Problem 5 - Campbell Biology

Question

Some plant species produce male and female individuals in which the males have an XY genotype and the females have an XX genotype. After double fertilization, what would be the genotypes of the embryos and endosperm nuclei? (A) embryo XY/endosperm XXX or embryo XX/endosperm XXY (B) embryo XX/endosperm XX or embryo XY/endosperm XY (C) embryo XX/endosperm XXX or embryo XY/endosperm XYY (D) embryo XX/endosperm XXX or embryo XY/endosperm XXY

Step-by-Step Solution

Answer

D - embryo XX/endosperm XXX or embryo XY/endosperm XXY

Step 1: Understand Double Fertilization

Double fertilization in plants involves two sperm cells. One fertilizes the egg to form the embryo, and the other fuses with two polar nuclei to form the endosperm.

Step 2: Determine Embryo Genotype

The embryo is formed from the fusion of a male sperm (either X or Y) and a female egg (always X). So, the embryo genotypes can be XX (female) or XY (male).

Step 3: Determine Endosperm Genotype

The endosperm results from the fusion of another sperm cell (either X or Y) with two polar nuclei (each X). Thus, endosperm genotypes can be XXX (fusion of X sperm with XX polar nuclei) or XXY (fusion of Y sperm with XX polar nuclei).

Step 4: Match Genotypes to Choices

From steps 2 and 3, possible combinations are: embryo XX and endosperm XXX or embryo XY and endosperm XXY. This matches with choice (D).

Key Concepts

Double Fertilization

Double fertilization is a unique and crucial process in the reproductive cycle of angiosperms (flowering plants). This process involves two sperm cells. The first sperm cell fertilizes the egg, leading to the formation of the embryo, which will develop into the new plant. The second sperm cell fuses with two polar nuclei found within the ovule's central cell to form a triploid cell, which will develop into the endosperm.

This method ensures that the embryo, the plant's future, has a food supply, thanks to the endosperm. The coordination of these events maximizes the efficiency of reproduction and resource allocation in flowering plants.

Plant Genotypes

Genotypes refer to the genetic makeup of an organism, specifically the combination of alleles inherited from the parents. In some plant species, males and females are distinct, and they carry different genotypes. Here, males have an XY genotype, and females have an XX genotype.

Understanding these genotypes helps determine the genetic outcomes after fertilization. In double fertilization, the male's sperm can carry either an X or Y chromosome. The female's egg cell will always carry an X chromosome. Hence, the possible embryo genotypes are either XX (female) or XY (male). This genetic mechanism laid down by the chromosomes dictates the sex and other genetic traits of the resultant plant.

Therefore, knowing the male and female genotypes allows for predicting outcomes in plant breeding and genetic studies.

Embryo and Endosperm Formation

The embryo and endosperm formation are crucial processes derived from double fertilization. The embryo arises from the fertilization of the egg by one of the sperm cells. In plants with separate sexes, the embryo's genotype can be either XX or XY, depending on whether the sperm cell carries an X or Y chromosome.

The endosperm, on the other hand, forms from the second sperm cell's fusion with the two polar nuclei, making it triploid (having three sets of chromosomes). Since the polar nuclei are both X, the endosperm will have the following possible genotypes:

• XXX - if the sperm cell carries an X chromosome
• XXY - if the sperm cell carries a Y chromosome

This newly formed endosperm acts as a nutrient-rich tissue that supports the growing embryo, ensuring it has enough energy for successful germination and growth. Understanding these processes is crucial for plant breeding and agricultural practices, as they directly impact the viability and health of the subsequent plant generation.