Reproductive interference and fecundity affect competitive interactions of sibling species with low mating barriers: experimental and theoretical evidence.

When allopatric species with incomplete prezygotic isolation come into secondary contact, the outcome of their interaction is not easily predicted. The parasitoid wasp Encarsia suzannae (iES), infected by Cardinium inducing cytoplasmic incompatibility (CI), and its sibling species E. gennaroi (EG), not infected by bacterial endosymbionts, may have diverged because of the complementary action of CI and asymmetric hybrid incompatibilities. Whereas postzygotic isolation is now complete because of sterility of F1 hybrid progeny, prezygotic isolation is still incipient. We set up laboratory population cage experiments to evaluate the outcome of the interaction between ES and EG in two pairwise combinations: iES vs EG and cured ES (cES, where Cardinium was removed with antibiotics) vs EG. We also built a theoretical model aimed at exploring the role of life-history differences and asymmetric mating on competitive outcomes. In three of four cages in each treatment, ES dominated the interaction. We found evidence for reproductive interference, driven by asymmetric mating preferences, that gave a competitive edge to ES, the species that better discriminated against heterospecifics. However, we did not find the fecundity cost previously shown to be associated with Cardinium infection in iES. The model largely supported the experimental results. The finding of only a slight competitive edge of ES over EG in population cages suggests that in a more heterogeneous environment the species could coexist. This is supported by evidence that the two species coexist in sympatry, where preliminary data suggest reproductive character displacement may have reinforced postzygotic isolation.

The Pseudomonas savastanoi tryptophan-2-mono-oxygenase is biologically active in Nicotiana tabacum.

It has been proposed that the "eukaryotic" T-DNA-encoded indole-3-acetic acid (IAA) biosynthesis genes of Agrobacterium tumefaciens and their prokaryotic counterpart in Pseudomonas savastanoi originated from common ancestor genes. This paper provides additional evidence for the functional similarity between the gene products. We have demonstrated that a chimeric gene consisting of the coding sequence of the P. savastanoi tryptophan-2-mono-oxygenase (iaaM gene) and a plant promoter encodes an active enzyme in Nicotiana tabacum. Transformants obtained with this chimeric gene grew as a callus on hormone-free media. No stably transformed plantlets could be isolated. The callus tissues contained extremely high levels of indole-3-acetamide and slightly elevated levels of IAA. Either indole-3-acetamide by itself has a low auxin activity or, alternatively, it is converted aspecifically and at low rates into IAA. The P. savastanoi tryptophan-2-mono-oxygenase activity in plants is also able to detoxify the amino-acid analogue 5-methyltryptophan. This property can be used for positive selection of transformed calli.

Isolation of a dual plant promoter fragment from the Ti plasmid of Agrobacterium tumefaciens.

The two most abundant transcripts derived from TR-DNA within plant cells transformed by an octopine strain of Agrobacterium tumefaciens arise from divergent transcription, both originating within an 500 bp section of the T-DNA. Using a combination of subcloning and exonuclease digestion, a 479-bp DNA fragment, directly flanked by the initiation codons for the two adjacent open reading frames, was isolated. The resulting DNA fragment was fused, in both orientations, to the neomycin phosphotransferase (NPT II) gene of the transposon Tn5 prior to introduction into Nicotiana tabacum cells via the Ti plasmid. The intergenic fragment was found to initiate expression of the NPT II gene in either orientation as assayed by kanamycin resistance of the transformed plant tissue as well as by enzymatic assay of the NPT II gene product. The plasmids described here are potential selection-expression vectors for plant systems.