Reproduction in Plants and Developmental Biology
What the exam tests
This domain covers flower development, microsporogenesis, megasporogenesis, pollination, double fertilization, endosperm and embryo formation, seed and fruit development, apomixis, polyembryony, and plant growth regulators.
Core rules and concepts
- Microspore mother cell (2n) undergoes meiosis to form microspores (n), which become pollen grains.
- Megaspore mother cell (2n) undergoes meiosis to form four megaspores; usually one survives and forms the embryo sac.
- A typical angiosperm embryo sac has 7 cells and 8 nuclei: one egg, two synergids, three antipodals, and a central cell with two polar nuclei.
- Double fertilization: one male gamete fuses with the egg (2n zygote); the other fuses with two polar nuclei to form triploid (3n) endosperm.
- Seed coat develops from integuments (2n); fruit develops from the ovary wall.
- Apomixis is asexual seed formation; polyembryony is the presence of more than one embryo in a seed.
Worked example
What is the ploidy of endosperm in an angiosperm? It is 3n because two polar nuclei (n + n) fuse with one male gamete (n). Do not apply this to gymnosperms: their endosperm is the female gametophyte and is haploid (n). The reasoning depends on the fertilization event.
Traps that cost marks
Do not call the endosperm 2n in angiosperms. The nucellus is 2n, not n. Perisperm develops from nucellus and is different from endosperm. Synergids guide the pollen tube but do not fuse with a male gamete in normal double fertilization.