Detecting a phylogenetic signal in the data
According to Laurin (2004)
, if the observed distribution does not significantly differ from a random one, then little confidence should be placed in the inference of ancestral states using parsimony, since state optimization might just as well be randomly inferred. The presence of a phylogenetic signal for a given character reflects its systematic utility (lack of homoplasy) and therefore gives more value to the inference of ancestral states. A phylogenetic signal was found in the three following characters: perianth merism 2, perianth symmetry and androecium organ number (P
< 0·005). No phylogenetic signal was detected in the evolution of the number of spurs (P
= 1·0) or in perianth merism 1 (P
= 0·33). The absence of a signal for the number of spurs is possibly due to the scarcity of transitions towards the presence of spurs. In contrast, there were many transitions among states for perianth merism 1, and >5 % of the simulated trees were shorter than the composite tree. However, the optimization of perianth merism 1 on the tree was only used descriptively, and this character was not used in the statistical tests as a consequence of its multistate coding.
Inference of ancestral states and independent evolution of characters
The inference of ancestral character states using maximum parsimony indicates that the hypothetical ancestral flower of Asteridae is pentamerous, actinomorphic and oligandrous. Tetramery evolved at least once in each order of the Asteridae and is the second-most frequent type of perianth merism after pentamery. Variable perianth merism evolved seven times independently (and each time from a pentamerous perianth merism; see ) which is surprisingly frequent for a clade where closed ground plans are supposed to be the basic floral Bauplan.
Zygomorphy evolved 15 times independently (see A), each time in the context of a fixed perianth merism as shown from the comparison with the tree displayed on . There were ten reversals to actinomorphy (see A). Zygomorphic families were especially abundant in the orders Lamiales and Dipsacales, where all the reversals to actinomorphy were found. The families and genera that showed a reversal to actinomorphy were Patrinia (Valerianaceae), Byblidaceae, Oleaceae, Peltanthera (Gesneriaceae), Plantago (Plantaginaceae), Nuxia (Stilbaceae), Anisacanthus, Sanchezia (Acanthaceae), Nashia and Petrea (Verbenaceae). It is noticeable that almost all zygomorphic flowers in this study are pentamerous. Zygomorphy was found in tetramerous taxa only when these tetramerous taxa were derived within a zygomorphic lineage, as revealed by the comparison between Figs and A.
Fig. 2. Mirror trees of the Asteridae showing the evolution of perianth symmetry and androecium organ number optimized using maximum parsimony. (A) Evolution of perianth symmetry. Black, Actinomorphy; red, zygomorphy. (B) Evolution of androecium organ number. (more ...)
Polyandry evolved three times in the Cornales and eight times in the Ericales, the two basalmost clades of Asteridae, yet only two times in the rest of the clade (see Endress, 2002
). Overall, oligandry was associated with zygomorphy (see A, B), with two exceptions, namely the genera Couroupita
(Lecythidaceae) and Tupidanthus
(Araliaceae), where zygomorphy was acquired in a polyandrous context.
Multiple spurs (as many spurs as perianth merism) evolved only twice in the whole Asteridae clade (see B), once within the genus Utleya (Ericaceae, Ericales) and once in Halenia (Gentianaceae, Gentianales). Both genera have actinomorphic flowers. Conversely, flowers with one or two spurs evolved seven times independently. Such flowers were zygomorphic (see A, B). The acquisition of zygomorphy always preceded that of the single spur except in Balsaminaceae (Ericales) where both transitions occurred simultaneously in the tree shown in .
Fig. 3. Mirror trees of the Asteridae showing the evolution of perianth symmetry and the number of spurs optimized using the maximum parsimony method. (A) Evolution of perianth symmetry. Black, Actinomorphy; red, zygomorphy. (B) Evolution of the number of spurs. (more ...)
The LR tests indicate that androecium organ number and the number of spurs are both correlated with perianth symmetry (LR = 15·84, P < 0·05; LR = 19·52, P < 0·001). summarizes the tests of character correlation conducted in this study as well as the tests performed to assess the transition rate values. The flow diagram of correlated evolution between symmetry and androecium organ number established on the basis of the temporal order test and the contingency test (see ) demonstrates that transitions towards a zygomorphic and polyandrous flower are scarce (q24 and q34 are not significantly different from zero, LRs = 0·56, 2·42; P = 0·454, 0·120, respectively). The transition rates indicated on the diagram, namely q12, q21, q13, q31, q43, are significantly different from zero (LRs = 5·72, 5·66, 8·26, 34·42, 4·87, P < 0·025, P < 0·0025, P < 0·005, P < 0·001, P < 0·05, respectively). The acquisition of zygomorphy or polyandry starting from an ancestral actinomorphic and oligandrous flower is equally probable (q12 is not significantly different from q13, LR = 3·34, P = 0·067). Reversals to the ancestral state starting from an actinomorphic and polyandrous flower are not more likely than reversals from a zygomorphic and oligandrous one (q31 is not significantly different from q21, LR = 3·44, P = 0·064). Because q12 was not significantly different from q13, it was not possible to detect any temporal order in the acquisition of zygomorphy vs. polyandry. Furthermore, changes in perianth symmetry from actinomorphy to zygomorphy and the reverse are equally probable (q21 is not significantly different from q12, LR = 1·80, P = 0·180). It is interesting to note that q31 is significantly different from q13 (LR = 25·51, P < 0·001), indicating that in an actinomorphic background, transitions from polyandry to oligandry are more likely than transitions from oligandry to polyandry. q43 is significantly superior to q21 (LR = 4·60; P < 0·05) suggesting that the reversal to actinomorphy is more likely when the flower is polyandrous than oligandrous.
Tests of character correlation and transition rates values under BayesDiscrete
Fig. 4. Flow diagram summarizing the transitions between the different states of androecium organ number and perianth symmetry. Parameter qij is the transition rate between state ‘i’ in androecium organ number and state ‘j’ in (more ...)
Similar tests were performed to examine the relative timing of evolution between perianth symmetry (considered as the independent state) and the number of spurs. The only transition rates that were significantly different from zero were those linking a zygomorphic flower without spurs [state (1,0)] and a zygomorphic flower with a single spur or two spurs [state (1,1)] (LR = 17·30, P < 0·001) and conversely (LR = 6·12, P < 0·025). In a zygomorphic context, the acquisition of one or two spurs occurs less frequently than reversals towards the absence of spurs (LR = 5·46, P < 0·025). Since the transition rates between the states (0, 0) and (1, 0); (0, 0) and (0, 1) were not significantly different from zero, the precedence of zygomorphy and the emergence of spurs could not be traced statistically.
According to the pairwise comparisons (Mesquite 2·01), the evolution of symmetry is correlated with androecium organ number, the number of spurs and perianth merism 2 (P < 0·01). Zygomorphy is strictly associated with oligandry and a fixed perianth merism, and a single nectar spur or a pair of oil spurs are only found in zygomorphic flowers.