Quantum dots shed light on angiosperm speciation

摘要： For all sexual organisms, mate selection is an important factor in preventing gene flow between species. This is because it typically involves assortative mating: choosing mates similar to yourself and rejecting other possibilities. For animals, reproductive barriers during this stage are termed ‘behavioral isolation’, whereas for flowering plants the corresponding barriers are termed ‘floral isolation’. However, these are not simply interchangeable terms depending on your taxonomic preference, as the biological functions are fundamentally different. Most plants actually depend on another organism to find potential mates and ferry their pollen to them. Thus finding mates depends on intrinsic factors for animals, such as its own appearance and behavior, and extrinsic factors for flowering plants, such as the availability and behavior of pollinators. It is not immediately clear whether this distinction should amplify or decrease speciation rates for angiosperms. On the one hand, it should make angiosperms more susceptible to ecological speciation, or speciation due to divergent selection in different habitats: range expansions to include new habitats are not likely to affect animal mate-seeking behavior significantly, but these expansions can expose plants to very different pollinator regimes. On the other hand, the pollination process is inherently messy – to what extent can the behavior or morphology of pollinators actually restrict pollen movement between incipient species? Unfortunately, the earliest stages of speciation are notoriously difficult to study, and there is a dearth of evidence on the topic. In this issue of New Phytologist, Minnaar et al. (2019, pp. 1160–1170) provide one of the clearest demonstrations to date of floral isolation during incipient speciation, showing how evolutionary shifts in flower tube length serve to greatly reduce pollen transfer between long- and short-tubed floral morphs. To track pollen movement, they debut a technique that Minnaar & Anderson (2019) recently developed to label pollen grains using quantum dots. The latest work by Minnaar et al. thus makes two contributions to the study of plant reproduction: it provides critical support for the potential role of floral isolation during speciation, and it illustrates the experimental use of an exciting new methodology, that has the potential to revolutionize the study of the male fitness component of plant reproductive success.