Seed camouflage to counteract birds hunger

All of us know that animals use camouflage and mimicry as strategies to protect themselves from predators. Many examples have become famous with many TV documentaries showing, among others, foxes and rabbits that in higher latitudes are white to be confused with the snow and thus, to become harder to be distinguished and caught. Insects that look like wooden sticks or leaves represent another widely known example, and of course the “king” of camouflage, the chameleon which has developed the capacity to modify the colour of his skin in function of his surrounding environment.

But animals are not the only organisms with the capacity to hide from predators. Mushrooms gatherers know very well that fungi have colours often very difficult to distinguish from their environment. In the plant kingdom there are also some famous examples of camouflage like stone plants from the Lithops genus. These succulent plants are native to dry regions of southern Africa and have evolved their shape, size and colour to resemble small stones in their native surroundings.

Besides, other less known examples of camouflage strategies also exist in the plant kingdom. Stephanie S. Porter ¹ recently published a nice study focused on the capacity of seeds to match the colour of their native soil to reduce their detection by animal predators, mainly birds that have been shown to preferentially consume seeds most divergent from the colour of the substrate.

In this sense, seed predators represent an important class of natural enemies of plants that can consume a large proportion of a plant´s reproductive output. However, despite their impact, little is known about how the selective pressure imposed by these seed predators may affect the local adaptation of seed defences. For instance, pine seeds are polymorphic in seed colour which offers a differential fitness in the presence of birds post-fire against a mosaic background of ash and exposed soil. Indeed, background colour matching has been demonstrated to affect seed fitness in nature supporting the advantage of color crypsis. This colour is given by different pigments including flavonoids and anthocyanins. However, the local adaptation of seed colour to a certain soil has not been already demonstrated.

With this aim the author chose to study the variation of the colour of Acmispon wrangelianus seeds in different California soils. A. wrangelianus is a small annual herb (Figure 1) native from California that belongs to the family of legumes (Fabaceae). It is highly self-fertile and produces seeds of 2 to 3.5 mm. Among others, this species was chosen for the study because it is widely spread and it is found in many habitats including disturbed zones. Importantly, after dispersal, A. wrangelianus seeds form a persistent seed bank within the soil.

Thus, to assess whether A. wrangelianos seed colour can be locally adapted to the colour of a native soil the author collected seeds in the McLauglin Reserve in California from 6 six different sites and 3 locations within each site 3. The distance between each site was relatively short (0.5 to 5 km; Figure 2). Three of these sites corresponded to serpentine soils and three to non-serpentine soils. Serpentine soils are formed by ultramafic rocks, with low nutrients and organic matter content and represent a difficult environment for plant growth. Thus, vegetation of a stressful serpentine soil is sparser than on nonserpentine soils and this kind of open habitats are often seed limited and subjected to higher pressure from seed predators, which could positively select seed colour camouflage.

To be sure that the variation in seed colour was due to genetic, rather than environmental effects, the field-collected seeds were grown for one generation in a glasshouse and the seeds were collected for the analysis. Colour measurements were captured as reflectance values quantified from calibrated digital images captured with a dissecting microscope and digital images analysis was performed with Image J, a public domain image processing program.

The results revealed that A. wrangelianus seeds exhibit significant variation in their colour and that the populations displayed adaptative divergence in seed colour camouflage between serpentine and nonserpetine soils (Figure 3). Therefore, despite the short distances between the sites, natural selection acted in favour for locally camouflaged colour. The authors show that A. wrangelianus seeds more closely matched the colour of their native site than other sites from the same soil type. Moreover, serpentine seeds matched more closely to their native soil colour than nonspertines did for their respective native soil. This suggests that predators would impose a selection eating seeds that differ from the soil colour thus maintaining genetic isolation by adaption.

Because seed coloration is a relatively inexpensive mechanism for defence that can be quickly effective. The authors suggested, that the potential of the rapid response to selection may explain how this genetic differentiation can occur on such a fine scale, 5 km maximum distance between soils, despite the gene flow among plants from different communities.