Corolla morphology influences diversification rates in bifid toadflaxes (Linaria sect. Versicolores)

Abstract

Background and aims: The role of flower specialization in plant speciation and evolution remains controversial. In this study the evolution of flower traits restricting access to pollinators was analysed in the bifid toadflaxes (Linaria sect. Versicolores), a monophyletic group of ~30 species and subspecies with highly specialized corollas.

Methods: A time-calibrated phylogeny based on both nuclear and plastid DNA sequences was obtained using a coalescent-based method, and flower morphology was characterized by means of morphometric analyses. Directional trends in flower shape evolution and trait-dependent diversification rates were jointly analysed using recently developed methods, and morphological shifts were reconstructed along the phylogeny. Pollinator surveys were conducted for a representative sample of species.

Key results: A restrictive character state (narrow corolla tube) was reconstructed in the most recent common ancestor of Linaria sect. Versicolores. After its early loss in the most species-rich clade, this character state has been convergently reacquired in multiple lineages of this clade in recent times, yet it seems to have exerted a negative influence on diversification rates. Comparative analyses and pollinator surveys suggest that the narrow- and broad-tubed flowers are evolutionary optima representing divergent strategies of pollen placement on nectar-feeding insects.

Conclusions: The results confirm that different forms of floral specialization can lead to dissimilar evolutionary success in terms of diversification. It is additionally suggested that opposing individual-level and species-level selection pressures may have driven the evolution of pollinator-restrictive traits in bifid toadflaxes.

Keywords: Convergence; Linaria sect. Versicolores; flower specialization; flower tube; nectar spur; pollination; reversal; speciation; species selection; toadflax; trait-dependent diversification.

Fig. 1.

Representatives of Linaria sect. Versicolores. Subsect. Versicolores: (A) L. algarviana; (B) L. bipartita; (C) L. clementei; (D) L. gharbensis; (E) L. incarnata; (F) L. maroccana; (G) L. multicaulis subsp. heterophylla; (H) L. onubensis; (I) L. pedunculata; (J) L. salzmannii; (K) L. spartea; (L) L. tenuis; (M) L. viscosa subsp. spicata; (N) L. viscosa subsp. viscosa; (O) L. weilleri. Subsect. Elegantes: (P) L. elegans; (Q) L. nigricans. Floral morphological types (see Fig. 4): Type I (broad tube, variable spur: A, D, F, G, I–O); Type II (broad tube, very short spur: C); Type III (narrow tube, variable spur: B, E, H, P, Q). Photographs by A. Fernández-Mazuecos (A, E), J. Quiles (B, F, K, N, O), J. Ramírez (C, I, J, M), J.L. Blanco-Pastor (D), E. Rico (G), P. Vargas (H, P, Q) and O. Fragman-Sapir (L).

Fig. 2.

Metric measures (spur length and tube width) and landmarks (1–10) employed in morphometric analyses, shown in two representative taxa of morphological Types I (L. spartea; A, B) and III (L. onubensis; C, D). Photos show lateral (A, C) and ventral (B, D) views. Notice the broad tube in B and the narrow tube in D.

Fig. 3.

Phylogenetic analyses of Linaria sect. Versicolores. (A, B) Gene trees of ITS (A) and ptDNA (B) sequences. The 50 % majority rule consensus trees obtained in Bayesian analyses are shown. A black dot indicates clade support in BI (PP > 0·95), ML (ML – BS > 70 %) and MP (MP – BS > 70 %) analyses. A grey dot indicates support only in BI and ML analyses. A white dot indicates support only in the BI analysis. A white square indicates support only in the ML analysis. (C) Time-calibrated maximum clade credibility species tree obtained in the Bayesian *BEAST analysis based on ITS and ptDNA sequences. Node bars represent the 95 % highest posterior density intervals for the divergence time estimates. Numbers along the branches are Bayesian posterior probabilities. Major clades are named, including subsections following Sutton (1988). The inset shows a log-lineage-through-time plot for Linaria sect. Versicolores, based on 1000 trees randomly sampled from the posterior distribution of the *BEAST analysis. The thick line corresponds to the MCC species tree.

Fig. 4.

Morphological traits of Linaria sect. Versicolores flowers. (A) Scatter plot of tube width versus spur length measured in 696 herbarium specimens representing the 30 species and subspecies of Linaria sect. Versicolores. Means (numbered dots) and standard deviations (bars) for each taxon are plotted. Notice the broad tubes of Types I and II, and the narrow tubes of Type III. (B) Canonical variate analysis of the landmark-based geometric morphometric dataset. Scatter plot of canonical variate 2 versus canonical variate 1 for 369 living specimens sampled in 18 populations belonging to the 12 Iberian species and subspecies (colours), which represent the full range of corolla shapes of Linaria sect. Versicolores. The variance explained by each axis is shown in brackets. In both plots, the three major morphological types discussed in the text are indicated, and representative taxa are shown.

Fig. 5.

Analyses of diversification rates and flower type transitions in Linaria sect. Versicolores. (A) Results of the binary-state speciation and extinction (BiSSE) analysis of the MCC species tree (Fig. 3C), considering two character states corresponding to morphological Types I/II (broad tube) and III (narrow tube). Posterior distributions of parameters (speciation rates, extinction rates, diversification rates and character transition rates) obtained in the MCMC-BiSSE analysis are shown. Horizontal bars indicate the 95 % credibility interval for each parameter. (B) Maximum likelihood ancestral state reconstruction of morphological Types I/II and III under state-dependent diversification (ASR-BiSSE), based on parameter estimates shown in A. The tree is the MCC species tree with nodes with PP < 0·5 collapsed. (C) Summary distributions of the number of shifts between flower types inferred when implementing parsimony optimization over the full posterior distribution of trees obtained in the *BEAST analysis.

Fig. 6.

Different behaviours of potential pollinators of morphological Types I (broad tube) and III (narrow tube). (A) Apis mellifera (Hymenoptera) in L. viscosa subsp. viscosa (Type I). (B) Euchloe sp. (Lepidoptera) in L. viscosa subsp. viscosa. (C) Bombyliidae (Diptera) in L. elegans (Type III). White arrows indicate pollen placement on pollinators. Photographs by M. Fernández-Mazuecos (A, B) and P. Vargas (C).