The vast majority of flowering plants are hermaphrodites, with both male and female parts in each of their flowers (Renner, 2014). This strategy is clearly a successful one, coming with several likely advantages including that, by producing both pollen and ovules, each flower has more opportunities to pass genes on to the next generation (Charnov et al., 1976). Although hermaphrodites might also benefit from the possibility of self-fertilization (and many do), most hermaphrodites do all they can to avoid ‘selfing’ because it is often detrimental to a plant’s fitness (Charlesworth and Willis, 2009; Barrett, 2002). Hermaphroditic plants have thus evolved a wonderful array of mechanisms that help them to promote outcrossing.
One of these mechanisms, known as heterostyly, involves male and female parts of the flower (the stamen and the stigma) being in different places in different flowers. The stigma sits on the end of a stalk called the style. Some flowers have long styles and hold their stamens, which produce pollen, deep in the floral tube, whereas others have short styles and hold their stamens much higher in the floral tube (Figure 1A). This means that pollen from a long-styled plant is more likely to pollinate a short-styled flower, and vice versa. Long- and short-styled plants may also differ in the size and colour of their pollen grains and the texture of their stigmas, and outcrossing is only possible between different morphs (Barrett, 2002). Remarkably, despite its complexity, heterostyly has evolved repeatedly in flowering plants and is found in at least 30 families, including the primroses (genus Primula).
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The vast majority of flowering plants are hermaphrodites, with both male and female parts in each of their flowers (Renner, 2014). This strategy is clearly a successful one, coming with several likely advantages including that, by producing both pollen and ovules, each flower has more opportunities to pass genes on to the next generation (Charnov et al., 1976). Although hermaphrodites might also benefit from the possibility of self-fertilization (and many do), most hermaphrodites do all they can to avoid ‘selfing’ because it is often detrimental to a plant’s fitness (Charlesworth and Willis, 2009; Barrett, 2002). Hermaphroditic plants have thus evolved a wonderful array of mechanisms that help them to promote outcrossing.
One of these mechanisms, known as heterostyly, involves male and female parts of the flower (the stamen and the stigma) being in different places in different flowers. The stigma sits on the end of a stalk called the style. Some flowers have long styles and hold their stamens, which produce pollen, deep in the floral tube, whereas others have short styles and hold their stamens much higher in the floral tube (Figure 1A). This means that pollen from a long-styled plant is more likely to pollinate a short-styled flower, and vice versa. Long- and short-styled plants may also differ in the size and colour of their pollen grains and the texture of their stigmas, and outcrossing is only possible between different morphs (Barrett, 2002). Remarkably, despite its complexity, heterostyly has evolved repeatedly in flowering plants and is found in at least 30 families, including the primroses (genus Primula).
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