This is an exciting time to study how different non-model species use common developmental genes to create such diversity in the plant world. In many cases researchers tend to identify, within newly available sequences, genes that putatively encode proteins similar to those genetically characterized in model species. A great toolbox of methods to study gene expression now exists, most of which can be outsourced: so, what is left is for researchers to choose and obtain the right samples and look at the correct genes. Samach focuses on the latter task and reminds us that it is not a trivial one, and that negligence can lead to publication of incorrect interpretations. A considered response is given by Muñoz-Fambuenaet al., whose paper (Annals of Botany 108: 511–519) is highlighted in Samach’s critique.
Most of the edible Citrus species are believed to be of interspecific origin as a consequence of reticulate evolution. Garcia-Lor et al. conduct phylogenetic studies at intergeneric level (Citrinae, Rutaceae) and within the Citrus genus based on targeted gene nuclear DNA sequences, SSR and indel nuclear markers. They clarify the phylogenetic relationships between the ancestral ‘true citrus fruit trees’ species and find that they appear consistent with their original geographic distribution. In addition, the phylogenetic origin of the genes analysed is inferred for the cultivated Citrus species resulting from interspecific hybridization, giving new insights in particular into the origin of C. sinensis.
Previous research has demonstrated that fruit load acts as a strong inhibitor of flowering in many fruit-tree species, including Citrus. Muñoz-Fambuena et al. analyse the expression of flowering-related genes at the meristem level in alternate-bearing citrus trees, and the interplay between buds and leaves in the determination of flowering. The results suggest that the presence of fruit affects flowering by greatly altering gene expression, not only at the leaf but also at the meristem level. Leaves are required for citrus flowering to occur, and their absence strongly affects the activity of floral promoters and identity genes.
The gene pool of Citrus and related genera of Aurantioideae is essentially diploid but tetraploid plants are frequently encountered in seedlings of diploid apomictic genotypes. Aleza et al. demonstrate that tetraploidization by chromosome doubling of nucellar cells is a frequent event in apomictic citrus, and is affected by genotypic and environmental factors, with colder conditions in marginal climatic areas appearing to enhance tetraploidization rates. Factors limiting the evolutionary role of polyploidization in citrus are probably related to problems of establishment and persistence of new polyploids.