Flower power


The binomial classification of flowering plants is strongly based on differences in flower design. Most species therefore differ in some observable design characteristics, a feature that evidently advertises diversification by natural selection, as extensively and experimentally investigated by Charles Darwin in the 19th century. His experimental approach to investigating the evolutionary consequences of floral mechanisms that lead to outbreeding and inbreeding continues to the present. Skimming through papers in a couple of recent New Phytologist issues shows that current methods of investigation are considerably more diverse, with opportunities to resolve the evolutionary quirks that have come into play to develop the magnificent diversity of floral design.

Floral design is an observable indicator of complex reproductive mechanisms that are generally difficult to identify and quantify. For example, the same red colouration in Mimulus flowers is achieved by different genetic pathways (Cooley & Willis, 2009). Flowers that trap their pollinating insects are easily recognizable, but even here the mechanisms are complex. In Aristolochia (Oelschägel et al., 2009), the inner walls of the mouth and long tube of the flower possess downward-facing trichomes that allow an easy entrance of insects and the opportunity to transfer pollen from another flower to the specialized stigma of the flower. At this stage the insect cannot escape past the downward-facing trichomes, but this does become possible during male anthesis when the trichomes wilt. The inducement of pollinators to visit flowers correlates with the production of nectar rewards; for the flower the key process is the loading of the pollinator with pollen and the lodging of pollen on the stigmatic surface. Ambruster et al. (2009) suggest that the dual requirements of pollen transfer from the flower to the pollinator and the forced transfer from the pollinator to the flower, en route to the nectar, should select for equal distances for reward to anther and to stigma distances, an adaptive relationship that should scale with flower size. However, many plant species have showy flowers but an absence of nectar rewards for pollinators. This is particularly the case for orchids, but in Dactylorhiza sufficient visits by naïve pollinators still have the potential to select for enhanced display and mechanical fit between flower and pollinator (Sletvold et al., 2010). Larger flower size is also selected in Mimulus guttatus when occurring in nonnative habitats (Murren et al., 2009), while it appears to be the general rule that invasion of a new habitat can also induce changes in flowering phenology (Levin, 2009). Variation in flower size and mating system is also found in cleistogamous species, such as the annual Impatiens capensis (McGoey & Stinchcombe, 2009) that produces large outcrossing, but small self-fertile, flowers, ensuring an annual production of fertile seed.

Pollen limitation is often the cause of the reproductive decline incurred as population size decreases, a major problem for the continued survival of rare species (Le Cadre et al., 2008). Pinus chiapensis has a restricted distribution in Central America and is classed as vulnerable. The species is self-fertile and suffers from inbreeding depression, with high variation within populations (del Castillo & Trujillo, 2008), reducing seed viability. In Pinus taeda (Williams, 2008) the selective elimination of selfed embryos and presumably deleterious alleles, occurs after fertilization up to embryo maturity, with a strong maternal influence. The loss, or virtual loss, of sexual reproduction that might occur at range boundaries or in isolated populations of species is expected to diminish adaptive evolution and increase the load of deleterious alleles, indicating an inevitable road to local extinction. However, work on the Oenothera genus (Johnson et al., 2010) and on clonal Vaccinium stamineum (Yakimowski & Eckert, 2008) indicates, like many other attempts at generalizations concerning plant reproduction, that the evolutionary consequences of reduced sexual reproduction at range boundaries are not that restrictive. Flower production in populations at their current low temperature limits is likely to be responsive to increasing temperatures and to the continuing increases in atmospheric CO2 concentration. Free-air CO2 and temperature-enrichment experiments in temperate grassland (Hovenden et al., 2008) showed precocious flowering in response to warming, but < 15% of the species exhibited any response to CO2 enrichment.

Research on flowers and their evolutionary and ecological interactions seems, more than any other area in plant research, to show that species differ, with a wide range of species-specific responses frequently showing quite different routes to the same apparent end point. The use of molecular techniques can identify how species differences arise but they require the application of a wide range of ecological, developmental and theoretical techniques to identify why such differences may have emerged.