Evolution of flower color pattern through selection on regulatory small RNAs

Abstract

One Sentence Summary:Selection acting on an inverted duplication that generates small RNAs leads to evolution of regulatory interactions and phenotypic change. LENGTH Main Text:A convenient system for studying selection in natural populations is afforded by hybrid zones, where closely-related species or populations come into contact (1). Such a hybrid zone has been described for two subspecies of Antirrhinum majus (snapdragon), that 4 differ in flower color (2), a trait involved in pollinator attraction (3-7). Both subspecies are pollinated by bees but have alternate patterns for guiding flower entry:A.m.pseudomajus flowers are magenta, with a patch of yellow highlighting the bee entry point (Fig. 1A), whereas A.m.striatum flowers are yellow with magenta veins at the entry point (Fig. 1B). The magenta and yellow flower color intensities show sharp clines at a hybrid zone (2) where the subspecies come into contact. Production of magenta is regulated by ROSEA (ROS) and ELUTA (EL) (8-10). ROS encodes a MYB-like transcription factor that promotes anthocyanin biosynthetic gene expression in A.m.pseudomajus and exhibits a steep cline in allele frequencies at the hybrid zone (2, 9).Distribution of yellow pigment is regulated by SULF (Fig. 1B, C), which represses production of the yellow flavonoid aurone in A.m.pseudomajus ( Fig. 1D) (2, 9, 10). Here we study the molecular nature of SULF.To isolate SULF we first mapped it to an interval of ~3Mb on chromosome 4 by sequencing pools of sulf and SULF phenotypes from a segregating population ( fig. S1).In parallel, we carried out a transposon mutagenesis experiment in A. majus (SULF) and isolated a mutant, sulf-660, that was both somatically and genetically unstable ( fig. S2A and Methods). Comparing the genome sequence of sulf-660 and its revertants revealed a single insertion site, within the mapped region of SULF, specific to sulf-660. Three independent revertants had different excision footprints at this site, confirming that the transposon was responsible for the sulf phenotype ( fig. S2B). 5BLAST searches of the sequence flanking the transposon insertion site revealed regions of 74-88% nucleotide sequence identity to A.majus chalcone 4'-O-gluosyltransferase (Am4'CGT), which encodes an enzyme involved in synthesis of the yellow pigment aurone ( Fig. 2A and table S1) (11). The regions of Am4'CGT homology were organized as an inverted duplication in the A. majus SULF genome. Both the left and right arms of the duplication carried deletions relative to intact Am4'CGT, suggesting they had independently degenerated from a more complete precursor. A contiguous region of inverted homology between the left and right arms spanned a ~590 bp region (red arrows, Fig. 2A), separated by a ~600 bp spacer region, which contained the transposon insertion site of sulf-660. Phylogenetic analysis indicated that the SULF inverted repeats were likely generated from Am4'CGT recently in the evolution of the Antirrhinum lineage ( Fig. 2B and fig. S3).To determine whether the inverted duplicatio...

The Evolution of Flavonoid Biosynthesis: A Bryophyte Perspective

“…AmAUS conducts oxygenation of the B-ring of the chalcone, which is followed by cyclization into the aurone (Nakayama et al, 2000) (Figure 6). Although it can use chalcone aglycones in vitro, in planta aurone production in A. majus requires the activity of the CHALCONE 4'-O-GLUCOSYLTRANSFERASE (C4'GT) since only the glucoside is transported into the vacuole (Ono et al, 2006;Bradley et al, 2017). Subsequently, PPOs that form aurones were identified in other species, with the AURONE SYNTHASE of Coreopsis grandiflora that makes 4-deoxyaurones being studied in detail (Molitor et al, 2015;Molitor et al, 2016).…”

New insight into the molecular mechanism of colour differentiation among floral segments in orchids

“…The anthocyanin biosynthesis pathway (ABP) is highly conserved and has been well studied in plants 6 . As crucial ABP regulators, the R2R3-MYB transcription factors can regulate the expression of structural genes to influence flower colour formation [8][9][10][11][12][13] . In Orchidaceae, some R2R3-MYBs have been verified to activate ABP structural genes to promote anthocyanin accumulation in Phalaenopsis 5 , Oncidium 14 and Dendrobium 15 flowers.…”

Genome structure and evolution ofAntirrhnum majusL

“…Furthermore, the absence of a recent WGT in Antirrhinum genome makes it a key diploid genome that promises to provide important insights into plant genome evolution. Recently, small RNAs in natural hybrid zone of A. majus using this genome sequence as a reference have revealed an inverted duplication acting as steep clines by nature selection in evolution of flower color pattern 25 .…”