Two species of a flowering plant, P ($2n = 20$ chromosomes) and Q ($2n = 30$ chromosomes) are reciprocally crossed with each other as male or female as shown below to produce F1 seeds.
Which of the following seed tissues from both the F1 seeds (R and S) will have the same chromosome numbers?
Show Hint
In reciprocal crosses, any tissue formed by symmetric genomic contributions (like the diploid embryo, which always receives exactly $1n$ from each parent) will have the same chromosome count regardless of who was the male or female parent.
Tissues with maternal bias (endosperm, which is $2/3$ maternal, or seed coat, which is $100\%$ maternal) will vary depending on the mother.
Step 1: Understanding the Question:
We are given two parent plants: P with a diploid number of $2n_P = 20$ chromosomes, and Q with a diploid number of $2n_Q = 30$ chromosomes.
They are crossed reciprocally:
- Seed R: Female P $\times$ Male Q
- Seed S: Female Q $\times$ Male P
We need to determine which of the resulting seed tissues (embryo, endosperm, and seed coat) will have the exact same chromosome number in both seeds R and S. Step 2: Key Formula or Approach:
Let us define the ploidy and origin of seed tissues in angiosperms:
1. Embryo ($2n$): Formed by fusion of one haploid female gamete (egg cell) and one haploid male gamete (sperm cell).
\[ \text{Ploidy} = n_{\text{female}} + n_{\text{male}} \]
2. Endosperm ($3n$): Formed by fusion of two haploid female polar nuclei (central cell) and one haploid male gamete.
\[ \text{Ploidy} = 2n_{\text{female}} + n_{\text{male}} \]
3. Seed coat (testa): Derived directly from maternal integuments (diploid tissue of the female parent).
\[ \text{Ploidy} = 2n_{\text{female}} \] Step 3: Detailed Explanation:
Let's calculate the chromosome counts for each tissue in both crosses:
- Haploid numbers of the parents:
- For P ($2n = 20$): $n_P = 10$.
- For Q ($2n = 30$): $n_Q = 15$.
- For Seed R (Female P $\times$ Male Q):
- Embryo: egg cell ($n_P = 10$) + sperm cell ($n_Q = 15$) $\to 10 + 15 = 25$ chromosomes.
- Endosperm: 2 polar nuclei ($2 \times n_P = 20$) + sperm cell ($n_Q = 15$) $\to 20 + 15 = 35$ chromosomes.
- Seed coat: maternal tissue from plant P $\to 2n_P = 20$ chromosomes.
- For Seed S (Female Q $\times$ Male P):
- Embryo: egg cell ($n_Q = 15$) + sperm cell ($n_P = 10$) $\to 15 + 10 = 25$ chromosomes.
- Endosperm: 2 polar nuclei ($2 \times n_Q = 30$) + sperm cell ($n_P = 10$) $\to 30 + 10 = 40$ chromosomes.
- Seed coat: maternal tissue from plant Q $\to 2n_Q = 30$ chromosomes.
- Comparison:
- Embryo: $25$ in R vs $25$ in S (Same).
- Endosperm: $35$ in R vs $40$ in S (Different).
- Seed coat: $20$ in R vs $30$ in S (Different). Step 4: Final Answer:
Only the embryo has the same chromosome number in both F1 seeds R and S. Therefore, the correct option is (A).
