Abstract
Platanthera bifolia and P. chlorantha are terrestrial and rewarding orchids with a wide Eurasian distribution. Although genetically closely related, they exhibit significant morphological, phenological and ecological differences that maintain reproductive isolation between the species. However, where both species co-occur, individuals with intermediate phenotypic traits, often considered as hybrids, are frequently observed. Here, we combined neutral genetic markers (AFLPs), morphometrics and floral scent analysis (GC-MS) to investigate two mixed Platanthera populations where morphologically intermediate plants were found. Self-pollination experiments revealed a low level of autogamy and artificial crossings combined with assessments of fruit set and seed viability, showed compatibility between the two species. The results of the genetic analyses showed that morphologically intermediate plants had similar genetic patterns as the P. bifolia group. These results are corroborated also by floral scent analyses, which confirmed a strong similarity in floral scent composition between intermediate morphotypes and P. bifolia. Therefore, this study provided a much more detailed picture of the genetic structure of a sympatric zone between two closely allied species and supports the hypothesis that intermediate morphotypes in sympatry could reflect an adaptive evolution in response to local pollinator-mediated selection.
Highlights
The evolution of reproductive isolation is a central topic in evolutionary biology
The analysis of similarities (ANOSIM) analysis was in agreement with the results of the Multiple Response Permutation Procedure (MRPP), since it showed a significant difference between those groups (Botton: R = 0.762, P < 0.001; Bois Niau R = 0.779, P < 0.001)
The multivariate analysis (PCA) showed that the analysed floral characters were exhaustively discriminant between the two Platanthera species, which were represented in two separate clusters, where intermediate morphotypes were closer to P. bifolia group (Fig. 3)
Summary
The evolution of reproductive isolation is a central topic in evolutionary biology. Flowering plants have evolved different ways to remain reproductively isolated from their congeners through various pre- and/or post-pollination barriers (Coyne & Orr, 2004; Rieseberg & Willis, 2007).Orchids, a large and widespread family of flowering plants, are well known for their remarkable floral diversity. The evolution of reproductive isolation is a central topic in evolutionary biology. Flowering plants have evolved different ways to remain reproductively isolated from their congeners through various pre- and/or post-pollination barriers (Coyne & Orr, 2004; Rieseberg & Willis, 2007). Much of this diversity is associated with intricate relationships with pollinators, and has often been attributed to adaptation to specific pollinators (e.g., Johnson, Linder & Steiner, 1998)—an estimated 60% of all orchid species only have a handful of recorded pollinator species (Tremblay, 1992)
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