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

Abstract

Plant species evolution is driven by many factors that have different roles in space and time. Using different field and laboratory methods, we studied reproductive patterns and their determinants in pure and mixed P. bifolia and P. chlorantha populations in different habitats. We also considered the probability of hybridisation between these two species and the role of intra-population processes in maintaining species integrity. Generally, we found a high level of reproductive success in both Platantherans. In both species, male (MRS) and female (FRS) reproductive success depended on floral display, and male reproductive success additionally on population structure. The flower traits were only weakly related to reproductive success. Moths’ assemblages varied spatially and temporally, and their diversity and numbers were correlated with MRS in the year, when their abundance was markedly lower. Analysis of patterns of pollen transfer showed that pollen was transported up to 25 m (average 8.2 ± 4.83 m) and showed gene exchange between these two Platanthera species. The germination level of both species was significantly lower than seed viability, although P. bifolia seed germinated with higher frequency than P. chlorantha seeds. We noted differences in viability and germination of seeds developed as an effect of experimental interspecies crossings and those developed from natural pollination. The presence of intermediate ecotypes together with observations of spontaneous interspecies crosses in the field and viability of seeds produced in interspecies crossing suggest that both pre- and postzygotic reproductive barriers are not complete and do not prevent hybrid production.

Trait analysis in a population of the Greater Butterfly-orchid observed through a 16-year period

“…Moreover, conventional wisdom states that the size of the overall floral display and the size and shape of those floral components that influence pollination success, notably the gynostemium and nectar-rich labellar spur, will be at least periodically subject to stabilizing selection, driven by the necessity of assuring frequent reproduction through seed dispersal (cf. Boberg and Ågren, 2009 ; Boberg et al., 2014 ; Ambroise et al., 2020 ; Brzosko et al., 2022 ). This innate conservatism inevitably limits variance in floral morphology, and therefore in our study it weakened all correlations that involved spur length – a phenomenon that is readily apparent in Tables 2 – 5 .…”