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‘When we see leaf-eating insects green, and bark-feeders mottled-grey; the alpine ptarmigan white in winter, the red-grouse the colour of heather, and the black-grouse that of peaty earth, we must believe that these tints are of service to these birds and insects in preserving them from danger.’
Darwin (1859) The Origin of Species
Heterogeneous selection as a result of variation in soil conditions can drive adaptive plant population divergence and result in ecotypes locally adapted to contrasting soil conditions despite the absence of physical barriers to gene flow (Linhart & Grant, 1996). Although abiotic soil factors are an important source of divergent selection (Kruckeberg, 1954; McNeilly, 1968; Brady et al., 2005), little is understood about the contributions of natural enemies to patterns of divergent selection across soil conditions (Cremieux et al., 2008). The effects of natural enemies can be spatially variable if the efficacy of prey defense is context dependent. Camouflage, the use of protective coloration to reduce the risk of detection by enemies (Stevens, 2007), is a highly context-dependent defense that depends upon the match of a prey's appearance to the coloration of a given environment (Endler, 1978).
A locally cryptic distribution of heritable color camouflage is strongly suggestive of heterogeneous natural selection via visually cued predation. There is both empirical (Hargeby et al., 2004; Sandoval & Nosil, 2005; Nosil & Crespi, 2006; Vignieri et al., 2010) and theoretical (Nilsson & Ripa, 2010) support for predation as a selective agent driving intraspecific polymorphic color camouflage. The degree of phenotypic divergence in color camouflage is related directly to the level of gene flow between habitats, with higher levels of gene flow requiring stronger selection to maintain color divergence (Mullen & Hoekstra, 2008; Nosil, 2009; Rosenblum & Harmon, 2011). Habitat-specific selection on color crypsis can be very strong, especially where driven by visually cued avian predators (Kettlewell, 1958; Clarke & Murray, 1962; Muggleton, 1978; Vignieri et al., 2010). In plants, camouflage of conspicuous structures, such as bracts (Klooster et al., 2009), trichomes (Weins, 1978; Lev-Yadun, 2006) and vegetative surfaces (Weins, 1978; Ellis et al., 2006), may allow plants to match the color of background substrates to reduce detection by herbivorous natural enemies. However, few studies have examined the natural microevolutionary origins of population divergence in seed coloration as a form of camouflage, despite the prominent role played by variation in seed color in evolutionary and agricultural genetics (Mendel, 1866; Winkel-Shirley, 2001; Armstead et al., 2007).
Seed predators are an important class of natural enemies of plants that can consume a large proportion of a plant's reproductive output (Janzen, 1971b). Post-dispersal seed predation can drastically reduce seed survival (Maron & Simms, 1997, 2001; Kauffman & Maron, 2006; Bricker et al., 2010). Many plant communities are seed limited and so this predation can exert a strong influence on population demography (Turnbull et al., 2000; Bricker et al., 2010). For example, seed predators can have a stronger effect than catastrophic fire on post-dispersal seed survival and regeneration in forests (Zwolak et al., 2010). A variety of seed defenses have evolved in response to selection as a result of seed predation, including toxins for chemical defenses (Janzen, 1969), protective tissues for mechanical defenses (Elliot, 1974), masting to satiate predators (Janzen, 1971a) and cryptic coloration for visual concealment (Cook et al., 1971). Despite the ecological impacts of seed predators and evolutionary impacts on seed defense traits, we know surprisingly little about how variation in the selection imposed by seed predators may affect the local adaptation of seed defenses.
Visually cued seed predators, such as birds (Marone et al., 2008), are important agents of selection on seed camouflage as they feed more rapidly and persistently upon less cryptic seeds (Jones et al., 2006). For example, coastal populations of dove weed experience high seed predation by doves and are variable in coloration and well camouflaged, whereas desert populations, where doves do not occur, have monomorphic, less well-camouflaged seeds (Cook et al., 1971; Cook, 1972). The seeds of some pines are polymorphic in color and seed colors may offer differential fitness in the presence of birds post-fire against a mosaic background of ash and exposed soil (Nystrand & Granstrom, 1997; Saracino et al., 2004). Background color matching has therefore been demonstrated to affect seed fitness in nature, supporting the selective advantage of seed color crypsis in the presence of avian seed predators. However, locally adaptive population divergence for cryptic seed color camouflage has not been demonstrated in plants.
Large-seeded plants, such as legumes, are particularly vulnerable to post-dispersal seed predators (Maron & Crone, 2006), and may therefore exhibit strong evolutionary responses to seed predators. Acmispon wrangelianus is a native annual legume inhabiting open grassland environments throughout California. Seeds develop in leguminous pods, which dehisce explosively. Seeds disperse onto the soil surface within meters of the maternal plant (Lau et al., 2008), which leaves them spatially clumped and exposed on the soil surface, putting them at high risk of predation (Brown, 1975; Mittelbach, 1984). Acmispon wrangelianus displays polymorphic seed color, with lineages bearing seeds of different colors, ranging from shades of gray to brown. Acmispon wrangelianus occurs on a variety of moderately disturbed or open soil environments, from gray–green, physiologically harsh serpentine soils to brown, more fertile nonserpentine soils. Outcrops of serpentine soils are commonly embedded in a matrix of various nonserpentine soils; however, neither soil condition is homogeneous in chemistry or color. The selective environment of this mosaic of soils is highly complex (Baythavong et al., 2009). We know little about the capacity of selection to generate fine-scale genetic differentiation within soil types in these open patchy environments, where gene flow may counter the effects of selection, or how the strength of adaptive differentiation could differ across conditions. Vegetation on the physiologically stressful serpentine soil is sparser than on nonserpentine soils at the McLaughlin Reserve (Harrison, 1999; Harrison et al., 2003), and open habitats are often seed limited and subject to higher effects from seed predators (Maron & Simms, 1997; Maron & Crone, 2006; Maron & Kauffman, 2006; Denham, 2008), which could drive closer color camouflage on serpentine soils. Alternatively, if harsh serpentine soils support smaller populations, low serpentine effective population sizes and swamping gene flow from nonserpentine populations could result in weaker color camouflage on serpentine soils (Kawecki, 1995; Leimu & Fischer, 2008).
As a prerequisite to the investigation of locally adaptive A. wrangelianus seed color camouflage, I first verified that there was significant color variation among the seeds and soils from different collection sites. To determine whether the spatial distribution of color morphs across soil environments coincides with the pattern of adaptive divergence expected to result from selection by visually cued seed predators, I examined the following questions. (1) Ecotypic color camouflage. Do lineages bear seeds with a closer color match to their native serpentine or nonserpentine soil type (excluding the native site) than to the opposing soil type? Do serpentine and nonserpentine soil populations differ in the accuracy of this color match? (2) Localized color camouflage. Do lineages bear seeds with a closer color match to the soil at their native site than at other sites of the same serpentine or nonserpentine soil type? Do serpentine and nonserpentine soil populations differ in the accuracy of this color match?
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Acmispon wrangelianus exhibits intraspecific genetic divergence, whereby lineages bear seeds with a striking concordance to the native soil color. This suggests that natural selection for locally camouflaged color morphs maintains adaptive divergence in pigmentation despite the relatively short distances (c. 0.5–5 km) separating the sites in this study. The divergent distribution of color morphs across soil environments coincides with the pattern expected to result from selection caused by visually cued seed predators. Selection against mismatched color morphs may counter gene flow and hence promote further adaptive divergence, despite the opportunity for migration between soils (Nosil et al., 2008).
Populations native to serpentine and nonserpentine soils exhibit soil type-specific differentiation in seed coloration. Overall, serpentine seeds more closely match soils from sites on serpentine than nonserpentine soil, and nonserpentine seeds more closely match soils from sites on nonserpentine than serpentine soil. The evolution of polymorphic crypsis can favor reproductive isolation and evolutionary divergence between populations originating in different habitat types. Assuming that seed predation would be stronger against seeds with a greater color mismatch, predation could impose stronger selection against migrants between populations of different soil types than migrants between populations of the same soil type. Selection against less cryptic immigrants can reduce gene flow between populations of contrasting habitat type, maintaining genetic isolation by adaptation (Bolnick & Nosil, 2007; Nosil et al., 2008).
Although color matching occurs at the level of serpentine and nonserpentine soil types, it also occurs at the level of local, subtler differences in soil color within a soil type. Seeds more closely match soil color from the native site than the soil color of other sites of the same soil type. Seed–soil color matching thus appears to have evolved in response to localized shifts in soil color beyond the general distinctions between the gray serpentine or brown nonserpentine soil. Seed–soil color matching at this fine scale suggests that locally cryptic seed color camouflage may be a widespread phenomenon driven by mosaics of selection on subtle aspects of seed pigmentation. This site-specific matching suggests that locally cryptic seed coloration may not be restricted to the unusually strong contrast in soil colors across the boundaries of serpentine soil outcroppings, and may be widespread in plants on a variety of soil chemistry environments.
Selection against mismatched color morphs inhabiting contrasting environments has often been found to be asymmetric (Hoekstra, 2004, Kettlewell, 1958), which could drive closer color matching in one environment than another. However, seeds from serpentine and nonserpentine soils do not appear to differ in the strength of ecotypic color camouflage. Habitat-associated differences in rates of predation may not result in differences in the strength or efficacy of selection on color camouflage if the relative fitness effects of predation are similar across habitats. For example, avian predators of rodents impose different rates of predation between habitats; however, in terms of relative fitness, it is equally disadvantageous for prey to be mismatched in either habitat (Vignieri et al., 2010). In addition, the impact of potentially greater predation rates in the sparsely vegetated serpentine habitat could be counterbalanced by gene flow from larger populations on nonserpentine soil into smaller serpentine populations.
Overall, seeds tend to more closely match the soil of their native site as opposed to other sites of the same soil type. However, populations of seeds from some sites are more closely matched to the color of their native soil type and site than others and, overall, populations from serpentine sites are more closely matched to the native site within this soil type than are nonserpentine populations. Site-to-site variation in the strength of color camouflage could reflect mosaics in the efficacy of selection exerted by seed predators. Future work could disentangle how ecological factors, such as patch size, gene flow, distance to contrasting soil color and predation rates, correlate with the accuracy of a population's color match, to suggest which factors have strong effects on the efficacy of selection for localized color camouflage.
Dormancy in seed banks can dampen the ecological effects of seed predation by buffering plant populations from predator-driven fluctuations in recruitment (Maron & Crone, 2006). However, seed banks may preserve the strong localized evolutionary effects of seed predation by providing a cumulative memory of its effects. The finding of locally cryptic seed color camouflage provides evidence suggesting that post-dispersal seed predators are an important agent of selection affecting the lifetime fitness of plants (Turnbull et al., 2000). Seed predation may be an especially effective agent of selection in annual seed-banked species, such as A. wrangelianus, because individuals often spend a much greater proportion of their lives as seeds than as vegetative plants. The cumulative risk of predation during the life history phase in the seed bank may be high, even if bouts of predation are infrequent. In addition, seed predation is a form of herbivory with maximal fitness costs to the dormant plant: if a seed is consumed, an individual is very likely to die outright, rather than experience some degree of reduction in fitness.
Seed coloration may be a relatively inexpensive form of defense that can respond quickly to selection. Seed color in legumes can be conferred by a handful of well-defined genes that regulate flavonoid and anthocyanin biosynthetic pathways in the maternal tissues during seed coat development (Vandenberg & Slinkard, 1990; McClean et al., 2002). Relatively few genes of major effect underlie patterns of locally adaptive color camouflage polymorphisms in other locally cryptic organisms, such as some vertebrates (Hoekstra, 2006; Rosenblum & Harmon, 2011). The potential for both strong selection and a rapid response to selection may explain how this genetic differentiation can occur on such a fine scale, despite the potential for swamping caused by gene flow. Future work should investigate whether color crypsis has evolved in the presence of strong or weak levels of gene flow between patches, which could allow an estimation of the strength and target of selection that could establish the observed patterns (Gray & McKinnon, 2007).
Avian seed predation probably contributes to selection for local seed color camouflage, as birds have been shown to preferentially consume seeds most divergent from the color of the substrate under controlled and field conditions (Nystrand & Granstrom, 1997; Saracino et al., 2004; Jones et al., 2006) and usually have well-developed color vision (Endler & Mielke, 2005). However, other seed-eating animals present at the McLaughlin Reserve with the potential to perceive color, such as harvester ants or even rodents (Kretz, 1979; Jacobs et al., 2001, 2004; Cammaerts & Cammaerts, 2009; Aksoy & Camlitepe, 2012), cannot be ruled out as potential sources of selection, especially given the high densities of some species at the study sites. The determination of the particular visually cued predators driving selection for color camouflage, the spatial and temporal patterns of seed consumption, and the selective advantage for locally matched seed color will be important steps towards the documentation of natural selection on seed color camouflage and the development of robust perceptual models of color discrimination (e.g. Spottiswoode & Stevens, 2012) for appropriate predators under ecologically relevant illumination (e.g. Chiao et al., 2000). In the developing field of seed defense theory, a correlate of plant defense theory specific to seeds and their antagonists (Dalling et al., 2011), seed camouflage may represent an important axis of seed defense.