Morphological and molecular evidences for DNA introgression in haploid induction via a high oil inducer CAUHOI in maize

The phenomenon of maternal haploid induction in maize was first described many years ago, but the underlying mechanism is still unclear. In this study, the Stock-6-derived, haploid-inducing line CAUHOI with high kernel oil content (KOC), was used as the pollinator to produce maternal haploids from the maize hybrid ZD958 with low KOC. CAUHOI is homozygous for the dominant marker gene R1-nj. Haploids were identified by morphological and cytological investigations. The frequency of haploid induction from this cross was 2.21%. Unexpectedly, many haploid kernels had weakly pigmented purple color on the embryo, and some haploid kernels had high KOC. Simple sequence repeat (SSR) analysis showed that 43.18% of the haploids carried segments from CAUHOI, and a small proportion (average 1.84%) of the genome of CAUHOI was introgressed into haploids. Haploid kernels with high KOC had a higher frequency of segment introgression from CAUHOI (2.92%) than that in haploid kernels with low KOC (1.79%), showing that the marker gene R1-nj and high-oil genes from CAUHOI were expressed during the development of some haploid embryos, and confirmed that the DNA introgression from the inducer parent occurred during maternal haploid induction. Together, these results suggested that the chromosome elimination was probably responsible for haploid induction in maize, and late somatic elimination might occur. Several possible mechanisms underlying haploid formation are discussed.