Similar genetic mechanisms underlie the parallel evolution of floral phenotypes

Abstract

The repeated origin of similar phenotypes is invaluable for studying the underlying genetics of adaptive traits; molecular evidence, however, is lacking for most examples of such similarity. The floral morphology of neotropical Malpighiaceae is distinctive and highly conserved, especially with regard to symmetry, and is thought to result from specialization on oil-bee pollinators. We recently demonstrated that CYCLOIDEA2-like genes (CYC2A and CYC2B) are associated with the development of the stereotypical floral zygomorphy that is critical to this plant-pollinator mutualism. Here, we build on this developmental framework to characterize floral symmetry in three clades of Malpighiaceae that have independently lost their oil bee association and experienced parallel shifts in their floral morphology, especially in regard to symmetry. We show that in each case these species exhibit a loss of CYC2B function, and a strikingly similar shift in the expression of CYC2A that is coincident with their shift in floral symmetry. These results indicate that similar floral phenotypes in this large angiosperm clade have evolved via parallel genetic changes from an otherwise highly conserved developmental program.

Figure 1. Floral morphology, development, and CYC2 expression of Malpighiaceae.

(A), Banisteriopsis argyrophylla illustrating the stereotypical New World floral morphology and pattern of CYC2 expression in New World Malpighiaceae (expression shown in grey). (B), Acridocarpus zanzibaricus, Sphedamnocarpus angolensis, and Tristellateia australasiae (from left to right) represent three Old World floral phenotypes that have evolved in parallel from a similar New World-type ancestor. (C–E), Scanning electron micrographs showing the typical orientation of the two dorsal petals at the earliest stage of floral development in the New World Malpighiaceae species Bunchosia glandulifera (C) and Heteropterys sp. (D), and in the Old World species, Tristellateia australasiae (E). (F), Phylogeny depicting relationships of the three focal Old World clades: Acridocarpus, African Sphedamnocarpus, and Tristellateia. Grey lines highlight the radially symmetrical sister groups of Malpighiaceae, Centroplacaceae, and Elatinaceae ; black lines highlight Malpighiaceae species with the stereotypical New World floral morphology; red highlights the three Old World clades with parallel floral morphologies that have departed from the New World morphology. For reference, the banner petal of the New World Malpighiaceae is highlighted in yellow (C and D). Dotted lines = initial axis of floral symmetry; solid lines = final axis of floral symmetry; arrows indicate the shift in the axis of symmetry that takes place just before anthesis in New World Malpighiaceae . Scale bars equal 100 μm.

Figure 2. Phylogeny of CYC2-like genes for Malpighiaceae.

Bayesian majority rule consensus topology shown; clades with >50% maximum likelihood (ML) bootstrap support and >60% Bayesian posterior probabilities depicted above lines, respectively. ML bootstrap support <50% indicated with a hyphen. Inferred gene tree is reflective of accepted species tree relationships . Accessions highlighted in red include the three Old World clades examined here that exhibit parallel floral phenotypes–Acridocarpus, African Sphedamnocarpus, and Tristellateia. Accessions labeled with dotted lines signify inferred gene losses in Acridocarpus natalitius, A. zanzibaricus, Tristellateia australasiae, and T. africana. See Supplementary Table S2 for species identities and voucher information. C, Centroplacaceae; E, Elatinaceae; M, Malpighiaceae; O, Oxalidaceae.

Figure 3. Quantitative RT-PCR (qRT-PCR) expression of CYC2-like genes for the parallel floral morphologies in the Old World Malpighiaceae Tristellateia australasiae (A) Acridocarpus natalitius (B) and Sphedamnocarpus pruriens (C).

Grayscale shading on floral diagrams summarizes the relative strength of the spatial pattern of CYC2 expression in the corolla and calyx. qRT-PCR expression data was determined for dissected floral organs at mid and late stages. Expression levels are relative to the control β-tubulin. Error bars represent standard errors. ds, dorsal sepal; ls, lateral sepal; vs, ventral sepal; dp, dorsal petal; lp, lateral petal; vp, ventral petal; st, stamens; ca, carpels; MB, medium buds ∼40–60% of full size buds; LB, large buds ∼70–90% of full size buds. Scale bars equal 5 mm.

Figure 4. Summary of CYC2-like gene expression.

Expression of CYC2-like genes in New World Malpighiaceae and in three parallel shifts in the Old World Malpighiaceae clades represented, from left to right, by Tristellateia australasiae, Acridocarpus natalitius, and Sphedamnocarpus pruriens. The blue shading of the New World Species indicates late stage CYC2 gene expression (Fig. S1 A). The gradient shading in Old World species, from white to black, indicates increasing intensity of CYC2 expression, respectively.