Multiple gene co-options underlie the rapid evolution of sexually deceptive flowers in Gorteria diffusa

Genome of the early spider-orchid Ophrys sphegodes provides insights into sexual deception and pollinator adaptation

Pollinator-driven evolution of floral traits is thought to be a major driver of angiosperm speciation and diversification. Ophrys orchids mimic female insects to lure male pollinators into pseudocopulation. This strategy, called sexual deception, is species-specific, thereby providing strong premating reproductive isolation. Identifying the genomic architecture underlying pollinator adaptation and speciation may shed light on the mechanisms of angiosperm diversification. Here, we report the 5.2 Gb chromosome-scale genome sequence of Ophrys sphegodes. We find evidence for transposable element expansion that preceded the radiation of the O. sphegodes group, and for gene duplication having contributed to the evolution of chemical mimicry. We report a highly differentiated genomic candidate region for pollinator-mediated evolution on chromosome 2. The Ophrys genome will prove useful for investigations into the repeated evolution of sexual deception, pollinator adaptation and the genomic architectures that facilitate evolutionary radiations.

A distinct foliar pigmentation pattern formed by activator-repressor gradients upstream of an anthocyanin-activating R2R3-MYB

The emergence of novel traits is often preceded by a potentiation phase, when all the genetic components necessary for producing the trait are assembled. However, elucidating these potentiating factors is challenging. We have previously shown that an anthocyanin-activating R2R3-MYB, STRIPY, triggers the emergence of a distinct foliar pigmentation pattern in the monkeyflower Mimulus verbenaceus. Here, using forward and reverse genetics approaches, we identify three potentiating factors that pattern STRIPY expression: MvHY5, a master regulator of light signaling that activates STRIPY and is expressed throughout the leaf, and two leaf developmental regulators, MvALOG1 and MvTCP5, that are expressed in opposing gradients along the leaf proximodistal axis and negatively regulate STRIPY. These results provide strong empirical evidence that phenotypic novelties can be potentiated through incorporation into preexisting genetic regulatory networks and highlight the importance of positional information in patterning the novel foliar stripe.

Complex petal spot formation in the Beetle Daisy (Gorteria diffusa) relies on spot-specific accumulation of malonylated anthocyanin regulated by paralogous GdMYBSG6 transcription factors

Gorteria diffusa has elaborate petal spots that attract pollinators through sexual deception, but how G. diffusa controls spot development is largely unknown. Here, we investigate how pigmentation is regulated during spot formation. We determined the anthocyanin composition of G. diffusa petals and combined gene expression analysis with protein interaction assays to characterise R2R3-MYBs that likely regulate pigment production in G. diffusa petal spots. We found that cyanidin 3-glucoside pigments G. diffusa ray floret petals. Unlike other petal regions, spots contain a high proportion of malonylated anthocyanin. We identified three subgroup 6 R2R3-MYB transcription factors (GdMYBSG6-1,2,3) that likely activate the production of spot pigmentation. These genes are upregulated in developing spots and induce ectopic anthocyanin production upon heterologous expression in tobacco. Interaction assays suggest that these transcription factors regulate genes encoding three anthocyanin synthesis enzymes. We demonstrate that the elaboration of complex spots in G. diffusa begins with the accumulation of malonylated pigments at the base of ray floret petals, positively regulated by three paralogous R2R3-MYB transcription factors. Our results indicate that the functional diversification of these GdMYBSG6s involved changes in the spatial control of their transcription, and modification of the duration of GdMYBSG6 gene expression contributes towards floral variation within the species.

The road less taken: Dihydroflavonol 4-reductase inactivation and delphinidin anthocyanin loss underpins a natural intraspecific flower colour variation

Visual cues are of critical importance for the attraction of animal pollinators, however, little is known about the molecular mechanisms underpinning intraspecific floral colour variation. Here, we combined comparative spectral analysis, targeted metabolite profiling, multi-tissue transcriptomics, differential gene expression, sequence analysis and functional analysis to investigate a bee-pollinated orchid species, Glossodia major with common purple- and infrequent white-flowered morphs. We found uncommon and previously unreported delphinidin-based anthocyanins responsible for the conspicuous and pollinator-perceivable colour of the purple morph and three genetic changes underpinning the loss of colour in the white morph - (1) a loss-of-function (LOF; frameshift) mutation affecting dihydroflavonol 4-reductase (DFR1) coding sequence due to a unique 4-bp insertion, (2) specific downregulation of functional DFR1 expression and (3) the unexpected discovery of chimeric Gypsy transposable element (TE)-gene (DFR) transcripts with potential consequences to the genomic stability and post-transcriptional or epigenetic regulation of DFR. This is one of few known cases where regulatory changes and LOF mutation in an anthocyanin structural gene, rather than transcription factors, are important. Furthermore, if TEs prove to be a frequent source of mutation, the interplay between environmental stress-induced TE evolution and pollinator-mediated selection for adaptive colour variation may be an overlooked mechanism maintaining floral colour polymorphism in nature.

Development and evolution of the Asteraceae capitulum

Asteraceae represent one of the largest and most diverse families of plants. The evolutionary success of this family has largely been contributed to their unique inflorescences, capitula that mimic solitary flowers but are typically aggregates of multiple florets. Here, we summarize the recent molecular and genetic level studies that have promoted our understanding of the development and evolution of capitula. We focus on new results on patterning of the enlarged meristem resulting in the iconic phyllotactic arrangement of florets in Fibonacci numbers of spirals. We also summarize the current understanding of the genetic networks regulating the characteristic reproductive traits in the family such as floral dimorphism and differentiation of highly specialized floral organs. So far, developmental studies in Asteraceae are still limited to a very narrow selection of model species. Along with the recent advancements in genomics and phylogenomics, Asteraceae and its relatives provide an outstanding model clade for extended evo-devo studies to exploit the morphological diversity and the underlying molecular networks and to translate this knowledge to the breeding of the key crops in the family.

Visual-, Olfactory-, and Nectar-Taste-Based Flower Aposematism

Florivory, i.e., flower herbivory, of various types is common and can strongly reduce plant fitness. Flowers suffer two very different types of herbivory: (1) the classic herbivory of consuming tissues and (2) nectar theft. Unlike the non-reversibility of consumed tissues, nectar theft, while potentially reducing a plant's fitness by lowering its attraction to pollinators, can, in various cases, be fixed quickly by the production of additional nectar. Therefore, various mechanisms to avoid or reduce florivory have evolved. Here, I focus on one of the flowers' defensive mechanisms, aposematism, i.e., warning signaling to avoid or at least reduce herbivory via the repelling of herbivores. While plant aposematism of various types was almost ignored until the year 2000, it is a common anti-herbivory defense mechanism in many plant taxa, operating visually, olfactorily, and, in the case of nectar, via a bitter taste. Flower aposematism has received only very little focused attention as such, and many of the relevant publications that actually demonstrated herbivore repellence and avoidance learning following flower signaling did not refer to repellence as aposematism. Here, I review what is known concerning visual-, olfactory-, and nectar-taste-based flower aposematism, including some relevant cases of mimicry, and suggest some lines for future research.

Evolution: The art of deceptive pollination

Beetle daisies evolved floral spots that mimic female bee flies to entice mate-seeking males for pollination. A new study shows that these deceptive spots emerged through stepwise co-option of multiple genetic elements, shedding light on the origin of complex phenotypic novelties.