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One-way traffic
Although some closely related species are inter-fertile, there are also many instances where specific incompatibility mechanisms have evolved to prevent inter-specific hybridizations. These are often a source of great frustration to plant breeders who may wish to introduce into a crop species a trait from a closely related wild species. However, those plant breeders will also know of situations in which attempted hybridizations work in one direction (pollen A to stigma B) but not in the other (pollen B to stigma A). Such unilateral incompatibility is particularly common in the Solanaceae, as described by Naci Onus and Barbara Pickersgill (University of Reading, UK, pp. 289–295). They have studied the phenomenon in Capsicum, a genus that is evolutionarily distant from other solanaceous genera that have been studied. The authors used all the readily available wild and cultivated Capsicum species and carried out all possible crosses in both directions. The results are very clear-cut and very interesting. All crosses were compatible (as indicated by pollen tube growth) except when species in the C. pubescens complex were pollinated by species outside the complex. The C. pubescens complex contains the only species in the genus that are self-incompatible (SI) and the authors thus state that unilateral incompatibility in Capsicum conforms to the ‘classic’ pattern in which pistils of SI species, or of self-compatible (SC) species closely related to SI species, inhibit the growth of pollen from more distantly related SC species, but not the other way round. The authors point out that the data raise further very interesting questions. In evolutionary terms, is this just a 'snapshot' in the divergence of the C. pubescens complex from the rest of the genus? And what exactly are the recognition mechanisms that permit this traffic of genes in one direction only? Clearly there is a lot of interesting research still to come. Professor J. A. Bryant, University of Exeter, UKj.a.bryant{at}exeter.ac.uk
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