Over the past 2-3 decades, hybrid varieties of vegetable crops have bceome commonplace. They are generally more productive and uniform than conventional open-pollinated varieties. Hybrid varieties are produced by crossing two contrasting and usually inbred parental lines. The plants resulting from this cross are genetically very similar to each other, and often have improved characteristics (such as increased yield) compared to either of the parental lines. The commercial interests of plant breeding companies are better protected by hybrids as the variety cannot be propagated by saved seed. Hybrids can be produced in a number of ways. The simplest types are the F1 hybrid varieties which are the result of a single cross between two parental lines. More complicated double and triple crosses may also be used, however the exact pedigree of a hybrid variety is rarely released into the public domain by commercial breeders.
While hybrids are of huge commercial importance in current vegetable breeding programmes, the conservation of older hybrid varieties presents problems for managers of plant genetic resources collections. Breeding companies rarely donate parental lines for these varieties to public collections for commercial reasons, making it impossible to ‘recreate’ the hybrids directly. Seed produced from hybrid plants will never be ‘true to type’ so it is therefore not possible for a gene bank to regenerate a hybrid variety using the same protocols as applied to open-pollinated material. The offspring produced from a cross between two F1 hybrid individuals would have different and variable characteristics due to the reassortment of genetic material (variants of genes or alleles) during reproduction.
Regeneration problems are increased by the use of self-incompatibility and/or male sterility by plant breeders to increase uniformity by controlling crosses. The problems associated with managing hybrid varieties are such that some genebanks do not accept them into their collections. However, this means that for brassica vegetables, along with other crops, the most modern varieties in the collection are 20-30 years old and more modern, highly adapted material is not preserved adequately for future use in research and breeding. Early hybrid varieties are likely to collectively represent a degree of variation not seen in more broadly adapted, newer hybrids. They are likely to have been bred for adaptation to a variety of local growing conditions within the UK and thus represent a source of useful traits in a form which can be integrated rapidly into new varieties. While more primitive cultivars and wild populations are more genetically diverse, it can take many generations to produce a marketable variety using such material. Using a donor accession with better agronomic traits could shorten this process. Thus there is a real need to address this issue to prevent the loss of highly derived, productive and well-adapted germplasm.
There are over 900 hybrid accessions of several crops in the Warwick HRI Genetic Resources Unit (GRU). The Brussels sprouts collection contains a significant proportion of these (338 unique accessions in total). There are also over 150 accessions of conventional open-pollinated Brussels sprouts of a UK origin in the GRU collection. Little is known about the genetic diversity of either of these groups. Around a third of the hybrid accessions are the result of the UK breeding programme, mainly from the 1980’s, and the UK is obliged to manage and conserve these under international legislation such as the International Treaty on Plant Genetic Resources and the Convention on Biological Diversity.
Many of these varieties were developed by breeding companies that no longer exist, so obtaining the parental lines used to create them is now not possible. Currently, these hybrid accessions are maintained in the long term store along with the rest of the seed collection. However, no regenerations are carried out and without replacement seed the accessions will eventually die or seed stock will become depleted as users request the material. Therefore the development of a regeneration method is a highly desirable goal for this group of accessions, and fits well with the remit of the GRU which is to collect, conserve, document and research a range of vegetable crops and their wild relatives. GRU accessions are often samples of commercial varieties, and our aim is to preserve the diversity within the collections rather than to maintain the individual varieties.
This project will test three different approaches (selfing, mixing accessions and treating as conventional open pollinated accessions) to regenerating F1 hybrid accessions. The three protocols vary in terms of the inensity of the labour inputs required and are expected to result in different amounts of seed with varying genetic compositions. The genetic diversity of the accessions and the offspring derived from two successive regenerations will be monitored using molecular markers, and this will provide a valuable insight into the genetic diversity present within hybrid and conventional Brussels sprouts accessions. The optimal protocol for managing F1 hybrid Brussels sprouts can then be chosen. Mathematical models based on this data will clarify the sampling strategies necessary to recover plants with a particular marker genotype from the accessions. The results of this project could be transferred to other crops where F1 hybrids are based on self-incompatibility as in other cruciferous vegetables such as cauliflower, cabbage, broccoli and radish. The project will enable the conservation of alleles in highly derived accessions, that can then continue to be made available to end users.