Floral symmetry genes and the origin and maintenance of zygomorphy in a plant-pollinator mutualism

Abstract

The evolution of floral zygomorphy is an important innovation in flowering plants and is thought to arise principally from specialization on various insect pollinators. Floral morphology of neotropical Malpighiaceae is distinctive and highly conserved, especially with regard to symmetry, and is thought to be caused by selection by its oil-bee pollinators. We sought to characterize the genetic basis of floral zygomorphy in Malpighiaceae by investigating CYCLOIDEA2-like (CYC2-like) genes, which are required for establishing symmetry in diverse core eudicots. We identified two copies of CYC2-like genes in Malpighiaceae, which resulted from a gene duplication in the common ancestor of the family. A likely role for these loci in the development of floral zygomorphy in Malpighiaceae is demonstrated by the conserved pattern of dorsal gene expression in two distantly related neotropical species, Byrsonima crassifolia and Janusia guaranitica. Further evidence for this function is observed in a Malpighiaceae species that has moved to the paleotropics and experienced coincident shifts in pollinators, floral symmetry, and CYC2-like gene expression. The dorsal expression pattern observed in Malpighiaceae contrasts dramatically with their actinomorphic-flowered relatives, Centroplacaceae (Bhesa paniculata) and Elatinaceae (Bergia texana). In particular, B. texana exhibits a previously undescribed pattern of uniform CYC2 expression, suggesting that CYC2 expression among the actinomorphic ancestors of zygomorphic lineages may be much more complex than previously thought. We consider three evolutionary models that may have given rise to this patterning, including the hypothesis that floral zygomorphy in Malpighiaceae arose earlier than standard morphology-based character reconstructions suggest.ost flowers are either bilaterally symmetrical (i.e., zygomorphic) and have a single plane of symmetry or radially symmetrical (i.e., actinomorphic) and have several planes of symmetry (1). Floral zygomorphy has evolved independently at least 38 times (2-4) and is a hallmark feature of the most diverse angiosperm clades, including Asteraceae (23,600 spp.), Orchidaceae (21,950 spp.), Fabaceae (19,400 spp.), and Lamiales (23,275 spp.) (5). Plant evolutionary biologists therefore propose that the origin of floral zygomorphy may have been a key innovation for promoting speciation throughout the course of angiosperm evolution (6). The driving force behind the origin of floral zygomorphy has long been thought to be a consequence of selection by specialization on certain insect pollinators (1, 7), which has recently gained experimental support (8).The tropical plant family Malpighiaceae exhibits a strong association between floral zygomorphy and insect pollinator attraction. The floral morphology of the more than 1,000 New World species of this clade is very distinctive and highly conserved, especially with regard to symmetry and pollinator reward. The single upright/dorsal banner petal is strongly differentiated from other petals in the corolla...

Long-term morphological stasis maintained by a plant–pollinator mutualism

“…Although such reorientations appear to be rare across angiosperms (18), they may be more readily accomplished in Malpighiaceae, where two dorsal petal primordia are the norm early in development and the dorsal medial position of the mature banner petal appears to be achieved by a twisting of the flower stalk later in development (23). Indeed, recent developmental genetic studies (38)(39)(40) have shown that the expression of CYCLOIDEA2-like (CYC2-like) genes, which are responsible for establishing floral symmetry in a wide range of angiosperms, has been altered in Malpighiaceae. This finding provides a potential mechanistic basis for changes in floral symmetry coinciding with the loss of oil-bee pollinators (39), and shows that development per se does not significantly constrain changes in this trait.…”

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

“…The CYC2 clade might have further experienced frequent duplications that gave birth to multiple copies of CYC2 genes in most members of zygomorphic clades within the Rosids and Asterids (Howarth and Donoghue, 2006). Of these, there is usually a pair of CYC2 clade genes involved in floral zygomorphy establishment redundantly, at least partially, with other copies having no or transient expression signals, such as CYC and DICH in snapdragon (Luo et al, 1996(Luo et al, , 1999, CYC1 and CYC2 in M. confertiflora (Hileman et al, 2003), CYC1 and CYC2 in L. japonicus (Feng et al, 2006;Wang et al, 2010), CYC1 and CYC2 in pea , CYC1A and CYC1B in Lupinus nanus (Fabaceae) (Citerne et al, 2006), CYC1C and CYC2A in O. dinghushanensis (Song et al, 2009), CYC2A and CYC2B in Byrsonima crassifolia (Malpighiaceae) (Zhang et al, 2010), and CYC2A and CYC2B in Lonicera (Caprifoliaceae) (Howarth et al, After expression initiation, CYC1C (blue box) and CYC1D (orange box) genes maintain expression throughout floral development via a double positive autoregulatory feedback loop by forming both homo-and heterodimers. The blue and orange circles represent the CYC1C and CYC1D proteins, and the blue and orange ovals represent the CYC binding sites found in the CYC1C and CYC1D promoters, respectively.…”

“…The dorsal identity function of CYC2 clade genes in patterning floral zygomorphy necessarily requires the coupling of at least two components (i.e., the expression maintenance by employing an autoregulatory loop and the expression location by evolving certain dorsal-specific cis-regulatory elements). Zygomorphy evolved independently several times from actinomorphic ancestors and is also frequently lost in the major zygomorphic lineages in angiosperms (Donoghue et al, 1998;Citerne et al, 2006;Zhou et al, 2008;Pang et al, 2010;Zhang et al, 2010). Therefore, it would be important to understand whether or how these secondary shifts are associated with a possible uncoupling of the two components, including a secondary change of the autoregulation loop and the regulatory alteration for CYC2 expression location, in the CYC2 gene regulatory pathway.…”

“…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).…”

“…Therefore, it is important to understand how the maintenance of expression of CYC2 clade genes has been achieved, as co-option of its dorsal-specific location could be functionally sufficient to generate morphological zygomorphy. Furthermore, transient or weak expression of CYC2 clade genes in young floral or floral organ meristems in the ancestral actinomorphic lineages usually has no or very weak morphological effects (Cubas et al, 2001;Zhang et al, 2010;Busch et al, 2012). Therefore, the regulatory changes required to maintain and strengthen CYC2 expression, co-opted along with the regulatory mechanisms that produce dorsal-specific CYC2 expression, may represent a major step in the evolutionary origin of floral zygomorphy.…”