Lena C. Hileman

Differential regulation of symmetry genes and the evolution of floral morphologies

Shifts in flower symmetry have occurred frequently during the diversification of angiosperms, and it is thought that such shifts play important roles in plant–pollinator interactions. In the model developmental system Antirrhinum majus (snapdragon), the closely related genes CYCLOIDEA (CYC) and DICHOTOMA (DICH) are needed for the development of zygomorphic flowers and the determination of adaxial (dorsal) identity of floral organs, including adaxial stamen abortion and asymmetry of adaxial petals. However, it is not known whether these genes played a role in the divergence of species differing in flower morphology and pollination mode. We compared A. majus with a close relative, Mohavea confertiflora (desert ghost flower), which differs from Antirrhinum in corolla (petal) symmetry and pollination mode. In addition, Mohavea has undergone a homeotic-like transformation in stamen number relative to Antirrhinum, aborting the lateral and adaxial stamens during flower development. Here we show that the patterns of expression of CYC and DICH orthologs have shifted in concert with changes in floral morphology. Specifically, lateral stamen abortion in Mohavea is correlated with an expansion of CYC and DICH expression, and internal symmetry of Mohavea adaxial petals is correlated with a reduction in DICH expression during petal differentiation. We propose that changes in the pattern of CYC and DICH expression have contributed to the derived flower morphology of Mohavea and may reflect adaptations to a pollination strategy resulting from a mimetic relationship, linking the genetic basis for morphological evolution to the ecological context in which the morphology arose.

Developmental genetics of floral symmetry evolution

The relative importance of convergence and parallelism in the independent evolution of similar traits remains an important question in evolutionary biology. Floral zygomorphy has evolved multiple times independently in different plant lineages through alterations in size, shape and/or number of spatially defined organs. In Antirrhinum majus (snapdragon) floral zygomorphy is controlled by CYCLOIDEA and DICHOTOMA, two recently duplicated TCP transcription factors that determine dorsal identity through their interaction with MYB and cell-cycle genes. Early on it was speculated that independent evolutionary transitions from floral actinomorphy to zygomorphy would probably result from unique developmental genetic mechanisms. Here, we review recent evidence supporting the parallel recruitment of CYCLOIDEA homologs in independent evolutionary transitions to zygomorphy in distantly related core eudicot lineages.

Functional analyses of genetic pathways controlling petal specification in poppy

MADS-box genes are crucial regulators of floral development, yet how their functions have evolved to control different aspects of floral patterning is unclear. To understand the extent to which MADS-box gene functions are conserved or have diversified in different angiosperm lineages, we have exploited the capability for functional analyses in a new model system, Papaver somniferum (opium poppy). P. somniferum is a member of the order Ranunculales, and so represents a clade that is evolutionarily distant from those containing traditional model systems such as Arabidopsis, Petunia, maize or rice. We have identified and characterized the roles of several candidate MADS-box genes in petal specification in poppy. In Arabidopsis, the APETALA3 (AP3) MADS-box gene is required for both petal and stamen identity specification. By contrast, we show that the AP3 lineage has undergone gene duplication and subfunctionalization in poppy, with one gene copy required for petal development and the other responsible for stamen development. These differences in gene function are due to differences both in expression patterns and co-factor interactions. Furthermore, the genetic hierarchy controlling petal development in poppy has diverged as compared with that of Arabidopsis. As these are the first functional analyses of AP3 genes in this evolutionarily divergent clade, our results provide new information on the similarities and differences in petal developmental programs across angiosperms. Based on these observations, we discuss a model for how the petal developmental program has evolved.

Functional evolution in the plant SQUAMOSA-PROMOTER BINDING PROTEIN-LIKE (SPL) gene family

The SQUAMOSA-PROMOTER BINDING PROTEIN-LIKE (SPL) family of transcription factors is functionally diverse, controlling a number of fundamental aspects of plant growth and development, including vegetative phase change, flowering time, branching, and leaf initiation rate. In natural plant populations, variation in flowering time and shoot architecture have major consequences for fitness. Likewise, in crop species, variation in branching and developmental rate impact biomass and yield. Thus, studies aimed at dissecting how the various functions are partitioned among different SPL genes in diverse plant lineages are key to providing insight into the genetic basis of local adaptation and have already garnered attention by crop breeders. Here we use phylogenetic reconstruction to reveal nine major SPL gene lineages, each of which is described in terms of function and diversification. To assess evidence for ancestral and derived functions within each SPL gene lineage, we use ancestral character state reconstructions. Our analyses suggest an emerging pattern of sub-functionalization, neo-functionalization, and possible convergent evolution following both ancient and recent gene duplication. Based on these analyses we suggest future avenues of research that may prove fruitful for elucidating the importance of SPL gene evolution in plant growth and development.

Phylogeny and Biogeography of the Arbutoideae (Ericaceae): Implications for the Madrean-Tethyan Hypothesis

Abstract Phylogenetic relationships within subfamily Arbutoideae (Ericaceae) were estimated using parsimony and maximum likelihood analyses of sequence data from the ITS region and part of the large subunit of nuclear ribosomal DNA. The data support the monophyly of Arctostaphylos, Arctous, and Comarostaphylis, but suggest that Arbutus is not monophyletic, with Mediterranean Basin species more closely related to the clade containing Arctostaphylos, Arctous, Comarostaphylis, Ornithostaphylos, and Xylococcus than to the western North American species of Arbutus. Calibration of branch lengths with the fossil record suggests that a vicariance event occurred among members of the Arbutoideae between western North America and the Mediterranean Basin at the Paleogene/Neogene boundary, consistent with the Madrean-Tethyan hypothesis. Communicating Editor: Kathleen A. Kron

Trends in flower symmetry evolution revealed through phylogenetic and developmental genetic advances.

A striking aspect of flowering plant (angiosperm) diversity is variation in flower symmetry. From an ancestral form of radial symmetry (polysymmetry, actinomorphy), multiple evolutionary transitions have contributed to instances of non-radial forms, including bilateral symmetry (monosymmetry, zygomorphy) and asymmetry. Advances in flowering plant molecular phylogenetic research and studies of character evolution as well as detailed flower developmental genetic studies in a few model species (e.g. Antirrhinum majus, snapdragon) have provided a foundation for deep insights into flower symmetry evolution. From phylogenetic studies, we have a better understanding of where during flowering plant diversification transitions from radial to bilateral flower symmetry (and back to radial symmetry) have occurred. From developmental studies, we know that a genetic programme largely dependent on the functional action of the CYCLOIDEA gene is necessary for differentiation along the snapdragon dorsoventral flower axis. Bringing these two lines of inquiry together has provided surprising insights into both the parallel recruitment of a CYC-dependent developmental programme during independent transitions to bilateral flower symmetry, and the modifications to this programme in transitions back to radial flower symmetry, during flowering plant evolution.

SQUAMOSA-PROMOTER BINDING PROTEIN 1 initiates flowering in Antirrhinum majus through the activation of meristem identity genes.

The degree to which developmental genetic pathways are conserved across distantly related organisms is a major question in biology. In Arabidopsis thaliana (L.) Heynh., inflorescence development is initiated in response to a combination of external and internal floral inductive signals that are perceived across the whole plant, but are integrated within the shoot apical meristem. Recently, it was demonstrated that SQUAMOSA-PROMOTER BINDING PROTEIN (SBP)-box proteins regulate A. thaliana flowering time by mediating signals from the autonomous and photoperiod pathways, and by directly activating key genes involved in inflorescence and floral meristem identity, including FRUITFULL (FUL), APETALA1 (AP1) and LEAFY (LFY). In the distantly related core eudicot species Antirrhinum majus L., paralogous SBP-box proteins SBP1 and SBP2 have likewise been implicated in regulating the AP1 ortholog SQUAMOSA (SQUA). To test the hypothesis that SBP-box genes are also involved in the floral induction of A. majus, we used a reverse genetic approach to silence SBP1. SBP1-silenced lines are late to nonflowering, and show reduced apical dominance. Furthermore, expression and sequence analyses suggest that the SBP1-mediated transition to flowering occurs through the positive regulation of FUL/LFY homologs. Together, these data outline the utility of virus-induced gene silencing in A. majus, and provide new insight into the conservation of flowering time genetic pathways across core eudicots.

Conservation and diversification of the symmetry developmental program among close relatives of snapdragon with divergent floral morphologies

Multiple evolutionary shifts in floral symmetry and stamen number have occurred in the snapdragon (Antirrhinum majus) family Veronicaceae. In Mohavea, Veronica and Gratiola there have been independent evolutionary reductions in stamen number and modifications to corolla shape. It is hypothesized that changes in the regulation of homologs of snapdragon dorsal flower identity genes CYCLOIDEA (CYC) and RADIALIS (RAD) underlie these floral transitions. CYC-like and RAD-like genes from Veronica montana and Gratiola officinalis were cloned and sequenced, compared with homologs from other Veronicaceae species using phylogenetic analysis, and their expression was investigated by reverse transcriptase-polymerase chain reaction (RT-PCR) and in situ hybridization. VmCYC1, GoCYC1, GoCYC2 and RAD-like genes are expressed exclusively in the dorsal region of floral meristems and developing flowers. Their expression patterns do not correlate with patterns of stamen arrest. VmCYC2 and GoCYC3 are expressed in both vegetative and floral tissues, with VmCYC2 being most abundant in all regions of the floral meristem and all petals. These results support conservation of the floral symmetry gene network for Veronicaceae RAD-like and some CYC-like paralogs, suggest regulatory evolution of other CYC-like genes following gene duplication and implicate different genetic mechanisms underlying dorsal versus ventral stamen abortion within Veronica and Gratiola.

Bilateral flower symmetry — how, when and why?

Bilateral flower symmetry has evolved multiple times during flowering plant diversification, is associated with specialized pollination, and is hypothesized to have contributed to flowering plant species richness. The genes and genetic interactions that control bilateral symmetry are well understood in the model species Snapdragon (Antirrhinum majus). I review recent insights into the genetic control of symmetry in Snapdragon. I summarize how this foundational genetic work has been integrated with mathematical modeling approaches, which together provided new insights into the control of quantitative aspects of petal shape. Lastly, I review how evolutionary studies, stemming from knowledge of the genetic control of symmetry in Snapdragon flowers, have revealed extensive parallel recruitment of a similar genetic program during repeated evolution of bilateral symmetry.

Gradual disintegration of the floral symmetry gene network is implicated in the evolution of a wind-pollination syndrome

Angiosperms exhibit staggering diversity in floral form, and evolution of floral morphology is often correlated with changes in pollination syndrome. The showy, bilaterally symmetrical flowers of the model species Antirrhinum majus (Plantaginaceae) are highly specialized for bee pollination. In A. majus, CYCLOIDEA (CYC), DICHOTOMA (DICH), RADIALIS (RAD), and DIVARICATA (DIV) specify the development of floral bilateral symmetry. However, it is unclear to what extent evolution of these genes has resulted in flower morphological divergence among closely related members of Plantaginaceae differing in pollination syndrome. We compared floral symmetry genes from insect-pollinated Digitalis purpurea, which has bilaterally symmetrical flowers, with those from closely related Aragoa abietina and wind-pollinated Plantago major, both of which have radially symmetrical flowers. We demonstrate that Plantago, but not Aragoa, species have lost a dorsally expressed CYC-like gene and downstream targets RAD and DIV. Furthermore, the single P. major CYC-like gene is expressed across all regions of the flower, similar to expression of its ortholog in closely related Veronica serpyllifolia. We propose that changes in the expression of duplicated CYC-like genes led to the evolution of radial flower symmetry in Aragoa/Plantago, and that further disintegration of the symmetry gene pathway resulted in the wind-pollination syndrome of Plantago. This model underscores the potential importance of gene loss in the evolution of ecologically important traits.