Genomic Structure of the Apomixis Locus in Pennisetum

Abstract

Apomixis is the term used for reproductive processes in flowering plants that result in asexual reproduction through seeds. This final outcome can be achieved through either sporophytic or gametophytic types of development (Nogler, 1984). The sporophytic type is lcnown as adventitious embryony and resembles somatic embryogenesis in that an embryo develops directly from a somatic cell (usually a nucellar cell) within the ovule. Such an embryo typically is nourished by endosperm derived through sexual reproduction, i.e., the meiotically derived embryo sac is fertilized, and adventitious embryos often coexist with zygotic embryos. Gametophytic apomixis retains the alternation of generations (sporophyte-gametophyte-sporophyte) within the apomictic process by replacing meiotically derived embryo sacs with megaspore mother cell- or somatic cell-derived embryo sacs that are unreduced in chromosome number. The egg cell in an unreduced embryo sac is genetically identical to the maternal parent and develops without fertilization into an embryo. The nutritive endosperm develops autonomously (without fertilization) in some apomicts, but in other apomicts, fertilization is required.