Summary1. Organisms with low mobility, living within ephemeral environments, need to find vehicles that can disperse them reliably to new environments. The requirement for specificity in this passenger-vehicle relationship is enhanced within a tritrophic interaction when the environment of passenger and vehicle is provided by a third organism. Such relationships pose many interesting questions about specificity within a tritrophic framework. 2. Central to understanding how these tritrophic systems have evolved, is knowing how they function now. Determining the proximal cues and sensory modalities used by passengers to find vehicles and to discriminate between reliable and non-reliable vehicles is, therefore, essential to this investigation.

Hybrids occasionally exhibit genetic interactions resulting in reduced fitness in comparison to their parents. Studies of Arabidopsis thaliana have highlighted the role of immune conflicts, but less is known about the role of other factors in hybrid incompatibility in plants. Here, we present a new hybrid incompatibility phenomenon in this species. We have characterized a new case of F hybrid incompatibility from a cross between the A. thaliana accessions Krotzenburg-0 (Kro-0) and BG-5, by conducting transcript, metabolite and hormone analyses, and identified the causal loci through genetic mapping. The F hybrids showed arrested growth of the main stem, altered shoot architecture, and altered concentrations of hormones in comparison to parents. The F phenotype could be rescued in a developmental-stage-dependent manner by shifting to a higher growth temperature. These F phenotypes were linked to two loci, one on chromosome 2 and one on chromosome 3. The F generation segregated plants with more severe phenotypes which were linked to the same loci as those in the F . This study provides novel insights into how previously unknown mechanisms controlling shoot branching and stem growth can result in hybrid incompatibility.

Temperature is a major determinant of plant growth, development and success. Understanding how plants respond to temperature is particularly relevant in a warming climate. Plant immune responses are often suppressed above species-specific critical temperatures. This is also true for intraspecific hybrids of Arabidopsis thaliana that express hybrid necrosis due to inappropriate activation of the immune system caused by epistatic interactions between alleles from different genomes. The relationship between temperature and defence is unclear, largely due to a lack of studies that assess immune activation over a wide range of temperatures. To test whether the temperature-based suppression of ectopic immune activation in hybrids exhibits a linear or non-linear relationship, we characterised the molecular and morphological phenotypes of two different necrotic A. thaliana hybrids over a range of ecologically relevant temperatures. We found both linear and non-linear responses for expression of immunity markers and for morphological defects depending on the underlying genetic cause. This suggests that the influence of temperature on the trade-off between immunity and growth depends on the specific defence components involved.

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