REPRODUCTIVE ISOLATION BETWEEN TWO CLOSELY RELATED HUMMINGBIRD POLLINATED NEOTROPICAL GINGERS

Reproductive isolation between two sympatric bat-pollinated Bauhinia (Leguminosae)

Several barriers contribute to reproductive isolation between plant species, which can be classified as pre- or post-pollination. Understanding the strength of these barriers could clarify the factors that maintain reproductive isolation and thus species integrity. In this study, we quantified reproductive isolation between two bat-pollinated co-occurring Bauhinia species (B. acuruana and B. pentandra) with similar flower morphology. Over the course of 18 months, we assessed reproductive isolation between these two Bauhinia species by quantifying the individual strengths and absolute contributions of five pre- and post- pollination barriers. Our data showed that both species are completely isolated in their reproduction by a combination of several barriers. Although they co-occur in a few populations, we found a high degree of geographic isolation between them. And although their flowering periods overlap, there is a significant difference in flowering peaks. Both species have the same pollinating bats, but the interspecific transfer of pollen between the plant species may be reduced due to the different length of the flower stamens, resulting in different pollen deposition on the bats' bodies. We have documented complete incompatibility between taxa and conclude that pre- and post-pollination barriers are important factors in preventing gene flow, even in contact zones between these two species of Bauhinia. We highlight that our work is the first study to use methods to estimate the strength of reproductive isolation barriers between bat-pollinated species.

Mechanisms of prezygotic post-pollination reproductive barriers in plants

Hybridisation between individuals of different species can lead to maladapted or inviable progeny due to genetic incompatibilities between diverging species. On the other hand, mating with close relatives, or self-fertilisation may lead to inbreeding depression. Thus, both too much or too little divergence may lead to problems and the organisms have to carefully choose mating partners to avoid both of these pitfalls. In plants this choice occurs at many stages during reproduction, but pollen-pistil interactions play a particularly important role in avoiding inbreeding and hybridisation with other species. Interestingly, the mechanisms involved in avoidance of selfing and interspecific hybridisation may work via shared molecular pathways, as self-incompatible species tend to be more 'choosy' with heterospecific pollen compared to self-compatible ones. This review discusses various prezygotic post-pollination barriers to interspecific hybridisation, with a focus on the mechanisms of pollen-pistil interactions and their role in the maintenance of species integrity.

Eco-Geography and Phenology Are the Major Drivers of Reproductive Isolation in the Royal Irises, a Species Complex in the Course of Speciation

The continuous nature of speciation implies that different species are found at different stages of divergence, from no- to complete reproductive isolation. This process and its underlying mechanisms are best viewed in incipient species. Moreover, the species complex can offer unique insight into how reproductive isolation (RI) has evolved. The royal irises (Iris section Oncocyclus) are a young group of species in the course of speciation, providing an ideal system for speciation study. We quantified pre- and post-zygotic reproductive barriers between the eight Israeli species of this complex and estimated the total RI among them. We tested for both pre-pollination and post-pollination reproductive barriers. Pre-pollination barriers, i.e., eco-geographic divergence and phenological differentiation were the major contributors to RI among the Iris species. On the other hand, post-pollination barriers, namely pollen-stigma interactions, fruit set, and seed viability had negligible contributions to total RI. The strength of RI was not uniform across the species complex, suggesting that species may have diverged at different rates. Overall, this study in a young, recently diverged group of species provides insight into the first steps of speciation, suggesting a crucial role of the pre-zygotic barriers.

Pollinator sharing between reproductively isolated genetic lineages of Silene nutans

High reciprocal pollination specialization leading to pollinator isolation can prevent interspecific pollen transfer and competition for pollinators. Sharing pollinators may induce mating costs, but it may also increase pollination services and pollen dispersal and offer more resources to pollinators, which may be important in case of habitat fragmentation leading to pollination disruption. We estimated pollen dispersal and pollinator isolation or sharing between two reproductively isolated genetic lineages of Silene nutans (Caryophyllaceae), which are rare and occur in parapatry in southern Belgium, forming two edaphic ecotypes. As inter-ecotypic crosses may lead to pollen wastage and inviable progeny, pollinator isolation might have evolved between ecotypes. Silene nutans is mainly pollinated by nocturnal moths, including nursery pollinators, which pollinate and lay their eggs in flowers, and whose caterpillars feed on flowers and seeds. Pollinator assemblages of the two ecotypes are largely unknown and inter-ecotypic pollen flows have never been investigated. Fluorescent powdered dyes were used as pollen analogues to quantify intra- and inter-ecotypic pollen transfers and seeds were germinated to detect chlorotic seedlings resulting from inter-ecotypic pollination. Nocturnal pollinators were observed using infrared cameras on the field, and seed-eating caterpillars were collected and reared to identify nursery pollinator species. No pollinator isolation was found: we detected long-distance (up to 5 km) inter-ecotypic dye transfers and chlorotic seedlings, indicating inter-ecotypic fertilization events. The rare moth Hadena albimacula, a nursery pollinator specialized on S. nutans, was found on both ecotypes, as well as adults visiting flowers (cameras recordings) as seed-eating caterpillars. However, S. nutans populations harbor different abundance and diversity of seed predator communities, including other rare nursery pollinators, suggesting a need for distinct conservation strategies. Our findings demonstrate the efficiency of moths, especially of nursery pollinators, to disperse pollen over long distances in natural landscapes, so to ensure gene flow and population sustainability of the host plant. Seed-predator specificities between the two reproductively isolated genetic lineages of S. nutans, and pollinator sharing instead of pollinator isolation when plants occur in parapatry, suggest that conservation of the host plant is also essential for sustaining (rare) pollinator and seed predator communities.

Unusual patterns of hybridization involving two alpine Salvia species: Absence of both F1 and backcrossed hybrids

Natural hybridization plays an important role in speciation; however, we still know little about the mechanisms underlying the early stages of hybrid speciation. Hybrid zones are commonly dominated by F₁s, or backcrosses, which impedes further speciation. In the present study, morphological traits and double digest restriction-site associated DNA sequencing (ddRAD-seq) data have been used to confirm natural hybridization between Salvia flava and S. castanea, the first case of identification of natural hybridization using combined phenotypic and molecular evidence in the East Asian clade of Salvia. We further examined several reproductive barriers in both pre-zygotic and post-zygotic reproductive stages to clarify the causes and consequences of the hybridization pattern. Our results revealed that reproductive isolation between the two species was strong despite the occurrence of hybridization. Interestingly, we found that most of the hybrids were likely to be F₂s. This is a very unusual pattern of hybridization, and has rarely been reported before. The prevalence of geitonogamy within these self-compatible hybrids due to short distance foraging by pollinators might explain the origin of this unusual pattern. F₂s can self-breed and develop further, therefore, we might be witnessing the early stages of hybrid speciation. Our study provides a new case for understanding the diversification of plants on the Qinghai-Tibet Plateau.

The Role of Hybridisation in the Making of the Species-Rich Arctic-Alpine Genus Saxifraga (Saxifragaceae)

Evolutionary processes fuelling rapid species diversification are not yet fully understood, although their major contribution to overall patterns of plant biodiversity is well established. Hybridisation is among the least understood of these processes, despite its multifaceted role in speciation processes being widely accepted. Species of the large arctic-alpine genus Saxifraga are notorious for their ability to hybridise; however, the overall role of hybridisation and polyploidisation for the diversification of this genus remains unknown. Here, we provide a comprehensive genus-wide review of hybridisation accounts and ploidy levels. We find that the sections of Saxifraga vary greatly in their propensity to hybridise. The majority of natural hybridisation accounts are from recent localised events (n = 71). Hybridisation hotspots were located in the Pyrenees and the European Alps, thus contrasting with the overall distribution of species richness in the genus. Hybrids or hybrid populations are often short-lived in Saxifraga due to a multitude of reproductive barriers, most commonly low F1 hybrid fertility. However, these barriers are not always fully effective, allowing for backcrossing and the formation of hybrid swarms. In addition, we find that the incidence of polyploidy varies widely across different sections of Saxifraga, with species-rich sections Porphyrion and Saxifraga showing divergent polyploidy proportions. Overall, we show that hybridisation and polyploidisation played differential roles in the diversification of this large genus. Nevertheless, a significant proportion of species are yet to be scrutinised, particularly among the Asian Saxifraga species, illustrating the need for systematic further study to fully unravel the role of hybridisation during the evolution of Saxifraga.

Reproductive ecology and isolation of Psittacanthus calyculatus and P. auriculatus mistletoes (Loranthaceae)

BACKGROUND

Relationships between floral biology and pollinator behavior are important to understanding species diversity of hemiparasitic Psittacanthus mistletoes (c. 120 species). We aimed to investigate trait divergence linked to pollinator attraction and reproductive isolation (RI) in two hummingbird-pollinated and bird-dispersed Psittacanthus species with range overlap.

METHODS

We investigated the phylogenetic relationships, floral biology, pollinator assemblages, seed dispersers and host usage, and the breeding system and female reproductive success of two sympatric populations of P. calyculatus and P. auriculatus, and one allopatric population of P. calyculatus. Flowers in sympatry were also reciprocally pollinated to assess a post-mating component of RI.

RESULTS

Hummingbird assemblages differed between calyculatus populations, while allopatric plants of calyculatus opened more but smaller flowers with longer lifespans and produced less nectar than those in sympatry. Bayesian-based phylogenetic analysis indicated monophyly for calyculatus populations (i.e. both populations belong to the same species). In sympatry, calyculatus plants opened more and larger flowers with longer lifespans and produced same nectar volume than those of auriculatus; populations shared pollinators but seed dispersers and host usage differed between species. Nectar standing crops differed between sympatric populations, with lower visitation in calyculatus. Hand pollination experiments indicated a predominant outcrossing breeding system, with fruit set after interspecific pollination two times higher from calyculatus to auriculatus than in the opposite direction.

CONCLUSIONS

Given the low genetic differentiation between calyculatus populations, observed trait divergence could have resulted from changes regarding the local communities of pollinators and, therefore, expected divergence for peripheral, allopatric populations. Using RI estimates, there were fewer heterospecific matings than expected by chance in P. calyculatus (RI4A = 0.629) as compared to P. auriculatus (RI4A = 0.20). When considering other factors of ecological isolation that affect co-occurrence, the RI4C values indicate that isolation by hummingbird pollinators was less effective (0.20) than isolation by host tree species and seed dispersers (0.80 and 0.60, respectively), suggesting that host usage is the most important ecological isolation factor between the two species. Accordingly, the absolute and relative cumulative strength values indicated that the host tree species' barrier is currently contributing the most to maintaining these species in sympatry.

Transitions between self-compatibility and self-incompatibility and the evolution of reproductive isolation in the large and diverse tropical genus Dendrobium (Orchidaceae)

BACKGROUND AND AIMS

The evolution of interspecific reproductive barriers is crucial to understanding species evolution. This study examines the contribution of transitions between self-compatibility (SC) and self-incompatibility (SI) and genetic divergence in the evolution of reproductive barriers in Dendrobium, one of the largest orchid genera. Specifically, it investigates the evolution of pre- and postzygotic isolation and the effects of transitions between compatibility states on interspecific reproductive isolation within the genus.

METHODS

The role of SC and SI changes in reproductive compatibility among species was examined using fruit set and seed viability data available in the literature from 86 species and ∼2500 hand pollinations. The evolution of SC and SI in Dendrobium species was investigated within a phylogenetic framework using internal transcribed spacer sequences available in GenBank.

KEY RESULTS

Based on data from crossing experiments, estimations of genetic distance and the results of a literature survey, it was found that changes in SC and SI significantly influenced the compatibility between species in interspecific crosses. The number of fruits produced was significantly higher in crosses in which self-incompatible species acted as pollen donor for self-compatible species, following the SI × SC rule. Maximum likelihood and Bayesian tests did not reject transitions from SI to SC and from SC to SI across the Dendrobium phylogeny. In addition, postzygotic isolation (embryo mortality) was found to evolve gradually with genetic divergence, in agreement with previous results observed for other plant species, including orchids.

CONCLUSIONS

Transitions between SC and SI and the gradual accumulation of genetic incompatibilities affecting postzygotic isolation are important mechanisms preventing gene flow among Dendrobium species, and may constitute important evolutionary processes contributing to the high levels of species diversity in this tropical orchid group.