Abstract
Plants that are highly selfing typically exhibit a suite of morphological traits termed a “selfing syndrome,” including reduced corollas and reproductive structures, loss of corolla pigmentation, little anther-stigma separation, and a lower pollen/ovule (P/O) ratio. While it is typically assumed that these changes are adaptive, few attempts have been made to determine whether they result from the operation of natural selection or genetic drift. In the southeastern United States, Ipomoea lacunosa has evolved a typical selfing syndrome compared to its close relative, Ipomoea cordatotriloba. Microsatellite markers confirmed that selfing rates are substantially higher in I. lacunosa. Furthermore, using a standard QST – FST comparison, we evaluated the relative importance of selection and drift in the evolution of selfing syndrome traits in I. lacunosa. The analysis demonstrated that natural selection is responsible for the evolution of reduced corolla size, anther-stigma distance, and style length in this species. By contrast, leaf characteristics unrelated to selfing were found to have diverged largely by genetic drift. Our study provides one of the first confirmations that natural selection drives the evolution of selfing-syndrome traits.
Highlights
The evolutionary transition from outbreeding to selfing is one of the most common changes in angiosperms, with an estimated 20% of all flowering plants having evolved autogamy as the predominant mode of reproduction (Barrett, 2002)
Selfing-syndrome characters may be divided into two categories: (1) those targeted by natural selection to increase selfing rate, and (2) those that evolved for other reasons
A recent analysis of shared genetic variation suggests that I. lacunosa and I. cordatotriloba are exchanging genetic material because they are genetically indistinguishable at neutral markers (Duncan, 2013, in press)
Summary
The evolutionary transition from outbreeding to selfing is one of the most common changes in angiosperms, with an estimated 20% of all flowering plants having evolved autogamy as the predominant mode of reproduction (Barrett, 2002). Autogamous plants are typically characterized by a “selfing syndrome,” which consists of having small, scentless, nectarless, and often white flowers, reduced anther-stigma distance, and a decreased pollen/ovule (P/O) ratio (Ornduff, 1969; Sicard and Lenhard, 2011; Kalisz et al, 2012). Selfing-syndrome characters may be divided into two categories: (1) those targeted by natural selection to increase selfing rate, and (2) those that evolved for other reasons. A likely example of a trait in the first category is anther-stigma distance. Once increased selfing has evolved, decreased P/O ratio may be the result of selection to redirect resources to other fitness-enhancing traits because less pollen is needed for effective self-pollination (Brunet, 1992)
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