Unique features of the plant life cycle and their consequences.

Continuous development, the absence of a germline, flexible and reversible cellular differentiation, and the existence of haploid and diploid generations--both of which express genes--are characteristics that distinguish plants from animals. Because these differences alter the impact of mutations, animals and plants experience varied selection pressures. Despite different life-cycles, both flowering plants and multicellular animals have evolved complex sensing mechanisms that act after fertilization as 'quality checks' on reproduction, and that detect chromosome dosage and the parent of origin for specific genes. Although flowering plant embryos escape such surveillance in vitro, embryo success in the seed often depends on a healthy endosperm--a nutritive tissue that is produced by a second fertilization event in which maternal and paternal gene contributions can be monitored immediately after fertilization and throughout development.