Elimination of Chromosomes as a Mechanism for the Formation of Diploid Plants in Diploid-Tetraploid Crosses in Maize (Zea mays L.)

One of the main components of apomictic plant reproduction is the formation of unreduced embryo sacs (ESs). Heteroploid crosses, in which maternal diploid plants are pollinated by pollen of tetraploids, can be used as an effective tool for identifying the ability to form such ESs. In maize, in 2n(female) x 4n(male) crosses, as a rule, shrunken kernels with a triploid embryo are formed, in which the development of the endosperm is impaired due to the deviation of the balance of the maternal (m) to paternal (p) genomes from the ratio of 2m : 1p. In our experiments, the formation of large plump kernels, from which diploid maternal-type plants developed, was observed in several diploid maize lines under their pollination with tetraploid pollen, while the maternal-type phenotype of plants, as well as the hybrid endosperm phenotype was confirmed by expression of genetic markers. It has been suggested that the plump kernels in 2n x 4n crosses arise based on unreduced ESs and pseudogamous apomixis, since the fusion of two diploid polar nuclei with a diploid sperm provides a ratio of maternal to paternal genomes in the endosperm of 2m : 1p. In order to clarify the genetic nature of diploid plants that are phenotypically similar to maternal lines that developed from plump kernels in 2n x 4n crosses, we carried out their genotyping for all ten chromosomes of the maize genome using polymorphic codominant SSR and Indel markers that differentiate the paternal pollinator line from maternal lines. As maternal forms, we used the HPL AT line, which has the ability for haploid parthenogenesis, plants from the F-2 generation of the hybrid between the lines B47 capable of forming unreduced ESs and HPL AT. In addition, the Korichnevyi Marker (KM) line carrying the marker gene B of brown stem coloring, as well as the YuV11 line was included in the crosses. As the paternal form, the Chernaya Tetra (ChT) tetraploid corn carrying the marker gene A1 of the black coloring of kernels was used. It was found that in all studied plants amplification of only maternal alleles was observed when using markers of five chromosomes: chromosome 1 (iDp525), chromosome 2 (iDP4004), chromosome 3 (JY_3:457), chromosome 4 (UflDP4_31.55), and chromosome 9 (INDEL_139329242). However, in each of the studied plants, cases of amplification of alleles characteristic of the paternal line were noted when using markers of other chromosomes. A hypothesis on the formation of diploid plants in 2n x 4n crosses in maize as a result of fertilization of unreduced ESs and the subsequent elimination of chromosomes predominantly of the paternal line is proposed.