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Seed predator/pollinator and host plant interactions, which may be considered as antagonistic, have the potential to provide good model systems for the study of the early stages of evolution towards mutualism. I describe a relationship between a seed predator, the geometrid moth Perizoma affinitatum, and the dioecious plant Silene dioica. The moth is an obligate seed predator on its host plant. The searching and ovipositing behaviour of the female moths, number of eggs deposited per flower, the pollinating ability of the moths and the seed consumption by the larvae are described as different parameters and studied in two Finnish coastal populations. A high pollinating ability and limited seed consumption by the predator was found and discussed in relation to fitness models of P. affinitatum and S. dioica. In a mutualistic relationship there must be a balance between the costs and benefits so that the seed production by the moths is larger than the seed consumption by the larvae, given a net seed output larger than zero. The data of the parameters included in a seed production/consumption model give a positive seed output when the proportion of S. dioica flowers pollinated by other non-predating insects is less than 60%. Accordingly, even if P. affinitatum would become the exclusive pollinator it would not endanger the survival of the host plant and both partners would benefit from this interaction. Limited seed consumption, high pollinating ability and host specificity as seen in the P. affinitatum and S. dioica interaction are considered to have been important pre-existing qualities in the evolution of the obligate mutualisms between yucca and yucca moths and fig and fig wasps. In isolated serpentine populations where the gene flow is restricted and co-pollinators are rare the interaction between P. affinitatum and S. dioica has the potential to shift from parasitism to mutualism.

The mutualisms between fig trees and their pollinator fig wasps and between yucca plants and yucca moths are spectacular examples of coevolution. The characteristics of these independently evolved mutualisms have resulted from long-term processes, the first stages of which are unknown. A fundamental question in the study of mutualism is how these interactions evolve. Seed predator/pollinator and host plant interactions, which may initially be considered as mainly antagonistic, have the potential to provide good model systems for the study of the first stages of evolution towards mutualism. We present here theoretical models assessing the consequences of interactions between specialized seed predator insects and their host plants. These models describe the parameters that affect the fitness of an individual female seed predator and her influence on the fitness of the host plant. In an optimal strategy for the seed predator, the number of eggs laid in each flower depends on the interaction between the adult and larva survival. Along with a growing predation pressure on adults and larvae several eggs must be laid in each flower by the female seed predator to enhance her fitness. However, in a situation where the host plant selectively aborts flowers with a high number of eggs the fitness of the seed predator will seriously decrease. If the cost of selective abortion is less than the cost of seed predation the host plant will maintain fitness. In a mutualistic relationship a balance between the cost and the benefit of the parameters in the fitness models of the seed predator and the host plant has to occur so that the net seed output is larger than zero (0). Any unselfish behaviour or quality of the seed predator that would benefit the host plant in such a way that the net seed output increases might be a first stage in an interaction becoming mutualistic. The models presented here will not only provide a platform for empirical studies on interactions that may swing from parasitism to mutualism, but also for seed predator/pollinator and host plant interactions in general.

Silene dioica (Caryophyllaceae) has evolved different morphotypes that vary in hairiness in different habitats in the Swedish mountains. A hairy form, var. serpentinicola and a hairless (glabrous) form, var. smithii grow on serpentine, i.e. a dry soil rich in heavy metals. Var. lapponica is a densely hairy morphotype on subalpine meadows. Crosses between these varieties showed that glabrousness has a simple Mendelian inheritance and that glabrousness is determined by a recessive allele. The crosses further suggested that the recessive allele has been present in the original population that invaded the mountain area. Glabrous plants have, however, never been found on subalpine meadows, even though an earlier study has shown that gene flow is relatively common among serpentine and nearby meadow populations. In addition to the genetic background of glabrousness, we studied the grazing on glabrous smithii and hairy serpentinicola and lapponica plants by the slugs Arion fasciatus and Arion subfuscus and the snail Arianta arbustorum in greenhouse and in nature. Arion slugs, abundant on subalpine meadows, preferred glabrous plants and, in particular, A. fasciatus consumed high amounts of S. dioica. Arianta snails, however, consumed irrespective of morphotype. The selective behaviour was seen both at the seedling stage and the adult plant stage. Arion slugs were more choosy when they were given a choice of glabrous smithii and densely hairy lapponica plants then when they were served smithii and the less hairy variety serpentinicola. This study implies that the Arion slugs are important agents of selection on subalpine meadows. The selective grazing seems to counteract the flow of the recessive allele that determines glabrousness, from serpentine into meadow populations. Gastropods are rare in the dry serpentine habitat. The evolution of a distinct glabrous morphotype on serpentine is therefore rather due to relaxation of selection than to adaptation to serpentine stress.

Silene dioica (L.) Clairv. (Caryophyllaceae) is an insect pollinated and obligately cross-fertilising herb commonly found on serpentine, i.e. soils rich in heavy metals, and on cultivated meadows. Groups of Silene populations were studied in two areas in the Scandinavian mountains. In a northern area Silene populations grow in an open pine and birch forest while in the south, populations are surrounded by a continuous and dense spruce forest. Gene flow, estimated on the basis of allele frequencies, is highly restricted in the south in comparison to the north. This has led to an extensive genetic differentiation among populations. The pollinator guilds (Thricops flies, syrphid flies and bumblebees) are similar in the northern and southern populations and can therefore not account for the difference in gene flow. The flowering in nearby populations overlaps broadly and is also not the reason for the genetic differentiation in the south. The differentiation is more likely due to vegetation that limits pollinator movement. The level of gene flow differs in the two habitats both in the north and in the south. The gene flow is restricted among serpentine populations but common among meadow populations. Pollen dispersal, and in particular, anthropogenic seed dispersal mediate gene flow among meadows. Serpentine populations are unaffected by human activity. Pollinators are therefore the main agent of gene flow on serpentine. This study shows the importance of the surrounding vegetation in influencing gene flow patterns among populations as well as that habitat fragmentation affects the genetic properties of plant populations. In addition, we have also observed a restricted gene flow within populations. Enzyme allele frequencies show an excess of homozygotes within most of the populations. This can be explained through limited pollen dispersal and differences in male and female flowering density.