Jun 17, 2005

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How do cross-bred progeny of summer rape and summer turnip rape react to plant pathogens?

(2001 – 2004) Max Planck Institute (MPI) for Plant Breeding Research, working group on plant diseases and plant protection, Cologne

Topic

Oilseed rape is one of the crop plants that can outcross to related species of wild flora. In this way, resistance genes, which protect the oilseed rape crop against pathogens, could for instance enter the gene pool of related wild species.

If this resistance were to integrate stably into cross-bred progeny of cultivated and wild species, it is conceivable that these, like the crop species, would prove less susceptible to pathogens. This could give them a fitness advantage in the community, leading them to spread in field locations and neighbouring natural spaces.

The experiments were carried out on conventional rape varieties.

Summary

Outcross progeny of rape and summer turnip rape (classic varieties) were examined for infestation after natural and artificial infection with various species of fungus.

It appeared that the F1 generation (first filial generation) was less affected than the parents, demonstrating increased resistance. This observation is attributed to heterosis. In the F2 generation (second filial generation) this higher resistance is lost again.

The yielding ability and vigour were severely reduced in the F1 and F2 generations. The investigations show that outcross hybrids from rape and summer turnip rape are not superior to the parents in terms of fitness.

Experiment description

Crossing summer rape with related wild species

Under greenhouse conditions summer turnip rape was selectively cross-bred with summer rape. The cross-bred progeny were then tested in the field and in the laboratory.

Disease infestation compared with related wild species

In the field trial, natural infestation was assessed for one summer turnip rape variety (Brassica rapa, Nokonava variety), four summer rape varieties (Jumbo, Lambada, Star and Tiger) and their hybrids by scoring (assessing) the disease symptoms of the whole plant.

Infestation tests under laboratory conditions

In the laboratory, infestation tests were carried out using artificial infection with six rape fungal pathogens (Alternaria brassicae, Botrytis cinerea, Phoma lingam, Pseudocercosporella herpotrichoides, Sclerotinia sclerotiorum and Verticillium longisporum). The infections were carried out on uninfested leaves, stalks and pods from field and greenhouse samples. The experiments on the cross-bred progeny took place over two years on the first generation (F1 generation) and in a third year on the progeny of the hybrids (the F2 generation).

Because of the close relationship between rape and the wild species, these plant species have many fungal pathogens in common. The aim here was to investigate to what extent the disease infestation of the parents differs from that of the hybrids. Since the hybrids combine the genomes of both parents, it is possible to calculate an expected value for the infestation of the hybrids. This involves taking the mean of the parental infestation. The results are based on a comparison between the expected value (parent average) and the infestation of the hybrid in question.

Results

F1 generation

A field trial was prepared with summer turnip rape and summer rape varieties in 2001. In 2002 they were joined by the hybrids produced by cross-breeding in the greenhouse.

In 2002 and 2003 the F1 generation was tested. The tests showed that the pathogen infestation on all plant organs from greenhouse and field cultivation from both years was much lower than the expected value (infestation average) of the parent varieties (Diagram 1). This result could not be explained sufficiently by the parental resistance, so heterosis is suspected.

F2 generation

In the F2 generation the increased resistance weakened considerably. On some hybrids the pathogen infestation was actually higher than the parent average (Diagram 2).

Yield and germination tests showed that the F1 hybrids have shorter pods and formed fewer and smaller seeds than their parent plants. The germinability also fell. The loss of vigour was even more dramatic among the seeds of the F2 hybrids.