Reproductive barriers and fertility of two neotropical orchid species and their natural hybrid

Hybridization in the Fringed Orchids: An Analysis of Species Boundaries in the Face of Gene Flow

Natural hybridization between closely related species in sympatry is an evolutionary process that is common in orchids. Once seen as a threat to parent species, interspecific genetic change is increasingly viewed as a source of novel variation in some ecological contexts. Terrestrial fringed orchids in the genus Platanthera contain several clades with high genetic compatibility among species and many putative hybrids. We used biallelic SNPs generated with 3RAD sequencing to study the hybrid complex formed from the parent species P. blephariglottis, P. ciliaris, and P. cristata with high resolution. The genetic structure and phylogenetic relationship of the hybrid complex revealed site-dependent gene flow between species. We documented extensive hybridization and cryptic hybrids in sympatric sites. Interspecific genetic exchange is particularly common between P. blephariglottis and P. ciliaris, with cryptic hybrids among putative P. ciliaris samples being more common than parental assignments in sympatric sites. Hybridization across the triad species complex can reticulate lineages and introduce adaptive alleles. Conversely, it can reduce diversification rates and introduce maladaptive alleles. Investigation into whether anthropogenic forces are eroding species boundaries, particularly the permeable P. blephariglottis and P. ciliaris boundary, is appropriate for conservation efforts.

An evolutionary framework for understanding habitat partitioning in plants

Many plant species with overlapping geographic ranges segregate at smaller spatial scales. This spatial segregation-zonation when it follows an abiotic gradient and habitat partitioning when it does not-has been experimentally investigated for over a century often using distantly related taxa, such as different genera of algae or barnacles. In those foundational studies, trade-offs between stress tolerance and competitive ability were found to be the major driving factors of habitat partitioning for both animals and plants. Yet, the evolutionary relationships among segregating species are usually not taken into account. Since close relatives are hypothesized to compete more intensely and are more likely to interact during mating compared to distant relatives, the mechanisms underlying habitat partitioning may differ depending on the relatedness of the species in question. Here, I propose an integration of ecological and evolutionary factors contributing to habitat partitioning in plants, specifically how the relative contributions of factors predictably change with relatedness of taxa. Interspecific reproductive interactions in particular are understudied, yet important drivers of habitat partitioning. In spatially segregated species, interspecific mating can reduce the fitness of rare immigrants, preventing their establishment and maintaining patterns of spatial segregation. In this synthesis, I review the literature on mechanisms of habitat partitioning in plants within an evolutionary framework, identifying knowledge gaps and detailing future directions for this rapidly growing field of study.

Microhabitat and Pollinator Differentiation Drive Reproductive Isolation between Two Sympatric Salvia Species (Lamiaceae)

Evaluation of multiple barriers contributing to reproductive isolation between sympatric plant species is key to understanding the mechanism of their coexistence; however, such investigations in biodiversity hotspots are still rare. In this study, we investigated and compared geography, microhabitat, phenology, flora, and pollinators, in addition to pollen–pistil interactions, seed production, and seed germination of the closely related sympatric Salvia digitaloides and S. flava on Yulong Snow Mountain, Southwestern Yunnan, China. The geographic distribution of these species overlapped, but their adaptation to physical and chemical properties of soil microhabitats differed. They shared the same flowering time but differed in flower size, style length, nectar volume, sugar concentration, and flower longevity. Both species shared bumblebees as effective pollinators, but flower constancy for the two species was relatively strong. Pollen tube growth, seed production, and seed germination were lower in interspecific than in intraspecific crosses. Our study suggested that microhabitat and pollinator isolation acted as the most important isolating barriers in maintaining the coexistence of the two Salvia species. Our study also highlighted that post-pollination barriers play an important role in preventing the gene flow between these two Salvia species.

Reproductive Isolation Among Three Nocturnal Moth-Pollinated Sympatric Habenaria Species (Orchidaceae)

Comparison and quantification of multiple pre- and post-pollination barriers to interspecific hybridization are important to understand the factors promoting reproductive isolation. Such isolating factors have been studied recently in many flowering plant species which seek after the general roles and relative strengths of different pre- and post-pollination barriers. In this study, we quantified six isolating factors (ecogeographic isolation, phenological isolation, pollinator isolation, pollinia-pistil interactions, fruit production, and seed development) that could possibly be acting as reproductive barriers at different stages among three sympatric Habenaria species (H. limprichtii, H. davidii, and H. delavayi). These three species overlap geographically but occupy different microhabitats varying in soil water content. They were isolated through pollinator interactions both ethologically (pollinator preference) and mechanically (pollinia attachment site), but to a variable degree for different species pairs. Interspecific crosses between H. limprichtii and H. davidii result in high fruit set, and embryo development suggested weak post-pollination barriers, whereas bidirectional crosses of H. delavayi with either of the other two species fail to produce fruits. Our results revealed that pollinators were the most important isolating barrier including both ethological and mechanical mechanisms, to maintain the boundaries among these three sympatric Habenaria species. Our study also highlights the importance of a combination of pre-and post-pollination barriers for species co-existence in Orchidaceae.