UC Davis

Haploid induction (HI) has been a cornerstone of potato and maize breeding for more than half a century. Haploids, organisms with gametic number of chromosomes, are obtained in these crops by crossing cultivars to special lines called haploid inducers. Among the outcomes of these crosses are seeds that lack a paternal genome. These result in embryos that only inherit half the maternal chromosomes and none from the paternal parent as is in regular crosses . Fascinating as this phenomenon is, it remains relatively understudied, particularly in potato. The aim of my studies was to review the literature on maize and potato haploid induction mechanisms, formulate hypotheses that explain the trait, and test some of these hypotheses. Beyond breeding interests, haploid induction is rarely studied. Haploids in plants occur in nature across dozens of seed plants that have been surveyed. However, haploids in nature are rare occurrences, perhaps due to their low fitness. In potato, many diploid accessions induce haploids when crossed to tetraploid cultivars. This common occurrence of haploids in potato crosses suggests that it is not an evolutionary fluke that has fixed deleterious haploid induction alleles in wild potato populations. Here, I put forth a hypothesis that explains the presence of these haploid induction alleles in populations, and their evolutionary significance. To further our understanding of potato haploid induction, I developed a diploid potato population segregating for the haploid inducer trait. This population was used to map a genetic locus that is associated with haploid induction in potato. Haploid induction was also found to be positively associated with seed death. Finally, it was observed that the population developed above was segregating for traits other than haploid induction. These included color, seed size, inheritance of the embryo spot, and the growth habit. I attempted to map two of these traits - seed size and the presence of the embryo spot. A region on chromosome ten that has previously been shown to harbor numerous anthocyanin biosynthesis genes was found to be associated with the seed embryo (anthocyanin) spot trait. This was a confirmation that my method and population is robust enough to pick up QTLs. As for the seed size, no association was found between the trait and a genomic locus. However, this was not surprising given the experimental setup. Overall, I was able to map for traits that have huge implications for diploid potato biology, breeding, and that might have evolutionary biology implications.