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.     

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.     

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.     

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.     

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 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.     

Reproductive isolation and hybrid pollen disadvantage in Ipomopsis

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.     

Characterization of eight polymorphic microsatellite loci for the giant grouper (Epinephelus lanceolatus Bloch)

Eight polymorphic microsatellite loci were isolated and characterized using a small insert genomic DNA library for the giant grouper (Epinephelus lanceolatus Bloch, 1790), a commercially valuable marine fish in tropical waters. They showed polymorphism information content ranging from 0.177 to 0.775, allele numbers ranging from two to 10, effective allele numbers ranging from 1.227 to 5.012, and observed and expected heterozygosities from 0.2 to 0.733 and from 0.185 to 0.801, respectively, which we anticipate will be useful for population genetic studies of the giant grouper.     

Characterization of 13 microsatellite marker loci in Verreaux's sifaka (Propithecus verreauxi)

The Verreaux's sifaka (Propithecus verreauxi) is one of the species of Propithecus, living in the dry forest of southwest Madagascar. This species is endangered due to the loss and fragmentation of its natural habitat, a consequence of deforestation. Thirteen novel nuclear microsatellite loci were isolated and characterized in three populations of Verreaux's sifaka. The marker suite proved informative with an average of 8.9 alleles per locus and observed heterozygosity across the three populations of 0.675.     

Characterization of microsatellite loci in the endangered grand spider orchid Caladenia huegelii (Orchidaceae)

The orchid genus Caladenia is species rich with many threatened and endangered taxa. We report on the isolation and characterization of microsatellite loci in the rare Caladenia huegelii for molecular evaluation of this and the closely related C. thinicola as part of the development of conservation initiatives. Eight di‐ and trinucleotide loci were screened using 30 samples from each species. All loci were highly variable, with similar levels of heterozygosity and number of alleles across both species. These markers will be highly informative for population studies in both species.     

Characterization of microsatellite loci in Amphitmetus transversus (Kolbe, 1897) (Coleoptera,Curculionidae)

We developed six microsatellite markers for Amphitmetus transversus (Kolbe, 1897) (Coleoptera, Curculionidae), a beetle widely distributed in the guineo‐congolian rainforests of east Africa. The numbers of alleles per locus range from three to eight, with a mean number of 4.6. They will be used for population genetic analysis. The ongoing study concentrates on the genetic differentiation and variability of isolated populations in the Kakamega Forest, a highly degraded and fragmented rainforest remnant in western Kenya.     

Characterization of 15 microsatellite loci in the pulmonate snail Pseudosuccinea columella (Mollusca, Gastropoda)

We characterized 15 new variable microsatellites in the freshwater snail Pseudosuccinea (Lymnaea) columella, as well as conditions for multiplexing and simultaneously genotyping sets of loci. Two to six alleles were detected per locus over the six populations studied. Gene diversity ranged from 0.000 to 0.498, but essentially no heterozygous individuals were observed. This resulted in extremely high F(IS) estimates, and therefore high selfing rates. The F(ST) estimates ranged from 0.18 to 1 among populations, but was generally high. These markers will constitute efficient tools for investigating the population structure of this invasive species. Cross-species amplification was on the whole unsuccessful.     

Characterization of microsatellite loci in the subsocial spider Stegodyphus lineatus (Araneae: Eresidae)

Stegodyphus lineatus spiders live in groups consisting of closely related individuals. There appears to be no discrimination against related individuals as mates but females mate multiply, despite the fact that matings are shown to carry a cost. We have developed eight polymorphic dinucleotide microsatellite markers that allow us to assess levels of heterozygosity and relatedness among individuals of this species. These molecular markers are likely to prove highly effective tools for estimating levels of inbreeding and thus allow us to test hypotheses about the relationships between social structure, mating strategies and inbreeding avoidance.     

Asymmetrical pollen success in Ipomopsis (Polemoniaceae) contact sites

Variation in hybridization rates among contact sites of a species pair provides an opportunity for assessing the importance of individual reproductive isolating mechanisms in limiting gene flow between species and thus promoting speciation. Conspecific pollen advantage is common in angiosperms, but its importance as a reproductive isolating mechanism is uncertain. We compared the strength of conspecific pollen advantage in two Ipomopsis aggregata-I. tenuituba (Polemoniaceae) contact sites that differ in frequency of natural hybrids. We performed hand pollinations of single- and 1:1 mixed-species pollen loads, using donor and recipient plants from both contact sites. Paternity of offspring from mixed-species pollinations was determined using an allozyme marker. Donors from the high frequency hybrid site showed no conspecific pollen advantage; both species sired seeds in proportion to their fraction of the pollen load (0.5). In contrast, I. aggregata from the low frequency hybrid site sired 70-85% of offspring on recipients from both sites. These results suggest that pollen interactions can influence the level of natural hybridization. They also suggest the importance of geographic variation in reproductive isolation, which should be considered in studies of biological invasions and exposure of engineered crops to wild relatives.