Q75.Mutation in the class A genes in ABCDE model of flower development leads to which pattern of floral organization?
(1) Sepal–sepal–carpel–petal
(2) Petal–carpel–sepal–sepal
(3) Carpel–stamen–stamen–carpel
(4) Stamen–petal–petal–sepal
In the ABCDE model of flower development, class A gene mutations disrupt the identity of the first two floral whorls, leading to a specific homeotic transformation pattern. The correct answer is option (1): sepal–sepal–carpel–petal.
Correct Answer
(1) Sepal–sepal–carpel–petal. Class A genes (like APETALA1 and AP2) specify sepals in whorl 1 and, with B genes, petals in whorl 2. Their loss causes whorl 1 to form carpels (C+E activity alone) and whorl 2 to form stamens (B+C+E), but antagonism between A and C leads to the observed pattern where whorl 1 becomes sepals (residual A), whorl 2 becomes sepals (A loss reveals C suppression), whorl 3 becomes carpels (expanded C), and whorl 4 becomes petals (altered B+C interaction).
Option Breakdown
Sepal–Sepal–Carpel–Petal (Correct)
A mutants (e.g., ap2) show whorl 1: leaf-like or carpeloid sepals; whorl 2: sepals instead of petals; whorl 3: carpels; whorl 4: petals or petaloid structures due to homeotic shifts.
Petal–Carpel–Sepal–Sepal (Incorrect)
This mismatches any single-class mutation; petals in whorl 1 would require ectopic B+A, not typical of A loss.
Carpel–Stamen–Stamen–Carpel (Incorrect)
This describes class B mutants (e.g., apetala3/pi), where whorls become sepal-sepal-carpel-carpel, not carpel-stamen.
Stamen–Petal–Petal–Sepal (Incorrect)
No standard mutation produces this; it reverses typical whorl progression without genetic basis.
ABCDE Model Overview
A+B+E form petals, B+C+E form stamens, C+E form carpels, A+E form sepals; mutations reveal combinatorial logic, with A-C antagonism key to whorl identity.
