POLLEN TRANSFER BY NATURAL HYBRIDS AND PARENTAL SPECIES IN AN IPOMOPSIS HYBRID ZONE

Models of hybrid zones differ in their assumptions about the relative fitnesses of hybrids and the parental species. These fitness relationships determine the form of selection across the hybrid zone and, along with gene flow, the evolutionary dynamics and eventual outcome of natural hybridization. We measured a component of fitness, export and receipt of pollen in single pollinator visits, for hybrids between the herbaceous plants Ipomopsis aggregata and I. tenuituba and for both parental species. In aviary experiments with captive hummingbirds, hybrid flowers outperformed flowers of both parental species by receiving more pollen on the stigma. Although hummingbirds were more effective at removing pollen from anthers of I. aggregata, hybrid flowers matched both parental species in the amount of pollen exported to stigmas of other flowers. These patterns of pollen transfer led to phenotypic stabilizing selection, during that stage of the life cycle, for a stigma position intermediate between that of the two species and to directional selection for exserted anthers. Pollen transfer between the species was high, with flowers of I. aggregata exporting pollen equally successfully to conspecific and I. tenuituba flowers. Although this study showed that natural hybrids enjoy the highest quality of pollinator visits, a previous study found that I. aggregata receives the highest quantity of pollinator visits. Thus, the relative fitness of hybrids changes over the life cycle. By combining the results of both studies, pollinator-mediated selection in this hybrid zone is predicted to be strong and directional, with hybrid fitness intermediate between that of the parental species.     

EFFECTS OF DIFFERENTIAL POLLEN-TUBE GROWTH ON HYBRIDIZATION IN THE LOUISIANA IRISES

To elucidate the importance of hybridization in evolution, it is necessary to understand the processes that affect hybridization frequency in nature. Here we focus on postpollination, prefertilization isolating mechanisms using two hybridizing species of Louisiana iris as a study system. We compared the effects of differential pollen-tube growth on the frequency of F₁ hybrid formation in experimental crosses between Iris fulva and Iris hexagona. Analyses of seed production in fruits from pure conspecific and heterospecific pollinations revealed that more seeds were produced in the top half than the bottom half of fruits for all four crosses. Heterospecific pollen was applied to flowers of each species at zero to 24 h prior to conspecific pollen, thereby giving a head start to the foreign pollen. Using diagnostic isozyme markers, the frequency of hybrid progeny was examined at the level of the whole fruit and separately for the top and bottom halves of fruits. In both species, the proportion of hybrid seeds per fruit increased significantly with increasing head starts, suggesting that differences in pollen-tube growth rates affect the frequency of hybridization. In I. fulva fruits, the increase in hybrid seeds occurred in both halves of the fruits, but in I. hexagona an increase was only detected in the top half of fruits. These findings are consistent with a model that assumes attrition of pollen tubes due to the greater length of I. hexagona styles. While pollen-tube growth rate appears to be the most important factor affecting hybridization frequency in I. fulva, both pollen-tube growth rate and pollen-tube attrition appear to be important in I. hexagona.     

Intergeneric hybridization between Diplotaxis siifolia,a wild species and crop brassicas

Summary Attempts were made to obtain intergeneric hybrids between Diplotaxis siifolia, a wild species, and cultivars of Brassica (B. campestris, B. juncea, and B. napus). The crosses showed unilateral incompatibility. When the wild species was used as female parent, pollen germination and pollen tube growth were normal, but hybrid seeds aborted due to post-fertilization barriers. Reciprocal crosses (cultivars as female parent) showed strong pre-fertilization barriers; although pollen grains showed germination, pollen tubes failed to enter the stigma. Hybrids were realized in two of the crosses, D. siifolia x B. juncea and D. siifolia x B. napus, through ovary culture. The hybrids were multiplied in vitro by multiplication of axillary shoots, or somatic embryogenesis. Detailed studies were carried out on the hybrid D. siifolia x B. juncea. F₁ hybrids had shrivelled anthers and were pollen sterile. Amphiploids of this hybrid showed 60% pollen fertility and produced seeds upon self-pollination as well as backcross pollination with the pollen of B. juncea.     

Context-dependent pollinator behavior: an explanation for patterns of hybridization among three species of Indian paintbrush

In some areas of sympatry, reproductively compatible plant species hybridize, but in other areas of sympatry, they do not and they remain reproductively isolated from one another. Explanations offered to explain patterns of hybridization that vary by population have usually focused on genetic or environmental factors. Instead, we examined whether different community contexts might change pollinator preference and constancy and thus influence the likelihood of hybridization among three Indian paintbrush species (Castilleja miniata, C. rhexifolia, and C. sulphurea). To determine whether visitation was context‐dependent, we observed pollinator behavior in experimental arrays (constructed using flowering stems of the three Indian paintbrush species) in different contexts. Contexts were defined by which Castilleja species occurred in the immediate neighborhood of the arrays. Specifically, we asked, does visitation to particular species in the arrays depend on context? In general, each Castilleja species was preferred when it matched the surrounding community context, as is predicted by optimal foraging theory. More interestingly, pollinator constancy was weakened in the hybrid context (an area where the three species co‐occurred with morphologically intermediate plants), which is likely to increase pollen flow among the species. Reduced pollinator constancy in hybrid zones could set up a positive feedback loop in which more flower diversity is created through hybridization, decreasing pollinator constancy, and leading to more hybridization. This self‐reinforcing mechanism could lead to “hybridization hot spots” and to a patchy distribution of hybrid populations. We expect that this mechanism may be important in other animal‐pollinated plant hybrid zones.     

Strong post-pollination pre-zygotic isolation between sympatric, food-deceptive Mediterranean orchids

Mediterranean orchids that grow in admixed, co-flowering populations, and frequently show hybrid progenies are interesting to use to study the nature and the strength of post-zygotic barriers. However, examination of pre- and post-pollination pre-zygotic isolating mechanisms requires sympatric, co-flowering species pairs that do not produce hybrid swarms. In this study, we analyzed a contact zone between Orchis italica and O. papilionacea, in which hybrid forms have never been reported, although hybridization between members of their groups of appurtenance has been signaled. We investigated pre-pollination barriers observing the floral phenology of both species and identified pollinators by means of molecular analysis of pollinaria collected on the insects captured in the study site. Post-pollination barriers were tested performing manual crosses in order to evaluate pollen germination/pollen tube growth in vivo and fruit and seed formation. Floral phenologies of O. italica and O. papilionacea display nearly overlapping trends, and two common pollinators have been identified by molecular analysis of pollinaria. Thus, pre-pollination barriers are very weak or nonexistent. Bidirectional crosses have shown that the growth of heterospecific pollen tubes is fully blocked in stigmatic cell layers. Since no fruit formation was detected in bidirectional interspecific crosses, we assume that reproductive isolation between the examined species is fully guaranteed by post-pollination pre-zygotic mechanisms acting at stigmatic level. Such condition has been rarely described and may mask the potential action of post-zygotic mechanisms.     

CPDNA INHERITANCE IN INTERSPECIFIC CROSSES AND EVOLUTIONARY INFERENCE IN LOUISIANA IRISES

Inheritance of chloroplast DNA haplotypes was determined for progeny from interspecific crosses involving Iris fulva and Iris hexagona. Polymerase chain reaction amplification of chloroplast DNA followed by restriction fragment length analysis of the amplification products was used to identify the haplotypes of 213 experimental hybrids. This analysis allowed a test for maternal, paternal, and biparental inheritance in the hybrid offspring. Two of the hybrid progeny possessed haplotypes that were combinations of those present in the pollen and seed parents. One of the offspring possessed only the paternal haplotype. The remaining 210 plants had the haplotypes characteristic of the maternal plant. Chloroplast DNA variation in iris populations has previously been used to infer not only introgressive hybridization between I. fulva and I. hexagona, but also the greater role of direct pollen transfer relative to seed dispersal as the avenue for interspecific gene flow. We reexamined the previous conclusions concerning the mode of introgressive hybridization between I. fulva and I. hexagona in light of the results from the chloroplast DNA inheritance analysis. The low level of paternal and biparental inheritance detected in this analysis suggests that previous analyses using the chloroplast DNA as a seed-specific marker were robust. Furthermore, data concerning barriers to hybridization between I. fulva and I. hexagona suggest that the probability of chloroplast DNA introgression via pollen is low.     

THE CONTRIBUTION OF MATING SYSTEM VARIATION TO REPRODUCTIVE ISOLATION IN TWO CLOSELY RELATED CENTAURIUM SPECIES (GENTIANACEAE) WITH A GENERALIZED FLOWER MORPHOLOGY

In closely related plant species that display strong similarities in phenology and pollinator communities, differences in breeding system and associated shifts in floral traits may have important effects on the magnitude and direction of heterospecific pollen flow and hybridization. Here, we quantified the strength of several pre‐ and postzygotic barriers acting between the facultatively outcrossing Centaurium erythraea and the predominantly selfing C. littorale via a suite of experiments, and estimated the frequency of hybridization in the field using molecular markers. The reproductive barriers primarily responsible for preventing hybridization were essentially prezygotic and these acted asymmetrically. Due to differences in floral display, pollen production, and pollen transfer rates, heterospecific pollen flow occurred predominantly from C. erythraea to C. littorale. In C. littorale, on the other hand, close anther–stigma positioning and resulting higher capacity for autonomous selfing functioned as an efficient barrier to counterbalance the higher risk for hybrid mating. In both species the action of all reproductive barriers resulted in a small opportunity for hybrid establishment, which was confirmed by the occurrence of only ～1% putative hybrids in the field. Our findings confirm that differences in breeding system affect heterospecific pollen transfer patterns and that autonomous selfing may efficiently prevent hybridization.     

The evolutionary potential of invasive Carpobrotus (Aizoaceae) taxa: are pollen-mediated gene flow potential and hybrid vigor levels connected?

Few studies have addressed the importance of native pollinators in shaping the breeding systems and evolutionary potential of invasive plants. We examined the pollination and gamete production of Carpobrotus affine acinaciformis (L.) L. Bol. and C. edulis (L.) N. E. Br. invading the coasts of southeast France (Provence), and found preliminary evidence that the pollen-mediated gene flow potential (PMGFP) within the four studied populations is positively correlated with their Relative Performance in Hybridization indices (RP_H), suggesting a link between pollinator services, gene flow, and Carpobrotus hybridization. Flower density (FD) may be a driver behind pollinator abundance and visitation patterns since it shows a significant, positive relationship with relative pollinator abundance (FR) and a significant negative relationship with per flower visit frequencies (FR/F). In the populations with the highest RP_H indices, and therefore high hybrid vigor, both taxa produce similar quantities of ovules and pollen per flower, whose ratios further indicate (facultative) xenogamy. Pollen diameter distributions include ∼25% micro-pollen, and overall viability is less than one half. Viable pollen profiles consist of 3.5% micro, 85.5% normal and 10.9% macro pollen (considered as diplogametes) on average per flower for C. affine acinaciformis, and 0.7% micro, 73.0% normal and 26.2% macro pollen for C. edulis. Given the co-occurrence of (1) (facultative) xenogamic breeding strategies, (2) a significant, positive relationship between RP_H and PMGFP, and (3) frequent pollen abnormalities, it is probable that hybrid dysgenesis mediated by the local pollinators occurs in these populations. Furthermore, the unusually high frequency of potentially viable, diploid macro-pollen underlines the evolutionary/polyploid potential of these invasive, introgressed populations. Native pollinator interactions may greatly affect the taxonomic status and evolutionary potential of invasive plant complexes.     

Shifts to earlier selfing in sympatry may reduce costs of pollinator sharing

Coexisting plant congeners often experience strong competition for resources. Competition for pollinators can result in direct fitness costs via reduced seed set or indirect costs via heterospecific pollen transfer (HPT), causing subsequent gamete loss and unfit hybrid offspring production. Autonomous selfing may alleviate these costs, but to preempt HPT, selfing should occur early, before opportunities for HPT occur (i.e., “preemptive selfing hypothesis”). We evaluated conditions for this hypothesis in Collinsia sister species, C. linearis and C. rattanii. In field studies, we found virtually identical flowering times and pollinator sharing between congeners in sympatric populations. Compared to allopatric populations, sympatric C. linearis populations enjoyed higher pollinator visitation rates, whereas visitation to C. rattanii did not differ in sympatry. Importantly, the risk of HPT to each species in sympatry was strongly asymmetrical; interspecies visits comprised 40% of all flower‐to‐flower visits involving C. rattanii compared to just 4% involving C. linearis. Additionally, our greenhouse experiment demonstrated a strong cost of hybridization when C. rattanii was the pollen donor. Together, these results suggest that C. rattanii pays the greatest cost of pollinator sharing. Matching predictions of the preemptive selfing hypothesis, C. rattanii exhibit significantly earlier selfing in sympatric relative to allopatric populations.     

Maintenance of species integrity in sympatrically occurring Rhinanthus minor and R. serotinus in The Netherlands

Summary In certain localities, R. minor and R. serotinus grow sympatrically and the flowering-periods overlap. The species hybridize but can still be recognized as distinct taxonomic entities. In the field s x m crosses can be expected to occur more frequently than the reverse, on the basis of flower morphology and pollinator (bumblebee) efficiency. Observation of pollen germination, pollen tube growth, seed set, and seed germination in artificial, reciprocal crosses permits the conclusion that a single m x s pollination leads to more offspring than a single s x m pollination. The two species are isolated from each other by a series of mechanisms none of which is 100% effective by itself, but their combined action comes close to that figure. The leakages in the ethological barrier against hybridization are closed, partly, by physiological mechanisms.     

ARTIFICIAL HYBRIDIZATIONS IN THE SUBTRIBE PERITYLANAE (COMPOSITAE–HELENIEAE)

Artificial hybridizations were conducted with 37 species of four genera of the subtribe Peritylanae (Compositae). Crossability, pollen stainability, and meiotic behavior were examined for most of the resulting 61 F₁ hybrid combinations. Intergeneric hybrids involving Amauria, Eutetras, Pericome, and Perityle, and infrageneric hybrids involving Perityle were readily obtained. Lower crossability and hybrid fertility were displayed by intergeneric crosses. The infrageneric crosses of Perityle were organized into two categories, intersectional and infrasectional, based upon the taxonomic sections that are recognized for the genus; sect. Pappothrix, sect. Laphamia, and sect. Perityle. In general, the crossability and hybrid fertility of intersectional crosses was markedly higher than that of intergeneric crosses. The results suggest a closer relationship between the three taxonomic sections of Perityle than between Perityle and other genera of the subtribe.     

小报春与岩生报春种间杂交亲和性研究

以报春花属报春花组的小报春(Primula forbesii)和指叶报春组的岩生报春(Primula saxatilis)为亲本,对种间杂交的结实性及花粉管行为进行了观察。结果表明:小报春与岩生报春种间杂交表现为不亲和,正反交组合正常结实率为0,但正交、反交组合花粉在柱头表面的萌发和花粉管伸长过程有明显差异,小报春花粉授粉4h后可以在岩生报春柱头表面萌发,但花粉管伸长的速度明显比对照组[岩生报春(P)×岩生报春(T)、岩生报春(T)×岩生报春(P)]慢,并且花粉管生长弯曲,授粉192h后花粉管仍未到达子房;岩生报春的花粉可以在小报春柱头上正常萌发,授粉48h后花粉管到达子房;4个对照授粉组合均分别于24h(短花柱为母本)、48h(长花柱为母本)时完成受精过程。研究表明,岩生报春×小报春杂交存在受精前障碍,小报春×岩生报春杂交亲和性较好,并可通过幼胚拯救的方法获得组间杂种后代;花柱长度可能是影响种间杂交结实能力的因素之一。     

AN INTERGENERIC HYBRID IN THE SAXIFRAGACEAE: EVIDENCE FROM RIBOSOMAL RNA GENES

The tandemly repeated multigene families encoding the 5S and 18S-25S ribosomal RNAs were studied at the restriction enzyme level in Tolmiea menziesii, Tellima grandiflora, and in a putative intergeneric hybrid. Using restriction endonucleases that cut once per repeat, the repeat lengths of the 5S and 18S-25S ribosomal genes were estimated. The 5S ribosomal gene repeat length is approximately 480 and 450 base pairs, respectively, in Tolmiea and Tellima. The repeat length of the 18S-25S ribosomal genes varied from 11–13 kb in Tolmiea, and was only about 9 kb in Tellima. The putative hybrid combined the repeat lengths of both Tolmiea and Tellima for both the 5S and the 18S-25S ribosomal genes. These data substantiate the occurrence of natural hybridization between Tolmiea and Tellima. For both the 5S and 18S-25S gene experiments, the hybrid appears to contain fewer repeats corresponding to Tolmiea than to Tellima.     

CHLOROPLAST DNA VARIATION WITHIN AND AMONG GENERA OF THE HEUCHERA GROUP (SAXIFRAGACEAE): EVIDENCE FOR CHLOROPLAST TRANSFER AND PARAPHYLY

The Heuchera group (Saxifragaceae) comprises Bensoniella, Conimitella, Elmera, Heuchera, Lithophragma, Mitella, Tellima, Tiarella, and Totmiea. Earlier studies employing morphology, karyology, and flavonoid chemistry indicated that these genera form a natural group, but failed to resolve relationships among them. Restriction site analysis of chloroplast DNA (cpDNA) suggests that Bensoniella, Tolmiea, and Lithophragma are close allies and form the sister group of a large clade containing the remaining six genera. Mitella and Heuchera are both paraphyletic based on cpDNA data. cpDNA data, in conjunction with morphological and allozyme data, suggest at least four examples of intersectional hybridization and subsequent chloroplast capture in Heuchera. Several of these events may be explained via a stepping stone model in which the chloroplast genome of a species was captured by a second species, and then ultimately by a third taxon. Two well-differentiated groups of Tellima populations were detected: one group has a unique chloroplast genome characterized by nine autapomorphies, and the second group has a chloroplast genome identical to that found in M. trifida and M. diversifolia. cpDNA and allozyme data suggest that some Tellima populations probably obtained their chloroplast genome via intergeneric hybridization with M. trifida, M. diversifolia, or the ancestor of these taxa. The occurrence of intergeneric chloroplast transfer in some populations of Tellima, as well as extensive intersectional chloroplast capture in Heuchera, not only suggests caution in the use of cpDNA restriction site data in phylogenetic reconstruction, but also demonstrates again the importance of adequate sampling of conspecific populations. If the intergeneric relationships in the Heuchera group suggested by cpDNA analysis are accurate, fundamental questions arise regarding the validity of certain morphological traits as good taxonomic characters in Saxifragaceae. Furthermore, significant taxonomic changes at the generic level would be necessary.     

FREQUENCY-DEPENDENT SEED PRODUCTION AND HYBRIDIZATION RATES: IMPLICATIONS FOR GENE FLOW BETWEEN LOCALLY ADAPTED PLANT POPULATIONS

Gene flow between genetically distinct plant populations can have significant evolutionary consequences. It can increase genetic diversity, create novel gene combinations, and transfer adaptations from one population to another. This study addresses the roles of frequency-dependent selection and mating system in gene exchange between two subspecies of Gilia capitata (Polemoniaceae). Long-distance migrants are likely to be rare in new habitats, and the importance of immigrant frequency to fitness, gene exchange, and ultimately introgression, has not been explored. To test for the importance of frequency in migration, a field experiment was conducted in which artificial populations (arrays) composed of different mixtures of the two subspecies were placed in the home habitats of both. Female function (seed production) and a portion of male function (hybridization rate) were compared for the two subspecies to assess the potential for gene exchange and introgression between them. Individual fitness (through both hybridization and seed production) for the inland subspecies varied with its frequency as an immigrant at the coastal site. Rare immigrants produced fewer seeds and fathered fewer hybrid offspring. In contrast, both forms of reproductive function were frequency independent for the coastal subspecies when it was an immigrant at the inland site. Seed production was high and insensitive to frequency, and immigrants from the coast never successfully fertilized the inland subspecies' seeds. To control for the effects of frequency-dependent pollinator behavior in the field, hand crosses were performed in the greenhouse using a range of pollen mixtures. The greenhouse experiment demonstrated that cross-fertilization is possible in only one direction, that cross-pollination in the other direction is only partially successful, and that pollen from the coastal subspecies has a strong negative effect on seed production by the inland subspecies. Experimental pollen supplementation in the field verified both the unilateral incompatibility and the negative effect of coastal pollen on inland plant seed production observed in the greenhouse. Contrasts between field array and greenhouse results suggest that pollinator behavior and other ecological factors act to exaggerate reproductive barriers between the two subspecies. In this system, immigrant frequency interacts with reproductive biology and pollinator ecology to enhance gene flow between the populations in one direction, while restricting gene establishment and introgression in the other direction.     

Genetic Evidence for Natural Hybridization between Species of Dioecious Ficus on Island Populations1

Natural hybrids between Ficus septica and two closely related dioecious species, F. fistulosa and F. hispida, were confirmed using amplified fragment length polymorphisms (AFLP) and chloroplast DNA markers. Ficus species have a highly species‐specific pollination mutualism with agaonid wasps. Therefore, the identification of cases in which breakdown in this sophisticated system occurs and the circumstances under which it happens is of interest. Various studies have confirmed that Ficus species are able to hybridize and that pollinator‐specificity breakdown can occur under certain conditions. This study is the first example in which hybrid identity and the presence of hybrids in the natural distribution of parental species for Ficus have been confirmed with molecular markers. Hybrid individuals were identified on three island locations in the Sunda Strait region of Indonesia. These findings support Janzen's (1979) hypothesis that breakdown in pollinator specificity is more likely to occur on islands. We hypothesized that hybrid events could occur when the population size of pollinator wasps was small or had been small in one of the parental species. Later generation hybrids were identified, indicating that backcrossing and introgression did occur to some extent and that therefore, hybrids could be fertile. The small number of hybrids found indicated that there was little effect of hybridization on parental species integrity over the study area. Although hybrid individuals were not common, their presence at multiple sites indicated that the hybridization events reported here were not isolated incidences. Chloroplast DNA haplotypes of hybrids were not derived solely from one species, suggesting that the seed donor was not of the same parental species in all hybridization events.     

Pollinator convergence and the nature of species' boundaries in sympatric Sardinian Ophrys (Orchidaceae)

Background and Aims

In the sexually deceptive Ophrys genus, species isolation is generally considered ethological and occurs via different, specific pollinators, but there are cases in which Ophrys species can share a common pollinator and differ in pollen placement on the body of the insect. In that condition, species are expected to be reproductively isolated through a pre-mating mechanical barrier. Here, the relative contribution of pre- vs. post-mating barriers to gene flow among two Ophrys species that share a common pollinator and can occur in sympatry is studied.

Methods

A natural hybrid zone on Sardinia between O. iricolor and O. incubacea, sharing Andrena morio as pollinator, was investigated by analysing floral traits involved in pollinator attraction as odour extracts both for non-active and active compounds and for labellum morphology. The genetic architecture of the hybrid zone was also estimated with amplified fragment length polymorphism (AFLP) markers, and pollination fitness and seed set of both parental species and their hybrids in the sympatric zone were estimated by controlled crosses.

Key Results

Although hybrids were intermediate between parental species in labellum morphology and non-active odour compounds, both parental species and hybrids produced a similar odour bouquet for active compounds. However, hybrids produced significantly lower fruit and seed set than parental species, and the genetic architecture of the hybrid zone suggests that they were mostly first-generation hybrids.

Conclusions

The two parental species hybridize in sympatry as a consequence of pollinator overlap and weak mechanical isolation, but post-zygotic barriers reduce hybrid frequency and fitness, and prevent extensive introgression. These results highlight a significant contribution of late post-mating barriers, such as chromosomal divergence, for maintaining reproductive isolation, in an orchid group for which pre-mating barriers are often considered predominant.Key words: AFLP markers, floral scent variation, hybrid zone, hybrid fitness, Ophrys iricolor, Ophrys incubacea, reproductive isolation, sexual deception     

Mating system and the hybridization between self-compatible Phlox cuspidata and self-incompatible Phlox drummondii

Mating system can impact the frequency of hybridization and therefore the maintenance of species diversity. I evaluate the effects of weak self-incompatibility (SI) in Phlox cuspidata and SI in Phlox drummondii on mating success within species and on hybridization dynamics between species under controlled conditions. The effects of SI on hybridization frequency were assessed by manipulating the relatedness of conspecific pollen and the relative timing of pollen deposition in mixed-donor interspecific pollinations. Selfing as opposed to outcrossing increased hybridization by 16% in P. cuspidata maternal plants and by 48% in P. drummondii maternal plants because self pollen did not compete as well against heterospecific pollen. The relative timing of conspecific versus heterospecific pollen deposition also impacted hybridization. In self-compatible P. cuspidata, the deposition of self pollen 5 h earlier than heterospecific pollen decreased hybridization by 28%. In self-incompatible P. drummondii, a 5 h delay in the deposition of compatible conspecific pollen increased hybridization by 32%. In this hybrid system, early self-pollination can decrease hybridization (but increase inbreeding) by P. cuspidata maternal plants, and SI may increase hybridization by P. drummondii maternal plants.     

Post-pollination mechanisms in Nicotiana longiflora and N. plumbaginifolia: pollen tube growth rate, offspring paternity and hybridization

In natural populations where interfertile species coexist, conspecific and heterospecific pollen can be delivered to the stigmas. Post-pollination mechanisms might determine the seed siring success of different pollen donors within species as well as the chances for hybridization between species. Nicotiana longiflora and N. plumbaginifolia occur in sympatry in Northwest Argentina, where they have overlapping flowering seasons and share floral visitors. We explored (1) pollen tube growth rates for outcross versus self pollen in single-donor pollinations; (2) siring success of self versus outcross pollen donors in competitive pollinations, and (3) possibilities for hybridization by performing two- (outcross conspecific vs. heterospecific) and three-pollen donor (self vs. outcross vs. heterospecific) crosses. In N. longiflora, both pollen tube growth rate and siring success favored outcross pollen over self pollen and strong rejection of heterospecific pollen. In N. plumbaginifolia, pollen tube growth rate was similar for self and outcross pollen, self pollen sired similar numbers of offspring than outcross pollen and heterospecific pollen sired roughly the same number of progeny than self pollen. Results suggest that in natural sympatric populations, interspecific crosses would likely lead to unidirectional hybridization with N. plumbaginifolia as the seed parent.     

Isolation mechanisms between two sympatric Sophronitis (Orchidaceae) species endemic to Northeastern Brazil

Sophronitis sincorana and S. pfisteri (Orchidaceae) are endemic to the campo rupestre vegetation, northeastern Brazil, occurring sympatrically and flowering synchronously. In this work we studied the reproductive biology of both species and investigated reproductive barriers between them. Both species are partially self-compatible, pollinator dependent, and are pollinated by males and worker bees of Bombus brevivillus. Pollinaria are deposited on the anterior portion of the mesonotum of these insects in S. pfisteri, and on the scutellum in S. sincorana. The flowers of both species are deceptive, as they have a dry cuniculus. Morphological differences in the labellum and the gynostemium impede inter-specific pollination of S. pfisteri with pollen from S. sincorana. Very low fruiting results from inter-specific crosses of S. sincorana with pollen from S. pfisteri. Although viable seeds can be produced from experimental inter-specific crosses, no natural hybrids were found. Species integrity appears to be maintained by additive unidirectional reproductive barriers between them.