Written by Merel Dagher (PEACE Advisor)
The transition from ovule to seed in gymnosperms results in a change in ploidy of the three layers of the ovule. In an unfertilized ovule, the megaspore is haploid, the layer that surrounds it (megasporangium) is diploid, and the outer layer (integument) is diploid. What are the names of the new structures in the seed and what is their ploidy?
a. embryo – 2n; food supply – n; seed coat – 3n
b. embryo – n; food supply – n; seed coat – 2n
c. embryo – n; food supply – 2n; seed coat – 2n
d. embryo – 2n; food supply – n; seed coat – 2n
a. embryo – 2n; food supply – n; seed coat – 3n
b. embryo – n; food supply – n; seed coat – 2n
c. embryo – n; food supply – 2n; seed coat – 2n
d. embryo – 2n; food supply – n; seed coat – 2n
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The correct answer is d. After fertilization (the fusion of sperm and egg), the embryo is a diploid structure; the seed coat is also diploid because it was originally the integument; the food supply, or endosperm, is now haploid as it develops from the megaspore before fertilization (remember the megasporangium undergoes meiosis to produce megaspores).