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Self-incompatibility in flowering plants. Evolution, diversity, and mechanisms.
V Franklin-Tong. ed. 2008.
Berlin, Heidelberg: Springer-Verlag. $219 (hardback). 314 pp.
A book about progress in understanding self-incompatibility is a good idea. This topic is a classic, with importance for several different areas within biology, ranging from population and evolutionary genetics to cell biology, and there has been an impressive history of breakthroughs that have, in turn, led to new questions, in a kind of ‘arms race’ where the plants have so far managed to guard the most fascinating secrets. The last time self-incompatibility was reviewed at length and in detail was in de Nettancourt's book, last revised in 2001, and even then it was clear that a single biologist could not review this whole field. This book, sensibly, contains chapters from experts in different aspects of self-incompatibility. Progress in the past 20 years, and the major current puzzles, are therefore well covered, but at a cost of a lack of integration. The editor has provided cross-references, but the disadvantage of a multi-author book is the lack of a clear overview. The final chapter on heteromorphic incompatibility in Primula, for instance, leaves it unclear how (or whether) each system can help us understand the other.
The book is therefore most suitable for researchers. Each chapter reviews the authors' area of expertise, and each provides a good account of the current information (often in great detail). Many chapters outline the questions that remain unanswered, but some do not explain the concepts discussed, or sometimes even the evidence for the conclusions stated, and so parts of many chapters will be understandable mainly by people already working on self-incompatibility, or who have already read de Nettancourt's book. These people will read review papers in specialized journals, and, for some topics, the authoritative reviews in the book may quickly be superseded.
These problems are perhaps inevitable. Progress in the past 25 years has been enormous – from being able to test different incompatibility types of individual plants, and determine the inheritance and roughly estimate the numbers of different alleles in populations with homomorphic systems, to knowing details of the molecular mechanism of self-incompatibility in different species. This knowledge should lead to new progress, and one value of gathering it into a book is to take stock of current understanding.
We still do not know the type of incompatibility inheritance in many taxa, and the genes responsible have been identified only in very few taxa (the sporophytic system of Brassica family members, and the very different gametophytic systems in Papaver rhoeas, and in some Solanaceae, Rosaceae and Antirrhinum). In some RNase-based systems, as clearly reviewed by J. Kohn, the pollen recognition gene remains puzzling, because the best candidates in most species do not seem to have the predicted high sequence diversity (which is found in the Solanaceae, P. rhoeas and the Brassica family members) and different mechanisms could be involved in different RNase-based systems. Genetic studies are still very laborious, even in short-lived plants, and most self-incompatible taxa remain unstudied. This book contains useful chapters on two groups whose incompatibility systems remain unknown at the molecular level, the two-locus gametophytic system in grasses (whose S and Z loci may soon be discovered using comparative mapping and genome sequencing) and sweet potato (where finding the loci seems tantalisingly close), and two chapters review heterostyled systems. The heterostyly genes seem likely to be discovered before long, now that closely linked regions have been identified. In heterostyly it is, however, possible that a physically large genome non-recombining region, similar to a sex chromosome, is involved, and testing this evolutionary hypothesis is one of the main reasons for studying these systems.
A major evolutionary puzzle is how new specificities arise in homomorphic multi-allele systems with linked pistil and pollen genes. This remains as mysterious as ever, as J. Kohn's chapter explains. It should be helpful to be able to identify alleles with different incompatibility types in natural samples, and it is now clear that alleles with different incompatibility types differ greatly in sequence (such polymorphism is now a criterion used along with functional tests to show that a candidate is indeed an incompatibility gene). If alleles of the same incompatibility type are generally very similar (as has also been shown), then sequences can be used to identify alleles. A book with a range as broad as this should show how the achievements so far can help answer the unresolved questions; however, by the end, it is hard to have a clear feeling for what questions should be asked next to further understanding of self-incompatibility.