Why Variation in Flower Color May Help Reproductive Success in the Endangered Australian Orchid Caladenia fulva

Abstract

Caladenia fulva G.W. Carr (Tawny Spider-orchid) is a terrestrial Australian endangered orchid confined to contiguous reserves in open woodland in Victoria, Australia. Natural recruitment is poor and no confirmed pollinator has been observed in the last 30 years. Polymorphic variation in flower color complicates plans for artificial pollination, seed collection and ex situ propagation for augmentation or re-introduction. DNA sequencing showed that there was no distinction among color variants in the nuclear ribosomal internal transcribed spacer (ITS) region and the chloroplast trnT-trnF and matK regions. Also, authentic specimens of both C. fulva and Caladenia reticulata from the reserves clustered along with these variants, suggesting free interbreeding. Artificial cross-pollination in situ and assessment of seed viability further suggested that no fertility barriers existed among color variants. Natural fruit set was 15% of the population and was proportional to numbers of the different flower colors but varied with orchid patch within the population. Color modeling on spectral data suggested that a hymenopteran pollinator could discriminate visually among color variants. The similarity in fruiting success, however, suggests that flower color polymorphism may avoid pollinator habituation to specific non-rewarding flower colors. The retention of large brightly colored flowers suggests that C. fulva has maintained attractiveness to foraging insects rather than evolving to match a scarce unreliable hymenopteran sexual pollinator. These results suggest that C. fulva should be recognized as encompassing plants with these multiple flower colors, and artificial pollination should use all variants to conserve the biodiversity of the extant population.

Keywords: DNA analysis; Hymenoptera; endangered; flower color; fruit set; orchid; pollination.

FIGURE 1

Polymorphic flower color categories of Caladenia species from Deep Lead imaged with white light.

FIGURE 2

Map of Caladenia categories at Deep Lead in patches 1–4 in 2000–2001. Key to flower types: Category 1 solid maroon circles, Category 2 white triangles with maroon borders, Category 3 yellow squares with maroon borders, Category 4 solid pink diamonds, Category 5 green circles with maroon borders.

FIGURE 3

Bayesian cladogram from concatenated ITS, trnT-F, and matK sequences, showing comparative relationships and intermixing of flower color categories and authentic samples of Caladenia fulva and C. reticulata collected in the same conservation reserve. Genetic distances derived from the posterior output are shown next to the branches.

FIGURE 4

Multivariate analysis of data from amplification of DNA of Caladenia species from Deep Lead and others by primers for three microsatellites: (GACA)₄, (CAT)₅, and (GTG)₅. Key to symbols: triangles, authentic species (black, Caladenia fulva; maroon, C. reticulata; gray, Caladenia hastata; teal, Caladenia venusta; green, C tentaculata); circles, specimens of polymorphic forms (red, Category 1; pink, Category 2; blue, Category 3; orange, Category 4; purple, Category 5).

FIGURE 5

Effect of (A) flower color category (1–5) and (B) patch (1–5) on percentage success in fruit set in Caladenia species at Deep Lead in 2003–2004.

FIGURE 6

Spectral qualities of Caladenia flowers from Deep Lead. Panel (A) depicts spectral profiles of tepal (solid black lines) and labellum (dashed black lines) regions of flowers from four different categories (1–4) of the orchid Caladenia fulva. Panel (B) shows the position of loci corresponding to the samples of C. fulva in the hexagon color model. In this panel, marker types identify the different flower regions: tepals (red circle markers) or labella (blue square markers). The spectral loci for Apis mellifera as a model of a trichromatic hymenopteran pollinator in the Hexagon color space is indicated by the solid, black line with cross markers.