Loss of staminodes in Aquilegia jonesii reveals a fading stamen-staminode boundary

Abstract

The modification of fertile stamens into sterile staminodes has occurred independently many times in the flowering plant lineage. In the genus Aquilegia (columbine) and its closest relatives, the two stamen whorls closest to the carpels have been converted to staminodes. In Aquilegia, the only genetic analyses of staminode development have been reverse genetic approaches revealing that B-class floral identity genes are involved. A. jonesii, the only species of columbine where staminodes have reverted to fertile stamens, allows us to explore the genetic architecture of staminode development using a forward genetic approach. We performed QTL analysis using an outcrossed F2 population between A. jonesii and a horticultural variety that makes fully developed staminodes, A. coerulea 'Origami'. Our results reveal a polygenic basis for staminode loss where the two staminode whorls are under some level of independent control. We also discovered that staminode loss in A. jonesii is not complete, in which staminode-like traits sometimes occur in the inner fertile stamens, potentially representing a fading boundary of gene expression. The QTLs identified in this study provide a map to guide future reverse genetic and functional studies examining the genetic basis and evolutionary significance of this trait.

Keywords: Columbine; Floral organ boundary; Floral organ loss; Quantitative trait locus; Staminodia.

Fig. 1

Floral phenotypes of ‘Origami’ (A–D) and A. jonesii (E–H). A and E Whole flowers (left) and after dissection to just the inner two staminode/stamen whorls and carpels (right). B and F Floral diagrams with the innermost whorl (I) and the next outer whorl (O) relative to the carpels indicated by arrows. For A. jonesii, morphological variation between the I-whorl (wavy symbol) and the O-whorl (flattened symbol) is depicted. C and G Dissected organs from the I and O whorls, labeled staminodia (std) on the left and a stamen (sta) on the right. Symbols under organs in G match those in F, depicting chimeric organs in A. jonesii. D, H Toluidine blue stained sections of young floral buds at stage 12 from Min & Kramer, 2017. H Image is from an F2 flower with the same phenotype as A. jonesii. Arrows indicate the inner whorl staminodes in panel D. Scale bars = 500 µm. Phenotypic scores for the A. jonesii flower are indicated in Additional file 1; Fig. S2 (AjBB3a). Phenotypic scores for the ‘Origami’ flower are all maximal staminode-like scores (Fig. 2)

Fig. 2

Examples of phenotype scores of subtraits. Organs were dissected from F2 plants representative of each phenotype score. Red text indicates an ‘Origami’-like (staminode-like) score and blue text indicates an A. jonesii-like (stamen-like) score. Intermediate colors are used for the intermediate LE scores of 1 and 2. Scale bar = 2 mm

Fig. 3

QTL maps for each subtrait that differentiate stamen and staminode identity for the inner and outer whorls of A. coerulea ‘Origami’ and A. jonesii, across the 7 Aquilegia chromosomes. LOD (logarithm of odds) profiles reflect the best multiple QTL model associated with each subtrait and whorl, with those affecting the inner whorl in grey and the outer whorl, or both whorls in black. LOD profiles are only shown for chromosomes where significance is reached and peak QTL are labeled by subtrait, whorl if applicable, and chromosome number. Shaded areas represent 1.5 LOD intervals for each QTL peak. Dashed horizontal line represents the LOD significance threshold of 3.37. A single QTL map for FU is shown since it combines organs from both the inner and outer whorls. Positions of potential candidate genes are indicated using vertical lines, with those falling within a 1.5 LOD interval (green), those outside a 1.5 LOD intervals but within a LOD profile above the significance threshold (orange) and B-class floral identity genes falling in a region with a LOD profile below the significance threshold (red). AqLOB: LATERAL ORGAN BOUNDARIES, AqPI: PISTILLATA, AqAP3-1&2: APETALA3-1&2, AqSEP3: SEPALLATA3, AqAGL9: AGAMOUS-LIKE9, AqCCR3: CRINKLY-RELATED3, AqGAUT4,10,12: GALACTURONOSYLTRANSFERASE4,10,12