Post-pollination barriers enable coexistence of pollinator-sharing ornithophilous Erica species

Synthesis and Scope of the Role of Postmating Prezygotic Isolation in Speciation

How barriers to gene flow arise and are maintained are key questions in evolutionary biology. Speciation research has mainly focused on barriers that occur either before mating or after zygote formation. In comparison, postmating prezygotic (PMPZ) isolation-a barrier that acts after gamete release but before zygote formation-is less frequently investigated but may hold a unique role in generating biodiversity. Here we discuss the distinctive features of PMPZ isolation, including the primary drivers and molecular mechanisms underpinning PMPZ isolation. We then present the first comprehensive survey of PMPZ isolation research, revealing that it is a widespread form of prezygotic isolation across eukaryotes. The survey also exposes obstacles in studying PMPZ isolation, in part attributable to the challenges involved in directly measuring PMPZ isolation and uncovering its causal mechanisms. Finally, we identify outstanding knowledge gaps and provide recommendations for improving future research on PMPZ isolation. This will allow us to better understand the nature of this often-neglected reproductive barrier and its contribution to speciation.

Pollinator shifts despite hybridisation in the Cape's hyperdiverse heathers (Erica, Ericaceae)

Interrogating the ecological and geographic factors that influence population divergence dynamics can reveal why some groups of organisms diversify more prolifically than others. One such group is the heathers (Erica, Ericaceae), the largest plant genus in the Cape Floristic Region. We study Erica abietina, a highly variable species complex with four subspecies differing in geographic range, habitat and pollination syndrome. We test for population differentiation, hybridisation, introgression and pollinator-driven divergence using genotyping-by-sequencing on samples across the entire distribution. We find five variably distinct genetic groups, with one subspecies comprising two independent lineages that are geographically isolated and occur on different soil types. Phylogenetic analysis suggests two independent shifts between bird and insect pollination, with accompanying genetic divergence. However, for one pair of populations with different pollinators, we uncover several individuals of hybrid origin at a site of sympatry. These results suggest that floral differentiation driven by divergent selection acts in concert with geographic isolation to maintain reproductive isolation and promote speciation. Our investigations reveal a highly dynamic system whose diversity has been shaped by a variety of interacting forces. We suggest that such a system could be a model for much of the diversification of the Cape flora.

Pollen transfer efficiency in Erica depends on type of pollinator

Pollen transfer efficiency (PTE; the proportion of pollen removed from flowers that reaches conspecific stigmas) is expected to vary with the type of pollinator and flower morphology, and to influence male siring success. Many species in the genus Erica are pollinated by bees (which consume pollen and should thus lower PTE) but during its radiation in the Cape, several independent shifts to both sunbird and long-proboscid fly (LP fly) pollinators, which do not consume pollen have taken place. Improvements in PTE could be one of the factors driving these pollinator shifts. PTE data for 15 Erica species (five for each of the three pollinator types) were collected and compared in relation to type of pollinator and anther exsertion. LP fly- and bird-pollinated species had higher PTE in comparison with bee-pollinated species. Species with inserted anthers had higher PTE than those with exserted anthers. This suggests that sunbirds and LP flies are more efficient pollinators than bees. Additionally, the study suggests that insertion of anthers within the corolla tube can reduce pollen losses.

Is self-incompatibility a reproductive barrier for hybridization in a sympatric species?

PREMISE

Barriers at different reproductive stages contribute to reproductive isolation. Self-incompatibility (SI) systems that prevent self-pollination could also act to control interspecific pollination and contribute to reproductive isolation, preventing hybridization. Here we evaluated whether SI contributes to reproductive isolation among four co-occurring Opuntia species that flower at similar times and may hybridize with each other.

METHODS

We assessed whether Opuntia cantabrigiensis, O. robusta, O. streptacantha, and O. tomentosa, were self-compatible and formed hybrid seeds in five manipulation treatments to achieve self-pollination, intraspecific cross-pollination, open pollination (control), interspecific crosses or apomixis, then recorded flowering phenology and synchrony.

RESULTS

All species flowered in the spring with a degree of synchrony, so that two pairs of species were predisposed to interspecific pollination (O. cantabrigiensis with O. robusta, O. streptacantha with O. tomentosa). All species had distinct reproductive systems: Opuntia cantabrigiensis is self-incompatible and did not produce hybrid seeds as an interspecific pollen recipient; O. robusta is a dioecious species, which formed a low proportion of hybrid seeds; O. streptacantha and O. tomentosa are self-compatible and produced hybrid seeds.

CONCLUSIONS

Opuntia cantabrigiensis had a strong pollen-pistil barrier, likely due to its self-incompatibility. Opuntia robusta, the dioecious species, is an obligate outcrosser and probably partially lost its ability to prevent interspecific pollen germination. Given that the self-compatible species can set hybrid seeds, we conclude that pollen-pistil interaction and high flowering synchrony represent weak barriers; whether reproductive isolation occurs later in their life cycle (e.g., germination or seedling survival) needs to be determined.

Flower orientation and corolla length as reproductive barriers in the pollinator-driven divergence of Erica shannonea and Erica ampullacea

A variety of reproductive barriers can enable reproductive isolation and stable coexistence of plant species. Differing floral traits might play an important role in reproductive isolation imposed by pollinators. Such shifts in pollinator use have been hypothesized to contribute to the radiation of Erica (Ericaceae) in the Cape Floristic Region, South Africa. The sister species Erica shannonea and Erica ampullacea co-occur and overlap in flowering phenology. Both have unscented long-tubed flowers consistent with adaptations for pollination by long-proboscid flies (LPFs), but differences in flower orientation and corolla tube length are indicative of a shift in pollinator species. We conducted controlled pollination experiments and pollinator observations to determine the breeding system and pollinators of the two species. Both species are self-incompatible and require pollinator visits for seed production, suggesting that pollinators could strongly influence flower evolution. The horizontally orientated flowers of E. shannonea were found to be pollinated by Philoliche rostrata (Tabanidae), which has a long, fixed forward-pointing proboscis, while the vertically upright orientated flowers of E. ampullacea were found to be pollinated by Prosoeca westermanni (Nemestrinidae), which has a shorter proboscis that can swivel downwards. The nemestrinid fly's proboscis is too short to access the nectar in the relative long-tubed flowers of E. shannonea and the tabanid fly's proboscis cannot swivel down to access the upright flowers of E. ampullacea. Consequently, these traits are likely to act as reproductive barriers between the two Erica species and thereby might have contributed to speciation and enable stable coexistence.

Genetic structure and trait variation within a maple hybrid zone underscore North China as an overlooked diversity hotspot

Tertiary relict flora in East Asia can be divided into northern and southern regions. North China is a diversity hotspot because it can be the secondary contact zone of ancient lineages from the two regions. To test the extent of ancient lineages hybridization and distinguish between the putative species pair Acer pictum subsp. mono and Acer truncatum, we conducted genetic and ecological studies within a maple hybrid zone in North China. Our results suggest that the two lineages of Acer coexist in the hybrid zone and that adult and offspring populations show typical bimodal genetic patterns. Hybrid individuals are established at intermediate altitudes between the two parental lineages. Flowering phenology is divergent between lineages, whereas the complex sexual system of Acer may ensure pollination among lineages. Leaf and fruit morphologies are different between the northern and southern origin lineages, corresponding to A. pictum subsp. mono and A. truncatum, respectively. Reduced gene flow between lineages suggests that they should be considered as two species. However, large morphological variations within each species and the existence of hybrids offer low reliability of species identification based solely on morphological traits. Our study underscores North China as an overlooked diversity hotspot that requires further study in the future.

Pollen competition in hybridizing Cakile species: How does a latecomer win the race?

PREMISE

Hybridization between cross-compatible species depends on the extent of competition between alternative mates. Even if stigmatic compatibility allows for hybridization, hybridization requires the heterospecific pollen to be competitive. Here, we determined whether conspecific pollen has an advantage in the race to fertilize ovules and the potential handicap to be overcome by heterospecific pollen in invasive Cakile species.

METHODS

We used fluorescence microscopy to measure pollen tube growth after conspecific and heterospecific hand-pollination treatments. We then determined siring success in the progeny relative to the timing of heterospecific pollen arrival on the stigma using CAPS markers.

RESULTS

In the absence of pollen competition, pollination time and pollen recipient species had a significant effect on the ratio of pollen tube growth. In long-styled C. maritima (outcrosser), pollen tubes grew similarly in both directions. In short-styled C. edentula (selfer), conspecific and heterospecific pollen tubes grew differently. Cakile edentula pollen produced more pollen tubes, revealing the potential for a mating asymmetry whereby C. edentula pollen had an advantage relative to C. maritima. In the presence of pollen competition, siring success was equivalent when pollen deposition was synchronous. However, a moderate 1-h advantage in the timing of conspecific pollination resulted in almost complete assortative mating, while an equivalent delay in conspecific pollination resulted in substantial hybrid formation.

CONCLUSIONS

Hybridization can aid the establishment of invasive species through the transfer of adaptive alleles from cross-compatible species, but also lead to extinction through demographic or genetic swamping. Time of pollen arrival on the stigma substantially affected hybridization rate, pointing to the importance of pollination timing in driving introgression and genetic swamping.

Hummingbird contribution to plant reproduction in the rupestrian grasslands is not defined by pollination syndrome

Floral traits mediate the roles of distinct animals as effective pollinators along a generalization/specialization continuum. Many plant species are visited by different pollinator functional groups and the specific contribution of each group is expected to reflect the set of floral characteristics defined by pollination syndromes. Although considered a highly specialized nectarivorous group, hummingbirds frequently visit flowers lacking apparent specialization to bird pollination. How they contribute to the reproduction of these plants, however, has not been evaluated through field experiments considering multiple non-related plant species simultaneously. Here, we investigated hummingbirds' contributions to the pollination of ten plant species comprising a gradient of adaptation to bird pollination in the Brazilian rupestrian grasslands. We excluded hummingbirds from flowers and evaluated their relative contribution in comparison to insects (mainly bees) on conspecific/heterospecific pollen deposition and fruit set. Floral traits that are typically associated with bird pollination were associated with increased pollen deposition, but not with fruit set in the presence of hummingbirds. With hummingbirds, conspecific and heterospecific pollen deposition increased in most species, while fruit set increased in four plant species with varying degrees of fit to ornithophily. Our results show that assessing the relative contribution of specific pollinator groups may depend on when this contribution is measured, i.e. pollen deposition or fruit set. Considering fruit set, our results indicate that hummingbirds contributed to plant reproduction independently of the fit to bird pollination syndrome. This emphasizes their importance as under-appreciated generalized pollinators in some communities.