Expression differentiation of CYC-like floral symmetry genes correlated with their protein sequence divergence in Chirita heterotricha (Gesneriaceae)

Abstract

CYCLOIDIEA (CYC) and its homologues have been studied intensively in the model organism Antirrhinum majus and related species regarding their function in controlling floral dorsoventral (adaxial-abaxial) asymmetry, including aborting the adaxial and lateral stamens. This raises the question whether the same mechanism underlies the great morphological diversity of zygomorphy in angiosperms, especially in Lamiales sensu lato, a major clade predominantly with zygomorphic flowers. To address this, we selected a representative in Gesneriaceae, the sister to the remainder of Lamiales s.l., to isolate CYC homologues and further investigate their expression patterns using locus-specific semiquantitative reverse transcriptase polymerase chain reaction. Our results showed that four CYC homologues in Chirita heterotricha differentiated spatially and temporally in expression, in which ChCYC1D was only expressed in the adaxial regions, and transcripts of ChCYC1C were distributed in both the adaxial and lateral regions, while ChCYC2A and ChCYC2B transcripts were only detected in the young inflorescences. ChCYC1C expression in the lateral regions correlated with abortion of the lateral stamens in C. heterotricha hinted at its gain of function, i.e., expanding from the adaxial to the lateral regions in expression. Correlatively, the protein sequences of ChCYC genes exhibited remarkable divergences, in which some lineage-specific amino acids between GCYC1 and GCYC2 in conserved functional domains and two sublineage-specific motifs between GCYC1C and GCYC1D in GCYC1 genes had further been identified. Our results indicated that ChCYC genes had probably undergone an expressional differentiation and specialization in establishing the floral dorsoventral asymmetry in C. heterotricha responding to different selective pressure after gene duplication.

Evolution of Double Positive Autoregulatory Feedback Loops in CYCLOIDEA2 Clade Genes Is Associated with the Origin of Floral Zygomorphy

“…Both P. heterotricha CYC1 and snapdragon CYC belong to CYC2 clade of TCP genes, which are widely conserved in floral zygomorphy patterning in the core eudicots (Luo et al, 1996(Luo et al, , 1999Feng et al, 2006;Zachgo, 2007, 2009;Broholm et al, 2008;Gao et al, 2008;Kim Howarth et al, 2011). In snapdragon, floral zygomorphy is established mainly through the CYC/DICH dorsal identity function in controlling the fate of dorsal organs, namely, promoting dorsal petal growth and arresting dorsal stamen development (Luo et al, 1996(Luo et al, , 1999.…”

“…Expression Analyses of CYC1C and CYC1D in Developing P. heterotricha Flowers Our previous RT-PCR results show that CYC2A and CYC2B are only transiently expressed in the very young inflorescences, but CYC1C and CYC1D are transcribed in both young inflorescences and developing flowers with specific expression in the dorsal/lateral floral organs in P. heterotricha (Gao et al, 2008). To explore how floral zygomorphy establishment depends upon specific CYC1 expression in P. heterotricha, here, we further examined their expression patterns in developing flowers by RNA in situ hybridization.…”

“…Phylogenetic analyses show that two major duplication events within the ECE lineage took place just before the radiation of core eudicots; these duplication events gave rise to three clades, CYC1, CYC2, and CYC3 (Howarth and Donoghue, 2006). Increasing evidence indicates that CYC2 clade genes, specific for the core eudicots, were repeatedly recruited to function in the control of floral zygomorphy based on their strong, dorsoventrally asymmetric expression (Luo et al, 1996(Luo et al, , 1999Feng et al, 2006;Zachgo, 2007, 2009;Broholm et al, 2008;Gao et al, 2008;Kim et al, 2008;Wang et al, 2008Wang et al, , 2010Preston and Hileman, 2009;Song et al, 2009;Zhang et al, 2010;Howarth et al, 2011). Nevertheless, as a linkage between genotype and phenotype, the maintenance of expression of a gene that controls a key developmental process would be crucial in generating phenotypic effects (Crews and Pearson, 2009).…”

“…As the basal-most group in the Lamiales, the family Gesneriaceae has diverse forms of zygomorphy relating to floral organ differentiation early in the order (Cubas, 2004). Recent studies show that CYC2 clade genes in Gesneriaceae play key roles in establishment of zygomorphy, depending on their dorsoventrally asymmetric expression (Du and Wang, 2008;Gao et al, 2008;Song et al, 2009). One zygomorphic member of the Gesneriaceae, Primulina heterotricha (formerly known as Chirita heterotricha; Wang et al, 2011) is characterized by a zygomorphic corolla with the abortion of both dorsal and lateral stamens.…”

“…One zygomorphic member of the Gesneriaceae, Primulina heterotricha (formerly known as Chirita heterotricha; Wang et al, 2011) is characterized by a zygomorphic corolla with the abortion of both dorsal and lateral stamens. Accordingly, two P. heterotricha CYC2 genes, CYC1C and CYC1D, are expressed in the corresponding floral organs from early to late stages (Gao et al, 2008). Therefore, P. heterotricha represents an ideal candidate to further explore the possible regulatory mechanisms for the expression maintenance of CYC2 clade genes.…”