Sympatric races of pea aphids on alfalfa and red clover are highly ecologically specialized and significantly reproductively isolated. Much of the restriction of gene flow between the specialized populations is due to habitat choice behavior of the winged colonizers (Via 1999). Here, we document additional pre-and postmating reproductive isolation through selection against migrants and hybrids in the parental environments. First, a group of randomly chosen genotypes from each race that were experimentally migrated between hosts had very low survival and reproduction on the alternate host relative to genotypes originating from that host (natives). Such selection against crosshost migrants forms a premating barrier to gene flow because it is likely to reduce migrant frequencies before the sexual forms are induced in the fall. Our reciprocal transplant experiment also shows that natural selection acts directly on individual migrants between the crops to favor host choice behavior: genotypes from each host suffered large losses of fitness when forced to migrate to the alternate host plant relative to the fitness they would have enjoyed had they been able to choose their native host. In a companion field study, sequential sampling throughout the summer in newly colonized fields of both alfalfa and clover revealed a decrease in the frequency of host-specific marker alleles characteristic of the alternate crop. These field data further support the hypothesis that selection disfavors migrants that cross between crops. Second, when two sets of F 1 hybrids between the races were reciprocally tested on alfalfa and clover, both sets had significantly lower average fitness than the specialized parent in each of the two environments. This demographic selection against hybrids in the parental environments is a source of postmating reproductive isolation between the specialized races. Finally, significant genetic variation in fitness traits was seen among F 1 hybrid genotypes from both crosses between alfalfa and clover specialists. Although this variation suggests that a generalized pea aphid could evolve, such generalists are not seen in field collections of these populations.

Genomics and bioinformatics have great potential to help address numerous topics in ecology and evolution. Expressed sequence tags (ESTs) can bridge genomics and molecular ecology because they can provide a means of accessing the gene space of almost any organism. We review how ESTs have been used in molecular ecology research in the last several years by providing sequence data for the design of molecular markers, genome-wide studies of gene expression and selection, the identification of candidate genes underlying adaptation, and the basis for studies of gene family and genome evolution. Given the tremendous recent advances in inexpensive sequencing technologies, we predict that molecular ecologists will increasingly be developing and using EST collections in the years to come. With this in mind, we close our review by discussing aspects of EST resource development of particular relevance for molecular ecologists.

The idea that natural hybridization has served as an important force in evolutionary and adaptive diversification has gained considerable momentum in recent years. By combining genome analyses with a highly selective field experiment, we provide evidence for adaptive trait introgression between two naturally hybridizing Louisiana Iris species, flood-tolerant Iris fulva and dry-adapted I. brevicaulis. We planted reciprocal backcross (BC 1 ) hybrids along with pure-species plants into natural settings that, due to a flooding event, favored I. fulva. As expected, I. fulva plants survived at much higher rates than I. brevicaulis plants. Backcross hybrids toward I. fulva (BCIF) also survived at significantly higher rates than the reciprocal backcross toward I. brevicaulis (BCIB). Survivorship of BCIB hybrids was strongly influenced by the presence of a number of introgressed I. fulva alleles located throughout the genome, while survivorship in the reciprocal BCIF hybrids was heavily influenced by two epistatically acting QTL of opposite effects. These results demonstrate the potential for adaptive trait introgression between these two species and may help to explain patterns of genetic variation observed in naturally occurring hybrid zones.

Abstract. Sympatric races of pea aphids on alfalfa and red clover are highly ecologically specialized and significantly reproductively isolated. Much of the restriction of gene flow between the specialized populations is due to habitat choice behavior of the winged colonizers (Via 1999). Here, we document additional pre-and postmating reproductive isolation through selection against migrants and hybrids in the parental environments. First, a group of randomly chosen genotypes from each race that were experimentally migrated between hosts had very low survival and reproduction on the alternate host relative to genotypes originating from that host (natives). Such selection against crosshost migrants forms a premating barrier to gene flow because it is likely to reduce migrant frequencies before the sexual forms are induced in the fall. Our reciprocal transplant experiment also shows that natural selection acts directly on individual migrants between the crops to favor host choice behavior: genotypes from each host suffered large losses of fitness when forced to migrate to the alternate host plant relative to the fitness they would have enjoyed had they been able to choose their native host. In a companion field study, sequential sampling throughout the summer in newly colonized fields of both alfalfa and clover revealed a decrease in the frequency of host-specific marker alleles characteristic of the alternate crop. These field data further support the hypothesis that selection disfavors migrants that cross between crops. Second, when two sets of F 1 hybrids between the races were reciprocally tested on alfalfa and clover, both sets had significantly lower average fitness than the specialized parent in each of the two environments. This demographic selection against hybrids in the parental environments is a source of postmating reproductive isolation between the specialized races. Finally, significant genetic variation in fitness traits was seen among F 1 hybrid genotypes from both crosses between alfalfa and clover specialists. Although this variation suggests that a generalized pea aphid could evolve, such generalists are not seen in field collections of these populations.

Summary Arnold, M. L., Kentner, E. K., Johnston, J. A., Cornman, S. & Bouck, A. C.: Natural hybridisation and fitness. – Taxon 50: 93–104. 2001. – ISSN 0040‐0262. There are several inferences that can be made from studies of the fitness of hybrid plants and animals. First, the fitness of hybrids varies. However, it is not possible to make a priori predictions concerning the relative fitness of a given hybrid genotype, or a series of genotypes. For example, plant hybrids are not more likely to have elevated fitness than animal hybrids. The variation in fitness for hybrids ranges from the highest fitness relative to parental genotypes to the lowest. The variation can be due to different environments or age classes. Furthermore, notwithstanding the paradigm that derived from the Neo‐Darwinian synthesis, some hybrid genotypes demonstrate elevated fitness relative to their parents. To test the evolutionary importance of hybridisation in a given species complex, it is critical that findings from greenhouse/population cage experiments be tested in the field. This latter statement is easier for a botanist to argue, but even with plants, the field experimentation needed for a rigorous test of fitness (i.e., reciprocal transplants) is at the best risky, and costly in time and resources. Yet, these are the types of studies needed to test predictions concerning the likeliest circumstances under which hybridisation will be promoted. The scientific literature over the past decade has seen a change in the tenor of papers describing natural hybridisation. Ten years ago, papers more often than not reported the use of natural hybridisation as a tool to understand divergent evolution. More recently, a majority of studies have discussed the evolutionary impact of natural hybridisation. We look forward to a continued increase in the frequency of studies that assume this process to have evolutionary importance per se.