Knowledge of mitochondrial gene evolution in angiosperms has taken a dramatic shift within the past decade, from universal slow rates of nucleotide change to a growing realization of high variation in rates among lineages. Additionally, evidence of paternal inheritance of plant mitochondria and recombination among mitochondrial genomes within heteroplasmic individuals has led to speculation about the potential for independent evolution of organellar genes. We report intraspecific mitochondrial and chloroplast sequence variation in a cosmopolitan sample of 42 Silene vulgaris individuals. There was remarkably high variation in two mitochondrial genes (atp1, atp9) and additional variation within a third gene (cob). Tests for patterns of nonneutral evolution were significant for atp1 and atp9, indicative of the maintenance of balanced polymorphisms. Two chloroplast genes (matK, ndhF) possessed less, but still high, variation and no divergence from neutral expectations. Phylogenetic patterns of organelle genes in both the chloroplast and mitochondria were incongruent, indicating the potential for independent evolutionary trajectories. Evidence indicated reassociation among cytoplasmic genomes and recombination between mitochondrial genes and within atp1, implying transient heteroplasmy in ancestral lineages. Although the mechanisms for long-term maintenance of mitochondrial polymorphism are currently unknown, frequency-dependent selection on linked cytoplasmic male sterility genes is a potential candidate.

Molecular studies of apomictic plant species often detect more genotypic variation than predicted from their assumed reproductive mode. The two most commonly invoked mechanisms to explain these high levels of variation are recombination, via facultative sexuality, and mutation. The potential for sexual reproduction in the facultative apomict Hieracium pilosella (Asteraceae) was determined at three field sites by artificially pollinating with the closely related, but morphologically distinct, H. aurantiacum. The level of genotypic variation at the three sites was recorded using inter-simple sequence repeats (ISSRs). There was a significant, positive relationship between the measured potential for sexual reproduction and population genotypic variability, indicating that sex has played a role in the structuring of these populations; however, a causal relationship cannot be stated because of the use of regression. We also applied the recently developed method of compatibility analysis. Compatibility analysis can determine, using the occurrence of "character incompatibilities," whether patterns of variation observed in populations are most parsimoniously explained by mutation or recombination. Compatibility analysis also indicated that sexual reproduction had played a role in generating genotypic diversity in these populations. Combining these different types of data may give a greater understanding of the potential for the generation of genotypic diversity in facultative apomictic populations.

Stigma colonization by Erwinia amylovora is the crucial first step in the development of most fire blight infections in apple and pear trees. Suppression at this point of the disease process by antagonists of E. amylovora, such as Pantoea agglomerans (Erwinia herbicola) strain Eh1087, is a rational approach to control fire blight. We tested the hypothesis that the ability of E. amylovora to compete with Eh1087 for colonization of a stigma is reduced by the potential for Eh1087 to produce the phenazine antibiotic, d-alanylgriseoluteic acid (AGA). In competition experiments on the stigmas of apple flowers, E. amylovora was significantly less successful against Eh1087 (AGA+) than against EhDeltaAGA (AGA-). Further experiments to test the importance of pre-emptive colonization of the stigma by either the pathogen or the antagonist suggested that AGA production significantly enhanced the competitiveness of Eh1087 when it was applied at the same time or 24 h before the pathogen. We also found that pre-emptive stigma colonization by either the pathogen or the antagonist resulted in a population that was resilient to subsequent invasion by a second species suggesting that niche exclusion has a dominant influence on the dynamics of bacterial populations on stigmas.

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