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Requests by patients for providers of specific demographic backgrounds pose an ongoing challenge for hospitals, policymakers, and ethicists. These requests may stem from a wide variety of motivations; some may be consistent with broader societal values, although many others may reflect prejudices inconsistent with justice, equity, and decency. This paper proposes a taxonomy designed to assist healthcare institutions in addressing such cases in a consistent and equitable manner. The paper then reviews a range of ethical and logistical challenges raised by such requests and proposed guidance to consider when reviewing and responding to them.

There is increasing recognition that palliative care interventions should be offered concurrent to cancer diagnosis and treatment. The model of service delivery has evolved throughout the years, from a primarily inpatient and crisis-driven service model to a model that encompasses both inpatient and ambulatory settings. This shift has allowed for the progression and development of early integrated palliative and oncology care. Studies using a variety of palliative care interventions, from traditional provider-based referrals to educational and case-management interventions, have shown clinically significant benefits for patients. Patients receiving early palliative care services demonstrate a more proactive approach-oriented coping style, report reduced rates of depression, report better quality of life, and use less aggressive treatments at the end of life, without detriment to survival. Although psychiatry has not been formally included in early palliative care trials, collaborative efforts with psychiatrists may offer creative ways of addressing psychosocial issues at the onset of cancer diagnosis.

Species are often defined by their ability to interbreed (i.e., Biological Species Concept), but determining how and why reproductive isolation arises between new species can be challenging. In the Mimulus tilingii species complex, three species (M. caespitosa, M. minor, and M. tilingii) are largely allopatric and grow exclusively at high elevations (>2000 m). The extent to which geographic separation has shaped patterns of divergence among the species is not well understood. In this study, we determined that the three species are morphologically and genetically distinct, yet recently diverged. Additionally, we performed reciprocal crosses within and between the species and identified several strong postzygotic reproductive barriers, including hybrid seed inviability, F1 hybrid necrosis, and F1 hybrid male and female sterility. In this study, such postzygotic barriers are so strong that a cross between any species pair in the M. tilingii complex would cause nearly complete reproductive isolation. We consider how geographical and topographical patterns may have facilitated the evolution of several postzygotic barriers and contributed to speciation of closely related members within the M. tilingii species complex.

Species are often defined by their ability to interbreed (i.e., Biological Species Concept), but determining how and why reproductive isolation arises between new species can be challenging. In theMimulus tilingiispecies complex, three species (M. caespitosa,M. minor, andM. tilingii) are largely allopatric and grow exclusively at high elevations (>2000m). The extent to which geographic separation has shaped patterns of divergence among the species is not well understood. In this study, we determined that the three species are morphologically and genetically distinct, yet recently diverged (M. tilingiicomplex would cause nearly complete reproductive isolation. We consider how geographical and topographical patterns may have facilitated the evolution of several postzygotic barriers and contributed to speciation of closely related members within theM. tilingiispecies complex.

Two distantly-related North American montane monkeyflower species,Mimulus lewisiiandMimulus tilingii,possess glandular trichomes. In this study, we characterized the morphological and histochemical features of these glandular trichomes. For each species, we used traditional light microscopy and scanning electron microscopy (SEM) to examine morphological characteristics, and determined the main components of the secretory products using histochemical and thin layer chromatography (TLC) staining techniques. We identified type VI glandular trichomes on leaf surfaces in both species of monkeyflowers. These trichomes exhibited stalk-cell lengths and head-cell counts that varied across adaxial and abaxial leaf surfaces, stems, and sepals. Both species contained secretory products within the cuticle of the trichome head, which releases the subcuticular metabolites when ruptured. Histochemical tests in bothM. lewisiiandM. tilingiiconfirmed that secretions contained lipids and polysaccharides. TLC plate staining indicated the presence of UV active compounds with polyalcohols, lipids, and amines. The common morphology and chemistry of the glandular trichomes suggests an analogous response to similar environmental conditions in these evolutionary distant montane monkeyflowers.

The genus Mimulus is a well-studied group of plant species, which has for decades allowed researchers to address a wide array of fundamental questions in biology (Wu & al. 2008; Twyford & al. 2015). Linnaeus named the type species of Mimulus (ringens L.), while Darwin (1876) used Mimulus (luteus L.) to answer key research questions. The incredible phenotypic diversity of this group has made it the focus of ecological and evolutionary study since the mid-20th century, initiated by the influential work of Clausen, Keck, and Hiesey as well as their students and collaborators (Clausen & Hiesey 1958; Hiesey & al. 1971, Vickery 1952, 1978). Research has continued on this group of diverse taxa throughout the 20th and into the 21st century (Bradshaw & al. 1995; Schemske & Bradshaw 1999; Wu & al. 2008; Twyford & al. 2015; Yuan 2019), and Mimulus guttatus was one of the first non-model plants to be selected for full genome sequencing (Hellsten & al. 2013). Mimulus has played a key role in advancing our general understanding of the evolution of pollinator shifts (Bradshaw & Schemske 2003; Cooley & al. 2011; Byers & al. 2014), adaptation (Lowry & Willis 2010; Kooyers & al. 2015; Peterson & al. 2016; Ferris & Willis 2018; Troth & al. 2018), speciation (Ramsey & al. 2003; Wright & al. 2013; Sobel & Streisfeld 2015; Zuellig & Sweigart 2018), meiotic drive (Fishman & Saunders 2008), polyploidy (Vallejo-Marín 2012; Vallejo-Marín & al. 2015), range limits (Angert 2009; Sexton et al. 2011; Grossenbacher & al. 2014; Sheth & Angert 2014), circadian rhythms (Greenham & al. 2017), genetic recombination (Hellsten & al. 2013), mating systems (Fenster & Ritland 1994; Dudash & Carr 1998; Brandvain & al. 2014) and developmental biology (Moody & al. 1999; Baker & al. 2011, 2012; Yuan 2019). This combination of a rich history of study coupled with sustained modern research activity is unparalleled among angiosperms. Across many interested parties, the name Mimulus therefore takes on tremendous biological significance and is recognizable not only by botanists, but also by zoologists, horticulturalists, naturalists, and members of the biomedical community. Names associated with a taxonomic group of this prominence should have substantial inertia, and disruptive name changes should be avoided. As members of the Mimulus community, we advocate retaining the genus name Mimulus to describe all monkeyflowers. This is despite recent nomenclature changes that have led to a renaming of most monkeyflower species to other genera.

Two distantly-related North American montane monkeyflower species, Mimulus lewisii and M. tilingii, possess glandular trichomes. In this study, we characterized the morphological and histochemical features of these glandular trichomes. For each species, we used traditional light microscopy and scanning electron microscopy (SEM) to examine morphological characteristics, and categorized the natural products on the leaf surface using histochemical and thin layer chromatography (TLC) staining techniques. We identified type VI capitate glandular trichomes on leaf surfaces in both species of monkeyflowers. These trichomes exhibited stalk-cell lengths and head-cell counts that varied across adaxial and abaxial leaf surfaces. Both species contained secretory products within the cuticle of the trichome head, which releases the subcuticular metabolites when ruptured. Histochemical tests of M. lewisii and M. tilingii indicated the presence of lipids and polysaccharides on the leaf surface. TLC plate staining further supported their presence, as well as UV active compounds and amines. The common morphology and chemistry of the glandular trichomes suggests an analogous response to similar environmental conditions in these evolutionary distant montane monkeyflowers.

PREMISE OF THE STUDY The introduction of nonnative taxa into areas occupied by conspecifics can lead to local extinction of native taxa via habitat modification and competitive dominance, and be exacerbated by outbreeding depression or the formation of invasive hybrid lineages following intraspecific gene flow. The expansion of Eurasian Phragmites australis into tidal wetlands of North America has been accompanied by a dramatic decline of native P. australis, with few relic populations remaining along the Atlantic coastline of the United States, particularly in the Virginia portion of the Chesapeake Bay. METHODS We sampled populations from the York River and its two major tributaries to determine the pattern of Phragmites invasion and identify remnant native populations that warrant conservation. We used chloroplast DNA haplotypes and nuclear DNA microsatellite profiles to classify individuals as belonging to the native or introduced lineage. KEY RESULTS Although native Phragmites stands were identified in the brackish upstream reaches of the two York River tributaries, the majority of Phragmites stands surveyed contained the introduced lineage. We also identified a single putative hybrid plant, based on its microsatellite profile. This plant possessed the native cpDNA haplotype and was located in an otherwise native Phragmites stand that is adjacent to an isolated patch of introduced Phragmites. CONCLUSIONS Although evidence of field hybridization between native and introduced lineages of Phragmites in North America is still relatively rare, the continued encroachment of the introduced lineage into native wetlands may increase the likelihood of future hybrid formation. Careful genetic monitoring to identify remnant native and potential hybrid Phragmites is essential for prioritizing ongoing management efforts.

Premise of the Study Novel nuclear microsatellite markers were developed for the invasive plant Oplismenus undulatifolius (Poaceae) to facilitate studies of invasion dynamics for this recently introduced, high‐risk invasive species in North American mid‐Atlantic forests. Methods and Results Sixteen polymorphic microsatellite loci were isolated and characterized from an Illumina paired‐end shotgun library of O. undulatifolius after initial assessment of 48 loci. When screened in three populations, these markers identified two to six alleles per locus. Observed heterozygosity ranged from 0.000 to 1.000. All loci were successfully amplified in the North American native congener O. hirtellus. Conclusions We provide the first nuclear genetic resources available for population genetic studies within the genus Oplismenus. These markers will serve as a critical tool for exploring patterns of gene flow and mechanisms of invasion of O. undulatifolius across its introduced range. These microsatellites should also be suitable for studies in other Oplismenus species.

Levels of hybridization between related species can vary in response to evolutionary history or local environmental conditions, such as pollinators and abiotic factors, that affect reproductive isolation. Contact sites between the herbs Ipomopsis aggregata and Ipomopsis tenuituba vary in habitat type, spatial distribution of the parental species, and level of hybridization, partly because of more species-specific pollinator foraging at the contact site with fewest hybrids. We hypothesized that such a contact site, with low hybridization and stronger genetic differentiation between the species, would also show greater divergence in ecophysiology, potentially contributing to reduced hybrid viability. We compared photosynthetic responses to temperature and water for the two species and their natural hybrids (where present) between the unusual site where hybrids are scarce and two contact sites with extensive hybridization. Measurements were made under field conditions and as a function of leaf temperature fo...

 Premise of the study: Fitness of plant hybrids often depends upon the environment, but physiological mechanisms underlying the differential responses to habitat are poorly understood. We examined physiological responses of Ipomopsis species and hybrids, including reciprocal F 1 s and F 2 s, to variation in soil moisture and nitrogen.  Methods: To examine responses to moisture, we subjected plants to a dry-down experiment. Nitrogen was manipulated in three independent experiments, one in the fi eld and two in common environments.  Key results: Plants with I. tenuituba cytoplasmic background had lower optimal soil moisture for photosynthesis, appearing better adapted to dry conditions, than plants with I. aggregata cytoplasm. This result supported a prediction from prior studies. The species and hybrids did not differ greatly in physiological responses to nitrogen. An increase in soil nitrogen increased leaf nitrogen, carbon assimilation, integrated water-use effi ciency, and growth, but the increases in growth were not mediated primarily by an increase in photosynthesis. In neither the fi eld, nor in common-garden studies, did physiological responses to soil nitrogen differ detectably across plant types, although only I. aggregata and hybrids increased seed production in the fi eld.  Conclusions: These results demonstrate differences in photosynthetic responses between reciprocal hybrids and suggest that water use is more important than nitrogen in explaining the relative photosynthetic performance of these hybrids compared to their parents.

Various models purporting to explain natural hybrid zones make different assumptions about the fitness of hybrids. One class of models assumes that hybrids have intrinsically low fitness due to genetic incompatibilities, whereas other models allow hybrid fitness to vary across natural environments. We used the intrinsic rate of increase to assess lifetime fitness of hybrids between two species of montane plants Ipomopsis aggregata and Ipomopsis tenuituba planted as seed into multiple field environments. Because fitness is predicted to depend upon genetic composition of the hybrids, we included F1 hybrids, F2 hybrids, and backcrosses in our field tests. The F2 hybrids had female fitness as high, or higher, than expected under an additive model of fitness. These results run counter to any model of hybrid zone dynamics that relies solely on intrinsic nuclear genetic incompatibilities. Instead, we found that selection was environmentally dependent. In this hybrid zone, cytoplasmic effects and genotype-by-environment interactions appear more important in lowering hybrid fitness than do intrinsic genomic incompatibilities between nuclear genes.

Hybrids between related species vary widely in relative fitness, and that fitness can depend upon the environment. We investigated aspects of physiology that might influence fitness patterns in a plant hybrid zone. Seeds of Ipomopsis aggregata, I. tenuituba, F1 hybrids, F2 hybrids, and offspring of crosses between natural hybrids were planted into the relatively mesic site of origin for I. aggregata and the drier site for natural hybrids. We measured rates of photosynthesis (A max), transpiration (E), instantaneous (A/E) and long-term (δ13C) indices of water use efficiency (WUE), and leaf nitrogen and carbon. We also examined correlations of these traits with plant size. Photosynthetic rate and A/E were higher in vegetative than flowering plants. WUE varied between sites and years, but differences among genotypic classes were spatially and temporally consistent. Instantaneous WUE was higher for F1 hybrids than for the average of the parental species, thereby showing heterosis. There was no evidence of hybrid breakdown, as WUE was no different in the F2 than the average across the F1 and parental species. Nor did WUE depend on cross direction in producing F1 progeny. Carbon isotope discrimination revealed higher long-term water use efficiency in I. tenuituba than I. aggregata. Leaf nitrogen was higher in I. tenuituba than I. aggregata, and higher in offspring of natural hybrids than in the F2. Results indicate heterosis for water use efficiency, with no hybrid breakdown. Heterosis in WUE may help to explain the relatively high survival of both reciprocal F1 hybrids in dry sites within the natural hybrid zone.

Spatial variation in natural selection may play an important role in determining the genetic structure of hybridizing populations. Previous studies have found that F1 hybrids between naturally hybridizing Ipomopsis aggregata and Ipomopsis tenuituba in central Colorado differ in fitness depending on both genotype and environment: hybrids had higher survival when I. aggregata was the maternal parent, except in the centre of the hybrid zone where both hybrid types had high survival. Here, we developed both maternally (cpDNA PCR-RFLP) and biparentally inherited (nuclear AFLP) species-diagnostic markers to characterize the spatial genetic structure of the natural Ipomopsis hybrid zone, and tested the prediction that the majority of natural hybrids have I. aggregata cytoplasm, except in areas near the centre of the hybrid zone. Analyses of 352 individuals from across the hybrid zone indicate that cytoplasmic gene flow is bidirectional, but contrary to expectation, most plants in the hybrid zone have I. tenuituba cytoplasm. This cytotype distribution is consistent with a hybrid zone in historical transition, with I. aggregata nuclear genes advancing into the contact zone. Further, nuclear data show a much more gradual cline than cpDNA markers that is consistent with morphological patterns across the hybrid populations. A mixture of environment- and pollinator-mediated selection may contribute to the current genetic structure of this hybrid system.

Premise of the study Novel nuclear microsatellite markers were developed for the invasive plant Oplismenus undulatifolius (Poaceae) to facilitate studies of invasion dynamics for this recently introduced, high-risk invasive species in North American mid-Atlantic forests. Methods and results Sixteen polymorphic microsatellite loci were isolated and characterized from an Illumina paired-end shotgun library of O. undulatifolius after initial assessment of 48 loci. When screened in three populations, these markers identified two to six alleles per locus. Observed heterozygosity ranged from 0.000 to 1.000. All loci were successfully amplified in the North American native congener O. hirtellus. Conclusions We provide the first nuclear genetic resources available for population genetic studies within the genus Oplismenus. These markers will serve as a critical tool for exploring patterns of gene flow and mechanisms of invasion of O. undulatifolius across its introduced range. These microsatellites should also be suitable for studies in other Oplismenus species.

 Premise of the study: Novel microsatellite primers were developed for the native wildfl ower Ipomopsis aggregata to facilitate ongoing studies of the genetics of local adaptation and patterns of hybridization with closely related species within the genus.  Methods and Results: Thirteen primer sets were successfully developed and tested using populations from the Rocky Mountains of Colorado, USA. The primers amplifi ed di-, tri-, and tetranucleotide repeats with 1–15 alleles per locus. All primers also amplifi ed fragments with varying success in closely related Ipomopsis species and more distant members of the Polemoniaceae.  Conclusions: The polymorphism levels observed across all loci suggest that these microsatellites may be useful for population genetic studies in Ipomopsis , as well as in studies of other related taxa in the Polemoniaceae.

Members of the Ipomopsis aggregata species complex in the phlox family (Polemoniaceae) often hybridize when they occur in sympatry, and thus have been extensively studied to examine processes involved in plant speciation. I developed 12 microsatellite loci in I. aggregata that are also polymorphic in closely related Ipomopsis tenuituba, producing an average of eight alleles per locus in test populations of 12 individuals per species. Several of these markers also successfully amplified in other Ipomopsis species and more distant members of the Polemoniaceae, suggesting they should prove useful for a broad range of evolutionary studies in this widely distributed system.

SummaryThe ocular motility disorder “Congenital fibrosis of the extraocular muscles type 1” (CFEOM1) results from heterozygous mutations altering the motor and third coiled-coil stalk of the anterograde kinesin, KIF21A. We demonstrate that Kif21a knockin mice harboring the most common human mutation develop CFEOM. The developing axons of the oculomotor nerve’s superior division stall in the proximal nerve; the growth cones enlarge, extend excessive filopodia, and assume random trajectories. Inferior division axons reach the orbit but branch ectopically. We establish a gain-of-function mechanism and find that human motor or stalk mutations attenuate Kif21a autoinhibition, providing in vivo evidence for mammalian kinesin autoregulation. We identify Map1b as a Kif21a-interacting protein and report that Map1b−/− mice develop CFEOM. The interaction between Kif21a and Map1b is likely to play a critical role in the pathogenesis of CFEOM1 and highlights a selective vulnerability of the developing oculomotor nerve to perturbations of the axon cytoskeleton.

Changes in chromosome number and structure are important contributors to adaptation, speciation and macroevolution. In flowering plants, polyploidy and subsequent reductions in chromosome number by fusion are major sources of chromosomal evolution, but chromosome number increase by fission has been relatively unexplored. Here, we use comparative linkage mapping with gene-based markers to reconstruct chromosomal synteny within the model flowering plant genus Mimulus (monkeyflowers). Two sections of the genus with haploid numbers ⩾14 have been inferred to be relatively recent polyploids because they are phylogenetically nested within numerous taxa with low base numbers (n=8–10). We combined multiple data sets to build integrated genetic maps of the M. guttatus species complex (section Simiolus, n=14) and the M. lewisii group (section Erythranthe; n=8), and then aligned the two integrated maps using >100 shared markers. We observed strong segmental synteny between M. lewisii and M. guttatus maps, with essentially 1-to-1 correspondence across each of 16 chromosomal blocks. Assuming that the M. lewisii (and widespread) base number of 8 is ancestral, reconstruction of 14 M. guttatus chromosomes requires at least eight fission events (likely shared by Simiolus and sister section Paradanthus (n=16)), plus two fusion events. This apparent burst of fission in the yellow monkeyflower lineages raises new questions about mechanisms and consequences of chromosomal fission in plants. Our comparative maps also provide insight into the origins of a chromosome exhibiting centromere-associated female meiotic drive and create a framework for transferring M. guttatus genome resources across the entire genus.

A long-standing question in evolutionary biology asks whether the genetic changes contributing to phenotypic evolution are predictable. Here, we identify a genetic change associated with segregating variation in flower color within a population of Mimulus lewisii. To determine whether these types of changes are predictable, we combined this information with data from other species to investigate whether the spectrum of mutations affecting flower color transitions differs based on the evolutionary time-scale since divergence. We used classic genetic techniques, along with gene expression and population genetic approaches, to identify the putative, loss-of-function mutation that generates rare, white flowers instead of the common, pink color in M. lewisii. We found that a frameshift mutation in an anthocyanin pathway gene is responsible for the white-flowered polymorphism found in this population of M. lewisii. Comparison of our results with data from other species reveals a broader spectrum of flower color mutations segregating within populations relative to those that fix between populations. These results suggest that the genetic basis of fixed differences in flower color may be predictable, but that for segregating variation is not.

Natural hybridization can produce individuals that vary widely in fitness, depending upon the performance of particular genotypes in a given environment. In a hybrid zone with habitat heterogeneity, differences in physiological responses to abiotic conditions could influence the fitness and spatial distribution of hybrids and parental species. This study compared gas exchange physiology of Ipomopsis aggregata, I. tenuituba, and their natural hybrids in situ and assessed whether physiological differences were consistent with their native environmental conditions. We also produced reciprocal F2s in a greenhouse study to test for cytonuclear effects on water-use efficiency (WUE). The relative performance of natural hybrids and parentals was consistent with their native habitats: I. aggregata at the coolest, wettest locations had the lowest WUE, while hybrids from the most xeric sites had the highest WUE. In hybrids, the mechanism by which both natural and experimental hybrids achieved this high WUE depended on cytotype: those with I. tenuituba cytoplasm had reduced transpiration, while those with I. aggregata cytoplasm had an increased photosynthetic rate, consistent with patterns in the cytoplasmic parent. The high WUE in hybrids may contribute to their high survival in the dry center of the natural hybrid zone, consistent with environment-dependent models of hybrid zone dynamics.

Soil moisture is a key factor affecting plant abundance and distribution, both across and within species. In response to water limitation, plants have evolved numerous morphological, physiological, and phenological adaptations. In both well-watered and water-limited conditions, we identified considerable natural variation in drought-related whole-plant and leaf-level traits among closely related members of the Mimulus guttatus species complex that occupy a diversity of habitats in the field. The self-fertilizing Mimulus nasutus and serpentine-endemic Mimulus nudatus demonstrated the overall greatest tolerance to soil water limitation, exhibiting the smallest reduction in seed set relative to well-watered conditions. This may be due in part to early flowering, faster fruit development, and low stomatal density. In contrast, flowering of coastal M. guttatus was so delayed that it precluded any seed production in water-limited conditions. This range of phenotypic responses to soil water deficit in Mimulus, coupled with developing genomic resources, holds considerable promise for identifying genomic variation responsible for adaptive responses to soil water availability.

Speciation is characterized by the evolution of reproductive isolation between two groups of organisms. Understanding the process of speciation requires the quantification of barriers to reproductive isolation, dissection of the genetic mechanisms that contribute to those barriers and determination of the forces driving the evolution of those barriers. Through a comprehensive analysis involving 19 pairs of plant taxa, we assessed the strength and patterns of asymmetry of multiple prezygotic and postzygotic reproductive isolating barriers. We then reviewed contemporary knowledge of the genetic architecture of reproductive isolation and the relative role of chromosomal and genic factors in intrinsic postzygotic isolation. On average, we found that prezygotic isolation is approximately twice as strong as postzygotic isolation, and that postmating barriers are approximately three times more asymmetrical in their action than premating barriers. Barriers involve a variable number of loci, and chromosomal rearrangements may have a limited direct role in reproductive isolation in plants. Future research should aim to understand the relationship between particular genetic loci and the magnitude of their effect on reproductive isolation in nature, the geographical scale at which plant speciation occurs, and the role of different evolutionary forces in the speciation process.

The plant genus Mimulus is rapidly emerging as a model system for studies of evolutionary and ecological functional genomics. Mimulus contains a wide array of phenotypic, ecological and genomic diversity. Numerous studies have proven the experimental tractability of Mimulus in laboratory and field studies. Genomic resources currently under development are making Mimulus an excellent system for determining the genetic and genomic basis of adaptation and speciation. Here, we introduce some of the phenotypic and genetic diversity in the genus Mimulus and highlight how direct genetic studies with Mimulus can address a wide spectrum of ecological and evolutionary questions. In addition, we present the genomic resources currently available for Mimulus and discuss future directions for research. The integration of ecology and genetics with bioinformatics and genome technology offers great promise for exploring the mechanistic basis of adaptive evolution and the genetics of speciation.

The recombination that follows natural hybridization may produce hybrid genotypes with traits that are intermediate or extreme relative to the parental species, and these traits may influence the relative fitness of the hybrids. Here we examined leaf ecophysiological traits that may influence fitness patterns in a natural plant hybrid zone. We compared the biochemical photosynthetic capacity of Ipomopsis aggregata, I. tenuituba, and early generation hybrids, as well as their photosynthetic responses to varying light and temperature, two abiotic factors found to differ among sites along the hybrid zone. In general, ecophysiological traits expressed in these plants were consistent with their natural habitat, even when grown under common greenhouse conditions. I. tenuituba reached higher photosynthetic rates (A) at higher light levels than I. aggregata, and also had a higher optimal temperature for photosynthesis (Topt). This suite of traits may reflect adaptations to the more exposed, rocky sites where I. tenuituba is found, compared to the more vegetated, mesic I. aggregata site. Hybrids had characters that were largely intermediate or tenuituba-like, but particular individual hybrids were extreme for some traits, including light saturation level, light-saturated A, and Topt. Many of these traits are consistent with adaptations reported for plants found in warm, dry sites, so they may put certain hybrids at an advantage at the relatively xeric center of the natural hybrid zone.

One cause of reproductive isolation is gamete competition, in which conspecific pollen has an advantage over heterospecific pollen in siring seeds, thereby decreasing the formation of F1 hybrids. Analogous pollen interactions between hybrid pollen and conspecific pollen can contribute to post-zygotic isolation. The herbaceous plants Ipomopsis aggregata and I. tenuituba frequently hybridize in nature. Hand-pollination of I. aggregata with pollen from F1 or F2 hybrids produced as many seeds as hand-pollination with conspecific pollen, suggesting equal pollen viability. However, when mixed pollen loads with 50% conspecific pollen and 50% hybrid pollen were applied to I. aggregata stigmas, fewer than half of the seeds had hybrid sires. Such pollen mixtures are frequently received if plants of the two species and F1 and F2 hybrids are intermixed, suggesting that this advantage of conspecific over hybrid pollen reduces backcrossing and contributes to reproductive isolation.