydew production. Under controlled temperatures, the number of honeydew droplets produced by these aphids per plant in 180 minutes with five plants per genotype offers a good criterion of this resistance.

An instant bioassay for resistance of lettuce to the leaf aphid Myzus persicae

Summary

The genetics of partial resistance (PR) of lettuce to the leaf aphid species Macrosiphum euphorbiae and Uroleucon sonchi was studied in F1 and F2 generations of six crosses between four cultivars. Partial resistance inherited mainly additively. With M. euphorbiae the estimates of broad-sense F2 heritability were low and non-significant in crosses between parents of the same resistance level (S x S and PR x PR), and high (0.69-0.76) in crosses between parents of different resistance level (PR x S). The level of resistance in lettuce cultivars ‘Charan’ and ‘Marbello’ led to a marked reduction in percentage of plants with colonies of M. euphorbiae in a field experiment. With U. sonchi the highest estimates of F2 heritability were found in ‘Charan x ‘Avoncrisp’ (0.51) and ‘Charan’ x ‘Chou de Naples' (0.48). In the cross ‘Charan’ x ‘Marbello’ (PR x PR) no significant segregation for resistance to M. euphorbiae was found, indicating that these cultivars may have the same genetic constitution for resistance to M. euphorbiae.‘Charan’ has additional genes for resistance to U. sonchi, that are not, or only weakly linked to the gene(s) for resistance to M. euphorbiae. The heritability estimates for the partial resistance to M. euphorbiae and U. sonchi indicate that it is possible to effectively select for these traits in plant breeding programmes.

R. Groenwold

Papers

Resistance of lettuce (Lactuca) to the leaf aphid Nasonovia ribis nigri. 1. Transfer of resistance from L. virosa to L. sativa by interspecific crosses

Resistance of lettuce (Lactuca) to the leaf aphid Nasonovia ribis nigri. 2. Inheritance of the resistance

Selection of lines of lettuce ( Lactuca sativa L.) with a high level of partial resistance to Myzus persicae (Sulzer)

An instant bioassay for resistance of lettuce to the leaf aphid Myzus persicae

Inheritance of partial resistance to the leaf aphids Macrosiphum euphorbiae and Uroleucon sonchi in lettuce.