Species-independent, geographical structuring of chloroplast DNA haplotypes in a montane herb Ipomopsis (Polemoniaceae)

Two hypotheses have been proposed to explain the occurrence of hybrid zones between red-flowered Ipomopsis aggregata and white-flowered I. tenuituba . Either local adaptation to hummingbird and hawkmoth pollinators has given rise to sympatric (or parapatric) divergence of flower colour and morphology (primary intergradation at hybrid zones), or alternatively two previously allopatric species are coming into contact at several geographical areas of secondary intergradation. We examined restriction site patterns in nuclear DNA (nrDNA), chloroplast DNA (cpDNA) and mitochondrial DNA (mtDNA) from populations of I. aggregata and I. tenuituba representing seven zones of sympatry. No variation was detected in a 350-bp fragment of mtDNA and uninformative levels of variation were observed for nrDNA. We detected 22 potentially informative restriction site polymorphisms in cpDNA, all of which united geographical areas containing populations of both species. We detected no informative species-specific markers. Studies of other species (e.g. oaks) have detected similar species-independent geographical structure of cpDNA. However, in these cases secondary interegradation could be inferred from species-specific nuclear alleles. The pattern in Ipomopsis is consistent with both primary intergradation (independent speciation in each area of sympatry) or secondary intergradation involving complete cytoplasmic replacement. Thus, additional data are needed to explain the origin of hybrid zones in Ipomopsis .     

Multiple paternity in fruits of Ipomopsis aggregata (Polemoniaceae)

Two different mechanisms can result in multiple paternity within fruits: deposition of a mixed pollen load due to carryover of pollen from flower to flower and multiple pollinator visits in close succession. I investigated the extent of multiple paternity within fruits of Ipompsis aggregata containing from 2 to 14 seeds. A paternity analysis based on ten polymorphic isozyme markers revealed multiple paternity in a minimum of 68% (based on simple paternity exclusion) and up to 100% (based on identification of the most likely father) of the multiseeded fruits. The estimated number of fathers increased with the number of seeds in a fruit, with an average of four sires, and up to nine sires, represented in a single fruit. To explore whether this level of multiple paternity could be explained solely by simultaneous deposition of a mixed pollen load, I constructed a computer simulation model based on previous measurements of movement patterns and pollen carryover by the hummingbird pollinators. Model predictions provided a good match to observed values for number of sires per fruit. Thus, the extensive pollen carryover in this species and consequent mixed pollen loads can explain the high levels of multiple paternity in natural populations.     

Maternal flower color,ultraviolet protection,and germination in Ipomopsis aggregata (Polemoniaceae)

Transgenerational interactions between flower color, seed quality, and seedling performance have rarely been investigated. The ecological model, Ipomopsis aggregata, is a great candidate for examining the maternal effects of flower color because it is a mostly scarlet-flowering plant which shows color polymorphism within natural populations. Anthocyanin, the red flavonoid pigment which gives these flowers color, has been shown to act as an ultraviolet (UV) protectant by shielding chloroplasts and acting as an antioxidant. This study was conducted on scarlet- and fuchsia-flowering maternal plants and their seeds from natural populations in Colorado. Dark-flowering (scarlet) maternal plants from these populations had consistently higher foliar anthocyanin content, photosystem efficiency, and chlorophyll content than light-flowering (fuchsia) plants over a 3-year period in the field. Seeds from a subset of these maternal plants were counted, weighed, and germinated in a growth chamber. Photosystem efficiency, vegetative anthocyanin content, chlorophyll content, and biomass were measured on germinated seedlings after the germination census was completed. Dark-flowering maternal plants yielded seeds and seedlings with higher biomass than light-flowering ones. Seeds from dark-flowering maternal plants also germinated faster than those from light-flowering maternal plants and seedlings had higher vegetative anthocyanin content. The hereditary nature of anthocyanin content thus suggests that higher anthocyanin levels (both floral and vegetative) are potentially linked to measures of fitness such as increased seed weight, germination rate, and seedling biomass. These data suggest that UV protection provided by anthocyanins potentially increases the realized fitness of maternal plants, thereby influencing life history.     

Patterns of color and nectar variation across an Ipomopsis (Polemoniaceae) hybrid zone

Hybridization may uncouple adaptive trait combinations that are present in parental species. I studied variation in flower color and reward quality across a hybrid zone of Ipomopsis aggregata and I. tenuituba. Individuals from hybrid populations showed considerable variation in flower color using corolla reflectance measurements. Flower spectra of such individuals were either intermediate or else resembled those flowers from the parental species. Ipomopsis aggregata populations had consistently higher nectar production rates and higher nectar standing crops than either I. tenuituba or hybrids. Ipomopsis aggregata flowers also produced more dilute nectar than those of hybrids and I. tenuituba, but the actual concentration values were quite variable among populations of the same type. Overall, the nectar quality of hybrid flowers most resembled that of I. tenuituba flowers. Based on the observed interpopulation patterns of color-reward associations in this hybrid zone, pollinators should be able to discriminate against I. tenuituba and hybrid populations and against most individuals within hybrid populations. However, they may visit those hybrids that resemble the most rewarding flower type (I. aggregata). The results emphasize the need for studies that address how hybridization affects subsequent plant fitness and the evolutionary dynamics of the species involved.     

PERMANENT GENETIC RESOURCES: Identification of nuclear microsatellite loci for Ipomopsis aggregata and the distribution of pairwise relatedness in a natural population

Nine microsatellite loci were developed from enriched libraries of scarlet gilia (Ipomopsis aggregata). A screen of 160 individuals from a population identified reduced levels of heterozygosity, low levels of relatedness, and weak spatial genetic patterns. The population inbreeding coefficient was calculated to be 0.19 (SE = 0.04). These patterns are consistent with those expected from low levels of biparental inbreeding in an obligate outcrosser and extensive seed and pollen dispersal. These preliminary data confirm the usefulness of microsatellite markers for breeding system studies of I. aggregata.     

Pollination of Ipomopsis aggregata (Polemoniaceae): effects of intra- vs. interspecific competition

Although plants may simultaneously experience intra- and interspecific competition for pollination, their relative strength has rarely been experimentally evaluated. Yet because intra- and interspecific competition can be caused by different mechanisms, their effect on the ecology and evolution of plants may differ. To determine the relative strength of intra- and interspecific competition for pollination, I manipulated the presence of heterospecifics and density of conspecifics using Ipomopsis aggregata as the focal species. All plots contained I. aggregata and Castilleja linariaefolia, but C. linariaefolia inflorescences were removed from half of the plots to create the heterospecifics-absent treatment. Within each plot, all I. aggregata inflorescences were removed from a 5-m radius around a focal plant to create a low conspecific density experimental unit, and a group of 12 I. aggregata plants/1 m² was designated as a high conspecific density unit. Conspecific pollen deposition was reduced when C. linariaefolia was present but was not influenced by I. aggregata density. Although seed set per fruit was reduced by 17% when C. linariaefolia was present, it was not significantly influenced by either treatment. Interspecific competition for pollination is stronger than intraspecific competition in the I. aggregata–C. linariaefolia system, but neither process appears to influence plant fitness.     

Variation in frequency of hybrids and spatial structure among Ipomopsis (Polemoniaceae) contact sites

Rates of hybridization vary among angiosperm taxa. Among-taxon variation in hybridization rate has been used to compare the importance of pre- and post-zygotic reproductive isolating mechanisms. Variation in rates of hybridization within a single-species pair would suggest that local conditions also affect reproductive isolation within a single taxonomic context. In this study, contact sites of Ipomopsis aggregata-Ipomopsis tenuituba were surveyed for variation in frequency of hybrids, and spatial structure. Floral morphology was used to identify parent species and hybrids in seven contact sites in the western Rocky Mountains, USA. Contact sites varied widely in elevational range, the degree to which morphological variation was clinal rather than mosaic and the frequency of hybrids. Two sites provided a strong contrast between a clinal, unimodal site and a mosaic, bimodal site. This natural variation among contact sites of the same species pair provides an opportunity to assess the effect of local ecological conditions and spatial structure of parent populations on reproductive isolation, while controlling for between-taxon variation.     

Absence of conspecific pollen advantage in the dynamics of an Ipomopsis (Polemoniaceae) hybrid zone

The frequency of hybrid formation in angiosperms depends on how often heterospecific pollen is transferred to the stigma and on the success of that heterospecific pollen at fertilizing ovules. Even if heterospecific pollen is capable of effecting fertilization it may perform poorly when conspecific pollen is also available on the stigma. We applied pollen mixtures to stigmas to determine how pollen interactions affect siring success and the frequency of hybrid formation between two species of Ipomopsis (Polemoniaceae) in Colorado. Plants of both parental species and natural hybrids were pollinated with I. aggregata and I. tenuituba pollen in ratios of 100:0, 80:20, 50:50, 20:80, and 0:100 by mass. Plants were homozygous for different alleles at an isozyme marker, allowing us to distinguish the type of pollen parent for 2166 viable seeds from 273 fruits. In contrast to studies of many other hybridizing taxa, there was no evidence of an advantage to conspecific pollen, nor did composition of the stigmatic pollen load affect seed set. Instead, the frequency of seeds sired by a given species was proportional to its representation in the pollen load. In this hybrid zone, both the frequency of first-generation hybrid formation and the relative male fitness of the two parental species should be predictable from the rates of pollen transfer to stigmas.     

Competition for pollination influences selection on floral traits of Ipomopsis aggregata

Although rarely tested, it is often assumed that interspecific competition results in the divergence of traits related to resource use. Using a plant-pollinator system as a model, I tested the prediction the presence of a competitor for pollination influences the strength and/or direction of pollinator-mediated selection on floral traits. I measured phenotypic selection via female fitness on five floral traits of Ipomopsis aggregata in seven populations. Four contained only conspecifics (I only) and three also contained the competitor Castilleja linariaefolia (C + I). Directional selection via fruits/plant and conspecific pollen deposited/flower on corolla length was positive and significantly stronger in C + I populations. This difference in selection was apparently driven by interpopulation variation in the degree to which reproduction of I. aggregata was pollen limited. Consistent with expectations of interspecific competition, I. aggregata plants in C + I populations received less conspecific pollen per flower and set fewer seeds per fruit and fruits per plant than those in I only populations. Ipomopsis aggregata's corollas were also significantly longer in C + I populations, suggesting that there had been a response to a similar selective regime in past generations. Phenotypic correlations between corolla length and width, which determine the variation in I. aggregata's flower shape, were significantly weaker in C + I populations. These data suggest that competition for pollination can influence the strength of selection on and patterns of correlations among floral traits of I. aggregata. If I. aggregata populations with and without competitors for pollination are linked by gene flow, then measuring selection in competitive and noncompetitive environments maybe necessary to accurately predict how floral traits will evolve.     

Regrowth following ungulate herbivory in Ipomopsis aggregata: geographic evidence for overcompensation

Studies were conducted on eight populations of scarlet gilia, Ipomopsis aggregata, across Colorado and in northern Arizona, to assess the fitness consequences of natural and simulated herbivory. To date, geographic studies have failed to incorporate treatment groups that included naturally browsed plants along with clipping treatments. The results presented here clearly demonstrate the importance of assessing, a priori, whether or not clipping experiments accurately reflect natural patterns of herbivory. Although because of the timing of the clip no evidence of overcompensation was found in any of the Colorado populations when ungulate herbivory was simulated in experimental clipping treatments, evidence for overcompensation was found in more than half the populations when plants that were naturally browsed and matched for size using root diameters were included early in the season. Matching plants for size based on root diameter late in the season would be problematic because root diameters significantly increased in size in all eight populations following high levels of ungulate herbivory and/or experimental clipping. Results from this study, and other recent studies on another biennial herb, the field gentian, clearly demonstrate that overcompensation is not only a real phenomenon but also is more widespread, both taxonomically and geographically, than previously thought. Received: 15 July 1998 / Accepted: 9 November 1998     

Characterization of six microsatellite loci in Echium vulgare (Boraginaceae)

Echium vulgare is a tetraploid plant with a very low selfing rate in the field. We suspect that cryptic self incompatibility plays a role in this species. In order to show lower success of self pollen/selfed embryos, after pollination with a mixture of self and outcross pollen, a paternity analysis has to be done. For the purpose of such analysis we developed microsatellites in E. vulgare. In this study, we report on six microsatellite loci which are easy to score, polymorphic, with a number of alleles per locus ranging from two to eight and, therefore, suitable for paternity analysis.     

Floral scent in natural hybrids of Ipomopsis (Polemoniaceae) and their parental species

Background and Aims

Floral traits, such as floral volatiles, can contribute to pre-zygotic reproductive isolation by promoting species-specific pollinator foraging. When hybrid zones form, floral traits could also influence post-zygotic isolation. This study examined floral volatiles in parental species and natural hybrids in order to explore potential scent mediation of pre-zygotic and post-zygotic isolation.

Methods

Floral bouquets were analysed for the sister species Ipomopsis aggregata and I. tenuituba and their natural hybrids at two contact sites differing in both hybridization rate and temporal foraging pattern of hawkmoth pollinators. Floral volatiles were quantified in diurnal and nocturnal scent samples using gas chromatography–mass spectrometry.

Key Results

The bouquets of parental species and hybrids showed qualitative overlap. All flowers emitted similar sets of monoterpenoid, sesquiterpenoid, aliphatic and benzenoid compounds, but separated into groups defined by multivariate analysis of quantitative emissions. The parental species differed most strikingly in the nitrogenous compound indole, which was found almost exclusively in nocturnal bouquets of I. tenuituba. Natural hybrid bouquets were highly variable, and showed emission rates of several compounds that appeared transgressive. However, indole emission rates were intermediate in the hybrids compared with rates in the parents. Volatile bouquets at the contact site with lower hybridization did not show greater species specificity in overall scent emission, but I. tenuituba presented a stronger indole signal during peak hawkmoth activity at that site.

Conclusions

The two species of Ipomopsis differed in patterns of floral bouquets, with indole emitted in nocturnal I. tenuituba, but not in I. aggregata. Natural hybrid bouquets were not consistently intermediate between the parents, although hybrids were intermediate in indole emission. The indole signal could potentially serve as a hawkmoth attractant that mediates reproductive isolation both before and after hybrid formation.     

Characterization of microsatellite loci in Sacramento perch (Archoplites interruptus)

We characterized 23 polymorphic tetranucleotide microsatellite loci for Sacramento perch (Archoplites interruptus). This species is extirpated in its native range, the Sacramento–San Joaquin Delta (California, USA), and is therefore targeted for recovery. A concerted effort is currently underway to re‐establish self‐sustaining populations of Sacramento perch in its native range. These microsatellites will be used to analyse the population structure of the species and, in conjunction with life history and physiological data, develop a comprehensive recovery plan.     

Characterization of 25 microsatellite loci in bowhead whales (Balaena mysticetus)

Bowhead whales (Balaena mysticetus) experienced a severe demographic population bottleneck caused by commercial whaling that ceased in 1914. Aboriginal subsistence whale harvests have continued and are managed by the International Whaling Commission. In an effort to provide management advice for bowhead whales, 25 microsatellite loci were isolated from genomic DNA libraries. This panel of markers will be utilized to analyse stock structure hypotheses of current bowhead whale populations.     

Characterization of microsatellite loci in Schoenoplectus americanus (Cyperaceae)

Schoenoplectus americanus is a model organism for studying ecological and ecosystem responses of salt marsh plant communities to global climate change. Here we characterize 16 microsatellite loci in S. americanus to facilitate studies on the genetic basis of phenotypic responses to changing climate conditions such as elevated atmospheric carbon dioxide. Most loci also amplified in the morphologically similar sister species, Schoenoplectus pungens. Five loci exhibited species‐specific alleles or distinct allelic size distributions that discriminate S. americanus from S. pungens.     

Isolation and characterization of microsatellite loci in hybridizing California and Gambel's quail (Callipepla californica and C. gambelii)

Callipepla californica and C. gambelii are sibling species of quail that hybridize throughout Southern California where their ranges overlap. We developed seven highly polymorphic microsatellite markers that will be used to assess introgression between these species. Numbers of alleles ranged in C. californica from seven to 24 and in C. gambelii from five to 18, with high expected levels of heterozygosity in both species. Alleles exclusive to each species are present at most loci.     

Characterization of microsatellite loci in Coenonympha hero (Lepidoptera: Nymphalidae)

Microsatellite markers are suitable tools for studying dispersal pattern among local populations. I report on the characterization of seven polymorphic microsatellite loci in the scarce heath butterfly (Coenonympha hero), from one unenriched and one enriched genomic library. Number of alleles ranged from two to 20 when 108 individuals from seven populations were screened. H_O ranged from 0.140 to 0.889. Primers were also tested on three other butterfly species. Amplification was successful for all loci in Erebia triaria, while only one gave products in Maculinea alcon and Maculinea rebeli. To my knowledge, these are the first microsatellite markers published in the Nymphalidae subfamily Satyrinae.     

Characterization of eight microsatellite loci in Steller sea lions (Eumetopias jubatus)

Steller sea lions (Eumetopias jubatus) are listed as an endangered species in western Alaska and have exhibited a significant population decline throughout their range. Eight microsatellite loci were isolated from genomic DNA libraries. In addition, all these markers were found to be variable in nine individuals of the California sea lion (Zalophus californicus). This panel of markers was developed to analyse population structure in Steller sea lions throughout their range.