Past exam of the plant Sciences course of the University of Cambridge / 2025 / Paper plm3 / 3 / Solution

Ancestral state reconstruction turns a difference between living flowers into a hypothesis about a particular evolutionary change; experiments must then establish the molecular cause. Without the historical step, two similar flowers may be mistaken for a retained ancestral condition, and a molecular difference may be assigned to the wrong evolutionary branch. floral symmetry provides a particularly instructive example because a return to radial appearance need not restore ancestral developmental identity.

Start by sampling the focal lineage and informative relatives, estimating a rooted phylogenetic tree, and recording the phenotypic trait at comparable developmental stages. For floral symmetry, distinguish the symmetry of the whole flower, the shapes and identities of individual petals, and the arrangement of stamens. A radial outline can be produced by several different organ-level changes. Coding every such flower as simply “radial” is a useful coarse hypothesis, but it can hide the very developmental mechanism one wishes to explain. Assess within-species variation and incomplete observations rather than forcing all uncertainty into a single state.

An explicit evolutionary model can assign transition rates between radial floral symmetry and bilateral floral symmetry. With state-change matrix $Q$, branch transition probabilities are $P(t)=e^{Qt}$ under a continuous-time Markov chain. For an ancestral node $v$, the likelihood-based weight of state $s$ combines the descendant observations, the rest of the tree and an assumed root-state distribution. In a Bayesian treatment,where $\eta$ includes transition rates and branch parameters, and $D$ denotes the comparative data used in the joint analysis. This expresses the need to carry phylogenetic and model uncertainty forward. A maximum parsimony reconstruction instead minimizes state changes; it does not automatically justify equal rates or identify the most probable historical path. stochastic character mapping can sample possible histories, including unobserved changes within branches, rather than equating endpoint differences with exactly one event.

The resulting reconstruction identifies where to look. If a lineage with radial flowers is nested within a group inferred to have had bilateral flowers, the natural molecular question is how a derived loss or transformation of asymmetry occurred. Candidate mechanisms include loss of a dorsal regulator, contraction or expansion of its expression domain, changes in downstream targets, or compensating changes elsewhere in a gene regulatory network. If several radial lineages evolved independently, their reconstructed branches provide replicates for testing repeated use of the same gene. They do not justify treating every living species in one derived clade as an independent origin.

Mutational work in Antirrhinum identified CYCLOIDEA as a regulator of dorsal floral growth and organ patterning.^[ref] This makes related regulators plausible candidates for symmetry evolution elsewhere, but does not prove that every bilateral flower uses precisely the same deployment. A gene tree is needed to distinguish corresponding copies from products of gene duplication; expression of an arbitrarily chosen member of the family would be an inadequate test.

The legume Cadia illustrates the payoff. Its flowers look radially symmetric, although its placement among relatives supports derivation from a lineage with bilateral flowers. A simple “loss of asymmetry” account might predict removal of the dorsal identity program. Instead, the expression domain of a CYCLOIDEA-like regulator was expanded into lateral and ventral parts of the corolla, supporting the interpretation that petals had acquired dorsal identity around the flower.^[ref] Thus a coarse character reconstruction can describe a return to radial symmetry while finer developmental characters indicate a new organization. The ancestral comparison generates competing molecular hypotheses; the expression data discriminate between their predictions.

Expression association remains weaker than a complete causal demonstration. To test the Cadia model, compare spatial gene expression at matched developmental stages in the derived lineage and a close bilateral relative. Use validated probes or RNA sequencing with controls for cross-detection among duplicated copies, cellular composition and developmental timing. Reducing the expanded regulator specifically in lateral and ventral primordia should alter those organ identities if the proposed deployment is necessary. Conversely, expanding the corresponding regulator in a bilateral relative tests sufficiency. Include unmodified siblings, independent edited lines, a restoration construct and assays of other developmental functions, because a global perturbation may change floral shape indirectly through poor growth.

One can next distinguish cis-regulatory evolution from altered trans-acting inputs. Candidate enhancers can be tested with matched reporters and endogenous sequence replacement; mapping an expression difference to a regulatory interval is more informative than comparing total transcript levels in whole flowers. A reporter's behavior outside its native genomic context establishes regulatory potential, not the necessity of that element. If a coding change is implicated, reconstruct ancestral sequences on the relevant gene tree, synthesize plausible alternatives at uncertain sites, and compare their activity in the same genetic background. Introducing derived substitutions into an ancestral sequence and reversing them in the derived sequence tests historical contingency and epistasis. A single best ancestral sequence should not be treated as an observed fossil protein.

Several limits govern interpretation. Reconstruction depends on the sampled relatives, rooting, character coding and the transition model; unequal gain and loss rates can change the inferred direction. Extinct intermediate lineages may be missing. A shift reconstructed on a long branch need not be simultaneous with every sequence substitution on that branch, and a high ratio of nonsynonymous to synonymous change does not itself identify a symmetry mechanism. Finally, explaining developmental origin is different from explaining adaptive significance. Pollinator behavior, pollen transfer and lifetime reproductive success require direct field or experimental tests, with floral display and reward controlled, before a symmetry shift can be called a pollination adaptation. The strongest account connects a probabilistic historical transition to a specific molecular change and an experimentally established developmental effect, while keeping appearance, mechanism and selective advantage distinct.