Self-compatible (SC) diploid potatoes allow innovative potato breeding. Therefore, the Sli gene, originally described in S. chacoense, has received much attention. In elite S. tuberosum diploids, spontaneous berry set is occasionally observed. We aimed to map SC from S. tuberosum origin. Two full-sib mapping populations from non-inbred diploids were used. Bulks were composed based on both pollen tube growth and berry set upon selfing. After DNA sequencing of the parents and bulks, we generated k-mer tables. Set algebra and depth filtering were used to identify bulk-specific k-mers. Coupling and repulsion phase k-mers, transmitted from the SC parent, mapped in both populations to the distal end of chromosome 12. Intersection between the k-mers from both populations, in coupling phase with SC, exposed a shared haplotype of approximately 1.5 Mb. Subsequently, we screened read archives of potatoes and wild relatives for k-mers specific to this haplotype. The well-known SC clones US-W4 and RH89-039-16, but surprisingly, also S. chacoense clone M6 were positives. Hence, the S. tuberosum source of SC seems identical to Sli. Furthermore, the candidate region drastically reduced to 333 kb. Haplotype-specific KASP markers were designed and validated on a panel of diploid clones including another renown SC dihaploid G254. Interestingly, k-mers specific to the SC haplotype were common in tetraploid varieties. Pedigree information suggests that the SC haplotype was introduced into tetraploid varieties via the founder "Rough Purple Chili". We show that Sli is surprisingly widespread and indigenous to the cultivated gene pool of potato.

In organic potato production, the need for varieties with durable late blight resistance developed through classical breeding programmes is urgent. Besides late blight resistance, other variety characteristics needed in organic potato production are early canopy closure for weed suppression and good tuber dormancy to eliminate the need for (chemical) sprouting inhibition during storage, amongst others. This paper is a unique example of collaboration between researchers, farmers and professional breeders of both large, medium and small breeding companies. The aim of the resulting breeding project, Bioimpuls, was to provide a substantial impulse to both the organic and conventional potato breeding sector by enlarging the access to various sources of late blight resistance. The Bioimpuls activities include providing true seed populations for variety selection with five available sources of R-genes against Phytophthora infestans, early and advanced introgression breeding with six new R-genes, and education and communication. The results achieved over the 11-year period (2009–2019) are analysed. Many true seed populations containing multiple resistance genes are produced and selected, and a constant flow of breeding clones is entering the evaluation and positioning trials of companies. However, it will still take a considerable amount of time before varieties with stacked resistance genes will replace the new resistant single gene varieties entering the market in the next few years. Five out of six new sources of R-genes need more years of backcrossing before they are ready for commercial use. Bioimpuls successfully introduced a training course for farmer breeders, and published a manual for potato breeding.

The balanced segregation of homologous chromosomes during meiosis is essential for fertility and is mediated by crossovers. A strong reduction of crossovers leads to desynapsis, a process in which pairing of homologous chromosomes is abolished before metaphase I. This results in a random segregation of univalent and the production of unbalanced and sterile gametes. However, if desynapsis is combined with another meiotic alteration that restitutes the first meiotic division, then uniform and balanced unreduced gametes, essentially composed of non-recombinant homologs, are produced. This mitosis-like division is of interest to breeders because it transmits most of the parental heterozygosity to the gametes. In potato, desynapsis is a recessive trait that was tentatively mapped to chromosome8. In this article, we have fine-mapped the position of the desynapsis locus and identifiedStMSH4, an essential component of the class I crossover pathway, as the most likely candidate gene. A seven base-pair insertion in the second exon ofStMSH4was found to be associated with desynapsis in our mapping population. We also identified a second allele with a 3820 base-pair insertion and confirmed that both alleles cannot complement each other. Such non-functional alleles appeared to be common in potato cultivars. More than half of the varieties we tested are carriers of mutational load at theStMSH4locus. With this new information, breeders can choose to remove desynaptic alleles from their germplasm to improve fertility or to use them to produce highly uniform unreduced gametes in alternative breeding schemes.