Characterization of sympatricPlatanthera bifoliaandPlatanthera chlorantha(Orchidaceae) populations with intermediate plants

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.

Nectar composition in moth-pollinated Platanthera bifolia and P. chlorantha and its importance for reproductive success

“…In effect, P. bifolia is predominantly adapted to pollination by hawk moths (Sphingidae) and P. chlorantha by noctuids (Noctuidae) (Nilsson 1983). The two Platanthera species also differ according to their floral scent chemistry; thus, they attract pollinators in different ways, which is an ethological mechanism of reproductive isolation that reduces hybridization (Nilsson 1983(Nilsson , 1985Esposito et al 2018).…”

“…The recent studies by Lindqvist et al (2018), which tested a novel technique, are methodical in their character and analysed sugars from only 18 flowers of the two Platanthera species. Both species were intensively investigated in the context of pollination biology, spatial variation of reproductive success (focusing on pollinator-mediated selection of floral traits), hybridization and genetic diversity (Nilsson 1983(Nilsson , 1985Brzosko 2003;Maad and Alexandersson 2004;Maad and Nilsson 2004;Bateman and Sexton 2008;Brzosko et al 2009;Bateman et al 2012 Esposito et al 2018;Mõtlep et al 2018). Our interest stems from the fact that this species pair is a suitable model system to study co-evolutionary processes.…”

Continuous video capture, and pollinia tracking, in Platanthera (Orchidaceae) reveal new insect visitors and potential pollinators

“…(Orchidaceae) contains many species that are reported to hybridize ( Brown, 2006 ; Argue, 2012b ). The genus is widespread globally and many aspects of the pollination biology of European species have been examined, especially for P. bifolia and P. chlorantha ( Nilsson, 1983 ; Maad & Nilsson, 2004 ; Steen, 2012 ; Boberg et al, 2014 ; Esposito et al, 2018 ). Two species within Section Limnorchis , P. dilatata (white bog orchid) and P. stricta (slender bog orchid), occur throughout North America, although P. stricta is restricted to western North America ( Brown, 2006 ).…”

Pollen transfer and patterns of reproductive success in pure and mixed populations of nectariferousPlatanthera bifoliaandP. chlorantha(Orchidaceae)

“…The example of long-spurred orchids are the objects of our studies— Platanthera bifolia and P. chlorantha , the most common representatives of Platanthera genus in Europe, occurring in sympatry. Their flowers differ in column structure, floral scent ( Nilsson, 1983 , 1985 ; Tollsten & Bergström, 1993 ; Esposito et al, 2018 ) and nectar chemistry ( Brzosko & Bajguz, 2019 ). According to the results of recent studies, the most important flower trait in attracting pollinators, thus in determining the fitness of these orchids, is spur length ( Nilsson, 1983 ; Maad & Nilsson, 2004 ; Boberg et al, 2014 ).…”

“…Despite the fact that flower traits constitute reproductive isolation, both Platantherans, at least partially, share pollinators, which increases the probability of hybridisation. Their hybrids have been found in different parts of geographic ranges ( Nilsson, 1983 , 1985 ; Bateman & Sexton, 2008 ; Schiestl & Schlüter, 2009 ; Moré et al, 2012 ; Esposito, Merckx & Tyteca, 2017 ; Esposito et al, 2018 ; Mõtlep et al, 2021 ). Hybridisation is a common phenomenon in plants and is often observed in orchids, which highlights its importance in evolutionary processes ( Campbell & Aldridge, 2006 ; Schiestl & Schlüter, 2009 ; Xu et al, 2011 ; Baack et al, 2015 ; Esposito et al, 2018 ; Mõtlep et al, 2021 ).…”