Elaboration of Bilateral Symmetry Across Knautia Macedonica Capitula Related to Changes in Ventral Petal Expression of CYCLOIDEA‑Like Genes Brent A
Total Page：16
File Type：pdf, Size：1020Kb
Berger et al. EvoDevo (2016) 7:8 DOI 10.1186/s13227-016-0045-7 EvoDevo RESEARCH Open Access Elaboration of bilateral symmetry across Knautia macedonica capitula related to changes in ventral petal expression of CYCLOIDEA‑like genes Brent A. Berger1,2*, Veronica Thompson2, Aedric Lim2, Vincent Ricigliano2 and Dianella G. Howarth2 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 dor- sally 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 expres- sion in internal florets compared to external florets. However, it was in the ventral petals where a statistically signifi- cant 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 dors- oventral morphological specialization. Keywords: CYCLOIDEA, Floral symmetry, Knautia, Capitula, qPCR Background radially symmetrical (actinomorphic) and bilaterally Shifts in dorsoventral asymmetric expression of tran- symmetrical (zygomorphic) flowers have been common scription factors affecting growth and patterning of the [1–3]. Such morphological changes are of special inter- corolla and androecium of a flower can give rise to a vast est in relation to pollination (e.g., see [4]) and perhaps array of different floral symmetries that potentially affect to rates of speciation [5]. Additionally, bilaterally sym- reproductive strategies and plant evolution. Across angi- metrical flowers vary significantly across groups in the osperms, major transitions in floral symmetry between degree of asymmetry along the dorsoventral axis. A lim- ited number of clades vary in floral symmetry across a *Correspondence: [email protected] single inflorescence, with multiple types of florets with 1 Department of Botany, University of Wisconsin-Madison, 430 Lincoln Dr., different degrees or types of symmetry occurring. Bilat- Madison, WI 53706, USA erally symmetrical flowers generally have three different Full list of author information is available at the end of the article © 2016 Berger et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Berger et al. EvoDevo (2016) 7:8 Page 2 of 10 morphological modules: dorsal petals, lateral petals, and 23]. In Caprifoliaceae, a duplication in CYC3 is correlated a single ventral petal [6], suggesting that genes regulate with the transition to bilateral symmetry, suggesting that the development of these petals differently. these paralogs may also play a role in determining floral Numerous studies across angiosperms show that symmetry. morphological shifts between radially symmetrical and Plants with capitula (compact floral head inflores- bilaterally symmetrical flowers are correlated with inde- cences) comprised of both radially and/or multiple pendent transitions in asymmetric dorsoventral expres- types of bilaterally symmetrical flowers, such as those sion of members of the ECE clade of TCP transcription in Asteraceae and Dipsacaceae, appear to have the factors [7–9]. CYCLOIDEA (CYC) was the first char- greatest number of CYC2 paralogs [23–25] with dif- acterized member [10] and has been shown repeatedly ferential expression tied to changes in floral symme- to play a role in specifying dorsal petal identity [9–12]. try across the inflorescence axis [18]. Although not as Around the divergence of the core eudicots, CYC-like commonly examined, Asteraceae and Dipsacaceae also genes duplicated twice, resulting in three copies—CYC1, have duplicated CYC1 and CYC3 clades [18, 23, 24]. A CYC2, and CYC3 [7]. Within the core eudicots, studies typical capitulum consists of many small flowers (flo- of the role of CYC-like genes in floral symmetry have rets) packed tightly into a condensed head that can focused on the CYC2 clade of core eudicots [7]. The closely resemble a single, large flower. In some groups dorsal restriction of CYC2 genes has been found in all (e.g., Asteraceae, Actinodium of Myrtaceae) capitula bilaterally symmetrical core eudicots to date [12, 13]. have small, similar florets throughout the inflorescence In contrast, in radially symmetrical groups, CYC2 clade with the exception of the outer ring of much larger, members either are not expressed in corolla tissue or are more strongly bilaterally symmetrical flowers [26]. In ubiquitously expressed [14, 15]. The localization of CYC2 others (e.g., Dipsacaceae), florets gradually increase in has been determined with in situ hybridization and/or size from the center (internal florets) to the outer ring semiquantitative RT-PCR from dissected petal lobes (see (external florets) [24]. [9, 12]). Additionally, it has been shown across core eud- Knautia macedonica (Dipsacaceae) has four-parted icots that loss of expression of CYC2 paralogs results in weakly zygomorphic internal florets that gradually ventralization of the flower [10, 11, 16–18]. All of these expand and diverge across the dorsoventral axis into data suggest that CYC2 genes are dorsal identity genes. strongly zygomorphic florets in the external ring (Fig. 1). In Dipsacales, the first transition from radial to bilateral This variation in the differentiation of dorsoventral spe- symmetry is correlated with a duplication event, resulting cialization across the inflorescence of K. macedonica in two CYC2 and two CYC3 gene copies [19]. Previous provides an opportunity to quantify gene expression of work indicates that the expression of both CYC2 cop- CYC-like paralogs among natural variation in bilateral ies is partially restricted to the dorsal and lateral corolla symmetry. Previous studies examining the CYC-like lobes [14]. Subsequent shifts to more strongly bilaterally gene phylogeny and duplication events in Dipsacaceae symmetrical flowers involved further restriction of the indicated that this family had perhaps the greatest num- duplicate copies and a decoupling of expression, such ber of paralogs of any examined group with as many as that one copy is more dorsally restricted than the other 17 paralogs across the ECE clade (CYC1 = 5, CYC2 = 9, [14]—a pattern observed in other core eudicots (Antir- CYC3 = 3) [24]. Not all paralogs were found in all spe- rhinum majus [11]; Malphigiaceae [15]; and Pisum sati- cies; however, at least a few could have been the result of vum [16]). This variation in paralog expression strongly allelic variation. Additionally, frequent changes in ploidy suggests that differentially restricted expression of CYC2 number, even within species [27], could be complicating copies occurs independently in the evolution of bilateral simple identification of gene number. symmetry from radially symmetric ancestors. The aim of the present study was to quantify ECE Additional independent duplications of CYC-like CYC-like paralogs from petal dissections of internal and genes have been common within bilaterally symmetrical external florets from K. macedonica. We used qPCR groups, especially in the CYC2 clade (e.g., [9, 20]). With with ANOVA and post hoc Tukey HSD statistical tests the exception of six genera in the Brassicaceae whose to examine significant changes in expression of 6 CYC- symmetry is regulated by a single CYC2 ortholog [21, 22], like paralogs across the dorsoventral axis. Our impor- all other examined bilaterally symmetrical core eudicots tant findings include that: (1) although weakly expressed, have two or more paralogs belonging to the CYC2 clade. the expression of 4 CYC-like paralogs is significantly The other two core eudicot clades of ECE CYC-like genes increased in the ventral petals of internal florets; (2)