|Sherwood R T,|
Submitted to: Sexual Plant Reproduction
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 31, 1994
Publication Date: N/A
Interpretive Summary: Some plant species form seed through an asexual process (apomixis) without fertilization, and their progeny have the same genetic constitution as the maternal parent. It has been assumed, but not proven, that the nucleus of the egg of certain apomictic grass species, including buffelgrass, is derived by normal mitotic division of the nucleus of the cell which gives rise to the embryo sac. But there are possible alternative pathways for formation of apomictic eggs, including doubling of chromosome numbers followed by abnormal meiosis (reduction division), or meiosis followed by merging of reduced nuclei. This study provided evidence that the egg of apomictic lines of buffelgrass is derived by mitosis. I found that the amounts of DNA in nuclei of the apomictic initial cells and of apomictic embryo sacs were the same as those in vegetative cells undergoing normal mitosis, and different from those of sexual cells undergoing meiosis. This sinformation may be helpful in providing new, and needed, ways to distinguish between sexual and asexual reproduction in buffelgrass lines which form both types of embryo sac
Technical Abstract: Nuclear DNA amount (C value) was estimated in Feulgen stained sections of sexual (B-2S, genotype aaaa) and aposporous (Higgins, genotype AAaa) buffelgrass ovules using image cytometry. The DNA amount in haploid products of meiosis in the G1 cell cycle phase is designated 1C. Embryo sac mother cells (2-4C) began enlargement and differentiation while the 2n nuclei were in G1 phase of the cell cycle. The dominant A allele of the apospory gene did not prevent meiosis. In both lines, degenerated micropylar members of the linear tetrads appeared as clusters of condensed chromatin with 1.75C DNA, indicating DNA synthesis (S phase) may be entered before programed death. Chalazal megaspores (functional only in B-2S) were 1-2C in both lines. Formation of reduced Polygonum type embryo sacs in B-2S involved three successive mitoses to form 8 haploid nuclei with 1C DNA in G1 phase. At the intermediate 2- and 4-nucleate stages, nuclear DNA showed bimodal distributions of C values with peaks corresponding to G1 (1C) and G2 (2C) phases characteristic of populations actively dividing by mitosis. Egg and polar nuclei were 1C (G1 phase), indicating chromosomes may not be duplicated before fertilization. Antipodal nuclei of older embryo sacs had values up to 36C, possibly due to polyteny, endopolyploidy or nuclear fusion. Formation of Panicum type aposporous embryo sacs in Higgins involved development of a prominently enlarged 1-nucleate unreduced (2n) nucellar aposporic initial, mitosis to form a 2-nucleate embryo sac and a second mitosis to form a 3- or 4-nucleate sac lacking antipodals. Bimodal distributions of 2n nuclei with 2C and 4C DNA (corresponding to G1 and G2 phases) were found for the aposporic initials and 2-nucleate embryo sacs. Products of the final mitosis remained in the G1 phase (2C DNA).