RESUMEN
Several models of hybrid zone evolution predict the same spatial patterns of genotypic distribution whether or not structuring is due to environment-dependent or -independent selection. In this study, we tested for evidence of environment-dependent selection in an Iris fulva x Iris brevicaulis hybrid population by examining the distribution of genotypes in relation to environmental gradients. We selected 201 Louisiana Iris plants from within a known hybrid population (80 m x 80 m) and placed them in four different genotypic classes (I. fulva, I. fulva-like hybrid, I. brevicaulis-like hybrid and I. brevicaulis) based on seven species-specific random amplified polymorphic DNA (RAPD) markers and two chloroplast DNA haplotypes. Environmental variables were then measured. These variables included percentage cover by tree canopy, elevation from the high water mark, soil pH and percentage soil organic matter. Each variable was sampled for all 201 plants. Canonical discriminant analysis (CDA) was used to infer the environmental factors most strongly associated with the different genotypic groups. Slight differences in elevation (-0.5 m to +0.4 m) were important for distinguishing habitat distributions described by CDA, even though there were no statistical differences between mean elevations alone. I. brevicaulis occurred in a broad range of habitats, while I. fulva had a narrower distribution. Of all the possible combinations, I. fulva-like hybrids and I. brevicaulis-like hybrids occurred in the most distinct habitat types relative to one another. Each hybrid class was not significantly different from its closest parent with regard to habitat occupied, but was statistically unique from its more distant parental species. Within the hybrid genotypes, most, but not all, RAPD loci were individually correlated with environmental variables. This study suggests that, at a very fine spatial scale, environment-dependent selection contributed to the genetic structuring of this hybrid zone.
Asunto(s)
Quimera/genética , Ambiente , Plantas/genética , Altitud , Quimera/fisiología , ADN de Cloroplastos/química , ADN de Cloroplastos/aislamiento & purificación , Concentración de Iones de Hidrógeno , Louisiana , Análisis Multivariante , Desarrollo de la Planta , Hojas de la Planta/química , Hojas de la Planta/genética , Hojas de la Planta/fisiología , Reacción en Cadena de la Polimerasa , Técnica del ADN Polimorfo Amplificado Aleatorio , Selección Genética , Suelo , Especificidad de la Especie , ÁrbolesRESUMEN
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 cross-host 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 F1 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 F1 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.