Diploid Inbreds For Fixation, and Unreduced GAmetes for Tetraploidy – A novel Fixation-Restitution Breeding method for potato
Conventional potato breeding takes place at the tetraploid level to exploit the yield advantage conferred by the naturally larger cells of tetraploids. In the highly successful inbreeding system used in diploid crops, genetic gain is driven by the accumulation and subsequent fixation of beneficial alleles over multiple recurrent breeding cycles. Potato is normally an outbreeding crop that is intolerant to inbreeding and mostly self-incompatible (SI) at the diploid level. Inbreeding potato at the tetraploid level results in the loss of vigour and fertility after few generations of selfing, making it impossible to practice recurrent selection efficiently, resulting in poor fixation of beneficial alleles and a high genetic load due to masking of deleterious alleles. Traditional potato breeding is based on segregating gametes from meiosis. Even when superior parents are combined, different specific combinations of beneficial alleles, initially accumulated in the elite parents, disperse in the progeny. As a result, the identification of an improved variety becomes a “numbers game”, where huge populations are screened to find varieties which, by chance, perform better than the parental pool in some, but generally not all characteristics. Given that more than 50 traits are important in potato breeding, such an approach is far from optimal.
A recently proposed alternative, F1 hybrid breeding, is a paradigm shift in the genetic approach of breeding potatoes. The use of self-compatible (SC) and inbreeding-tolerant potato diploids allows the incremental accumulation of beneficial alleles via rapid backcrossing schemes. Diploid potato is generally a strict outbreeder due to gametophytic SI, but rare mutants lack this self-pollen arrest and are SC. Thus, highly inbred parental genotypes can be hybridized to generate uniform F1 botanical seed as propagating material and any residual inbreeding depression is dealt with by exploiting the effect of heterosis. However, initiating F1 hybrid breeding in potato is an enormously expensive endeavour and out of the reach of most potato breeding companies.
We propose Fixation - Restitution (Fix-Res) Breeding as an innovative potato breeding system where, similar to F1 hybrid breeding, self-compatible diploids are used to accumulate favourable alleles via rapid backcrossing schemes. However, these lines are subsequently used to rapidly transfer and fix the traits into tetraploid breeding populations by virtue of the diploid male's ability to produce unreduced (2n) diploid pollen, thus producing viable tetraploids when used in 4x X 2x crosses. Fix-Res breeding will also allow efficient recurrent selection to be harnessed in potato, but will be more feasible to implement than F1 hybrid breeding for SME potato breeders.
The goal of this 3 year project is the development of all of the components that are required to enable the initiation of Fix-Res breeding. We will (i) develop genetic markers for self-compatibility and for the formation of 2n gametes; (ii) develop a genome-wide SNP-based marker system that allows both the accurate tracking of heterozygosity status and diagnosis of multiple favourable trait alleles simultaneously, to drive the development of Fix-Res inbreds in as short a period as possible; (iii) maximise our ability to stack multiple disease resistance and abiotic stress tolerance alleles, by developing material in which many beneficial alleles residing on the same chromosome are placed into the cis-configuration on the same haplotype; (iv) develop early stage diploid inbred germplasm (incorporating SC) that conforms to the ideotypes for multiple different market segments (eg. processing, starch, ware potatoes) that breeders target; and (v) develop a data repository based on the coordinates of the potato reference genome. By the end of the project, the participants will all be in an immediate position to initiate Fix-Res breeding programmes.
Dr Dan Milbourne
Teagasc, Crop Science, IRELAND
Dr Herman van Eck
Wageningen University, Plant Breeding, NETHERLANDS
Dr Vanessa Prigge
SaKa Pflanzenzucht GmbH & Co. KG, GERMANY
Dr Hanne Grethe Kirk
Danespo R & D, DENMARK
Prof Kåre Lehmann Nielsen
Aalborg University, Chemistry and Biosciences, DENMARK