Q.55 In wild-type Arabidopsis thaliana, the four types of floral organs
(sepal, petal, stamen, carpel) are arranged in concentric whorls from outside to inside.
With reference to the ABC model of floral organ patterning, match the homeotic
mutants in Group 1 with their respective arrangements of organs in the four whorls given
in Group 2.

Options:

1. P–iv, Q–ii, R–i
2. P–iii, Q–i, R–ii
3. P–ii, Q–i, R–iii
4. P–iii, Q–i, R–iv

   The correct matching is: P–iii, Q–iv, R–i, which corresponds to option (B).​

   ---

   Introduction

   In wild‑type Arabidopsis thaliana, floral organ identity follows the ABC model of floral organ development, where combinations of A, B and C class genes specify sepals, petals, stamens and carpels in four concentric whorls. Mutations in each gene class cause characteristic homeotic transformations, so the organs in two adjacent whorls are replaced by organs typical of other whorls.​

   ---

   ABC model and wild‑type flower

   In the classical ABC model for Arabidopsis:

   • Whorl 1: A alone → sepals

   • Whorl 2: A + B → petals

   • Whorl 3: B + C → stamens

   • Whorl 4: C alone → carpels​

   A and C functions are mutually antagonistic, so loss of one allows expansion of the other into additional whorls. These principles are the basis for predicting the phenotype of A, B and C class mutants.​

   ---

   Phenotypes of A, B and C class mutants

   A class mutants

   In A‑class mutants (e.g. apetala2), A function is lost from whorls 1 and 2, allowing C function to expand outward into these whorls.​

   • Whorl 1: C alone → carpels

   • Whorl 2: B + C → stamens

   • Whorl 3: B + C → stamens

   • Whorl 4: C alone → carpels​

   Thus the arrangement becomes carpel, stamen, stamen, carpel, which corresponds to pattern (iii) in Group 2. Therefore:
   P (A class mutants) → (iii).

   B class mutants

   In B‑class mutants (e.g. apetala3, pistillata), B function is absent, so only A or C acts where they are normally present.​

   • Whorl 1: A alone → sepals

   • Whorl 2: A alone (no B) → sepals

   • Whorl 3: C alone (no B) → carpels

   • Whorl 4: C alone → carpels​

   So the sequence is sepal, sepal, carpel, carpel, which matches pattern (i). Hence:
   Q (B class mutants) → (i).

   C class mutants

   In C‑class mutants (e.g. agamous), C function is lost from whorls 3 and 4, and A function expands inward into these whorls.​

   • Whorl 1: A alone → sepals

   • Whorl 2: A + B → petals

   • Whorl 3: A + B (no C) → petals

   • Whorl 4: A alone (no C) → sepals, often with extra repeated whorls​

   So the basic sequence is sepal, petal, petal, sepal, which is pattern (ii). Therefore:
   R (C class mutants) → (ii).

   Putting these together gives P–iii, Q–i, R–ii, which is option (B).

   ---

   Option‑wise explanation

   The patterns in Group 2 are:

   • (i) sepal, sepal, carpel, carpel

   • (ii) sepal, petal, petal, sepal

   • (iii) carpel, stamen, stamen, carpel

   • (iv) sepal, sepal, petal, petal

   Checking each option

   Option	Mapping given	Why it is correct/incorrect
   (A)	P–iv, Q–ii, R–i	A‑class mutants never give outer whorl sepals; instead, whorl 1 becomes carpel‑like, so P–iv is wrong.​
   (B)	P–iii, Q–i, R–ii	Matches the known phenotypes: A mutant → carpel, stamen, stamen, carpel (iii); B mutant → sepal, sepal, carpel, carpel (i); C mutant → sepal, petal, petal, sepal (ii).​
   (C)	P–ii, Q–i, R–iii	Assigns the C‑mutant pattern (ii) to A mutants and the A‑mutant pattern (iii) to C mutants, reversing their identities and violating A–C antagonism.​
   (D)	P–iii, Q–i, R–iv	Correctly matches A (P–iii) and B (Q–i) but assigns pattern (iv) to C mutants, which is inconsistent because inner whorls in C mutants are petal and sepal, not two successive petal whorls.​

   Because only option (B) gives the correct assignment for all three classes, it is the right answer.