Elaboration of bilateral symmetry across Knautia macedonica capitula related to changes in ventral petal expression of CYCLOIDEA-like genes

Abstract

Background: Shifts in floral form across angiosperms, particularly from radially symmetrical to bilaterally symmetrical flowers, are often associated with shifts in speciation rates and changes in pollination syndrome. Growing evidence across both rosids and asterids indicates that CYCLOIDEA (CYC)-like transcription factors from the TCP gene family play a role in establishing the dorsoventral pattern of flower symmetry, which affects the development of both the corolla and androecium. Previous studies of CYC-like genes, especially of the CYC2 clade, indicate that these genes are dorsally restricted in bilaterally symmetrical flowers. Also, gene duplication of CYC-like genes often correlates with shifts in floral form in both individual flowers and head-like inflorescences (capitula).

Results: Here, we compared the expression patterns of six CYC-like genes from dorsal, lateral, and ventral petals of internal and external florets across capitula of Knautia macedonica (Dipsacaceae). We demonstrate that multiple copies of CYC-like genes are differentially expressed among petal types and between internal and external florets. Across paralogs, there was a general trend toward a reduction in dorsal expression and an increase in ventral expression in internal florets compared to external florets. However, it was in the ventral petals where a statistically significant increase in expression correlates with a less zygomorphic flower. We also show for the first time lateral-specific expression of a CYC-like gene. Additionally, dorsoventral asymmetric expression of a CYC3 paralog indicates that this understudied gene clade is likely also involved in floral symmetry.

Conclusions: These data indicate that the elaboration of bilateral symmetry may be regulated by the dorsoventral gradient of expression, with statistically significant changes in ventral expression correlating with changes in dorsoventral morphological specialization.

Keywords: CYCLOIDEA; Capitula; Floral symmetry; Knautia; qPCR.

Fig. 1

Floral images of Knautia macedonica capitulum development. a–c Developmental series of capitulum development. d Cross section through a mature capitulum. e, f Representative examples of internal and external florets. Diagrams show orientation of petals in e, f with D dorsal, LL left lateral, RL right lateral, and V ventral. (Scale bars 5 mm)

Fig. 2

Putative gene tree of CYC-like paralogs in Knautia macedonica. Duplication events that span the core eudicots are shown in black (CYC1), blue (CYC2), and red (CYC3). Black circles label duplications that span the Caprifoliaceae. Gray circles indicate a putative Dipsacaceae duplication. A single paralog was successfully cloned and used in qPCR experiments from each major clade. Additional paralogs are hypothesized within other groups of Dipsacaceae and are shown as dashed lines with putative copy numbers indicated inside triangles

Fig. 3

qPCR relative expression of CYC-like paralogs for external and internal florets. a Relative expression pattern for external petal types by paralog. b Relative expression pattern for internal petal types by paralog. c Relative expression pattern for dorsal petals. d Relative expression pattern for ventral petals (note difference in y axis values). Abbreviations for petal types are as follows: D dorsal petals, LL left lateral petals, RL right lateral petals, and V ventral petals. Standard error bars are shown. Statistical significance among petal types is as follows: *p ≤ 0.05; **p ≤ 0.01; ***p ≤ 0.001

Fig. 4

Knautia macedonica expression model for internal and external florets. Color shade indicates level of expression with darker shades signifying higher expression relative to other petals in that flower. Significance shown in blue: circles (not significant) and blue asterisk (*p ≤ 0.05; **p ≤ 0.01; ***p ≤ 0.001). Blue circles or asterisks show dorsal to lateral (top), dorsal to ventral (middle), and lateral to ventral (bottom). Green arrows highlight petals with a significant change in expression between external and internal florets

Fig. 5

Model of CYC-like paralog expression across the inflorescence. In the transition from internal to external florets, there is a slight reduction in expression of four CYC-like paralogs in the dorsal petals. Conversely, there is a slight increase in all six paralogs (three of which are significant) in ventral petals. This results in an evening out of asymmetry along the dorsoventral axis in internal florets. Asterisk denotes level of significance: *p ≤ 0.05; **p ≤ 0.01; ***p ≤ 0.001