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1 Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, California 93106 USA
Received for publication August 27, 1998. Accepted for publication June 10, 1999.
ABSTRACT
Many species exhibit reduced siring success of self-relative to outcross-pollen donors. This can be attributed either to postfertilization abortion of selfed ovules or to cryptic self-incompatibility (CSI). CSI is a form of self-incompatibility whereby the advantage to outcross pollen is expressed only following pollinations where there is gametophytic competition between self and outcross pollen. Under the definition of CSI, this differential success is due to the superior prefertilization performance (pollen germination rate and pollen tube growth rate) of outcross pollen relative to self pollen. Although CSI has been demonstrated in several plant species, no studies have assessed among-population variation in the expression of CSI. We conducted a greenhouse study on Clarkia unguiculata (an annual species with a mixed-mating system) to detect CSI, and we compare our observations to previous reports of CSI in C. gracilis and another population of C. unguiculata. In contrast to these previous studies of CSI in Clarkia, we used genetic rather than phenotypic markers to measure the relative performance of selfed vs. outcross pollen. In this study, we measured the intensity of CSI in C. unguiculata from a large population in southern California and we determined whether the magnitude of pollen competition (manipulated by controlling the number of pollen grains deposited on a stigma) influenced the outcome of competition between self and outcross pollen. In contrast to previous investigations of Clarkia, we found no evidence for CSI. The mean number of seeds sired per fruit did not differ between self and outcross pollen following either single-donor or mixed pollinations. In addition, the relative success of selfed vs. outcross pollen was independent of the magnitude of pollen competition. These results suggest that: (1) one of the few nonheterostylous species previously thought to be cryptically self-incompatible is completely self-compatible (at least in the population studied here) or (2) phenotypic markers may be problematic for the detection of CSI.
Key Words: Clarkia • cryptic self-incompatibility • gametophytic competition • mating system • Onagraceae • outcrossing • pollen performance • selfing • siring success
Cryptic self-incompatibility (CSI) is a type of self-incompatibility resulting in higher rates of ovule fertilization by outcross relative to self pollen following pollinations in which both types of pollen are deposited on a stigma (Bateman, 1956
). In contrast to self-incompatible species, cryptically self-incompatible species produce fruits with full seed set following self-pollination (Bateman, 1956
), but outcross pollen sires more seeds per fruit than self pollen when the two types of pollen are applied simultaneously to a stigma. Previous studies have found that, in species with CSI, outcross donors sire an average of 76–92% of the seeds per fruit following pollinations with equal amounts of self and outcross pollen (Bateman, 1956
; Weller and Ornduff, 1977
; Bowman, 1987
). The incompatibility of self pollen is regarded as cryptic because there is a reduction in the siring success of self pollen only when there is gametophytic competition between self and outcross pollen.
CSI has been documented in a relatively small number of plant species. In several species, self pollen germinates more slowly or produces pollen tubes that grow more slowly than outcross pollen tubes (Waser et al., 1987
; Weller and Ornduff, 1989
; Cruzan, 1989
; Aizen, Searcy, and Mulcahy, 1990
; Eckert and Allen, 1997
). In a handful of studies designed to detect differences between the siring success of self vs. outcross pollen, self donors sired fewer seeds per fruit than did outcross donors. These siring success differences indicated CSI because there were no differences in seed set when self and outcross pollen were applied to separate flowers (Bateman, 1956
; Weller and Ornduff, 1977
; Bowman, 1987
; Eckert and Barrett, 1994
; Jones, 1994
). Studies of two heterostylous species, Amsinckia grandiflora (Weller and Ornduff, 1977, 1989
) and Decodon verticillatus (Eckert and Barrett, 1994
; Eckert and Allen, 1997
) measured both pollen performance (a combination of pollen germination rate, pollen tube growth rate, and ovule fertilization rate) and siring success of self and outcross donors. These studies also were able to rule out inbreeding depression as an explanation for siring success differences by comparing intra- and inter-morph outcross siring success. In both species, reduced pollen performance of self relative to outcross pollen was correlated with lower percentages of selfed seeds per fruit following pollinations with mixed (self and outcross) pollen loads. By contrast, no differences in seed production were detected between self and outcross single-donor pollinations, supporting CSI as the process observed. Moreover, intra-morph pollen sired fewer seeds relative to inter-morph pollen following mixed pollinations in Amsinckia grandiflora, suggesting that outcrossing is favored even in the absence of inbreeding effects on seed development (Weller and Ornduff, 1977
).
The evolutionary mechanisms that lead to the evolution of CSI are poorly understood. CSI may evolve in response to inbreeding depression. In species with a significant genetic load, individual plants that avoid selfing and the production of inferior offspring due to inbreeding depression will have higher fitness than those that self, assuming that pollen is not limiting to seed production. However, this does not explain why cryptically self-incompatible species do not express complete self incompatibility. There may be selection for CSI in species that experience variable spatial or temporal availability of outcross pollen (Bowman, 1987
; Cruzan and Barrett, 1993
; Eckert and Allen, 1997
). Due to the facultative nature of outcrossing in CSI species, they can still reproduce (by selfing) in environments or during time periods that are characterized by little pollen movement between plants. However, CSI also ensures that maternal plants receiving a mixture of self and outcross pollen will have a higher proportion of seeds sired by outcross donors than if they were completely self-compatible. In other words, CSI provides reproductive assurance when pollen is limiting, while it minimizes selfing when stigmas are saturated with pollen.
The intensity of CSI can be defined as the deviation from 1:1 of the ratio of outcross siring success to self-siring success when equal amounts of both types of pollen are placed on a stigma. This ratio ranges from 3.2 to 11.5 among cryptically self-incompatible species (Bateman, 1956
; Weller and Ornduff, 1977
; Bowman, 1987
). The variation in this ratio among conspecific populations, however, is unknown. There are reasons to believe that the intensity of CSI is likely to differ among populations. For example, the advantage of outcross relative to self pollen may well vary among populations that differ in the amount of outcross pollen typically deposited on stigmas. Alternatively, populations that differ in their genetic load will differ in the strength of selection favoring processes (such as CSI) that lower the rate of inbreeding. Environmental differences among populations could also lead to differences among populations in the intensity of selection on CSI. In environments in which inbreeding depression is relatively high, CSI may be expected to evolve more rapidly or to a higher intensity. To date, our knowledge of cryptically self-incompatible species is based exclusively on studies of single populations. Information concerning differences among populations in the intensity of CSI would be of great interest because it may shed light on the mechanisms that promote its evolution.
In this study, we measure the degree of CSI in Clarkia unguiculata L. (Onagraceae), one of the few nonheterostylous species determined to be cryptically self-incompatible. Two studies of Clarkia have detected that gametophytic competition between self and outcross pollen results in differential siring success. In a study of 25 hand-pollinated fruits representing 18 plants of C. unguiculata, Bowman (1987)
found that, on average, outcross pollen sires 85% of the seeds per fruit when applied in equal proportions with self pollen to receptive stigmas. This result corresponds to an intensity of CSI equivalent to a 5.7:1 ratio, well within the range of values reported by Bateman (1956)
. Bowman (1987)
concluded that C. unguiculata is cryptically self-incompatible, because self donors sire a reduced number of seeds when in competition with outcross pollen while single-donor self-pollinations produced equal numbers of seeds as outcross pollinations. Similarly, in a study of 72 hand-pollinated fruits representing 72 plants of Clarkia gracilis, outcross donors sired 77.4% of the seeds per fruit (corresponding to an intensity of CSI equivalent to a 3.4:1 ratio) following pollinations with equal mixtures of self and outcross pollen (Jones, 1994
).
In our study, we used genetic markers to determine the siring success of self vs. outcross pollen. We also designed the experiment reported here to distinguish between the relative roles of prezygotic and postzygotic mechanisms in determining the siring success of self vs. outcross pollen. Our objectives were: (1) to measure the intensity of CSI in C. unguiculata from a large population of C. unguiculata in southern California, (2) to determine whether there are differences between self and outcross pollen in pollen performance or in the rate of seed abortion, and (3) to determine whether the magnitude of pollen competition (controlled by manipulating the number of pollen grains deposited on a stigma) influenced the outcome of competition between self and outcross pollen.
MATERIALS AND METHODS
Clarkia unguiculata (Onagraceae) is an annual hermaphroditic herb that grows throughout central and southern California in disturbed roadsides and in oak woodland (Lewis and Lewis, 1955
). The showy, purple to pink flowers are visited by native bees, bumble bees, honey bees, and occasionally hummingbirds (MacSwain, Raven, and Thorpe, 1973
). Vasek (1965)
found that C. unguiculata has one of the highest outcrossing rates (96%) of any species in the genus. Despite mechanisms (e.g., protandry and herkogamy) promoting outcrossing in C. unguiculata, selfing is still possible in this species. Pollen transfer between flowers within a plant (geitonogamy) may often lead to pollinations with mixed pollen loads of self and outcross pollen. It is common to observe individual C. unguiculata plants in the field with multiple flowers at different developmental stages open at the same time, providing the opportunity for geitonogamy.
The parental plants used in this experiment were grown from seed in the glasshouse facilities at Cornell University in September 1997. The seeds were collected from a single population of C. unguiculata located in an oak woodland in Santa Barbara County, California (Fremont campground: 34°32'30'' N by 119°50'0'' E, elevation 350 m; San Marcos Pass USGS quadrangle—Santa Barbara County, California). The population from which the seeds of experimental plants were collected included over a thousand individuals. Several other large populations were within a 4-km radius of the sampled population. The maternal plants from which seeds were collected occupied a partly shaded, rocky hillside near a gap in the oak woodland and were visited regularly by bumble bees and smaller native bees during the day (S. E. Travers, personal observation).
Following collection from the field, seeds were stored in paper envelopes at room temperature for 2 mo and were then sown in 20 cm long tapering plastic tubes 2–3 cm in diameter and filled with fritted clay. Seedling tissue was sampled in October 1997 for allozyme analysis. The seedlings were ~10 cm tall when their leaves were sampled.
Seedlings from 55 maternal families (ten seeds per family) were screened for their PGI genotype using cellulose acetate gel electrophoresis (CAGE). Only those plants that were homozygous for B or C alleles at the Pgi-c1 locus (Gottlieb and Ford, 1996
) were used as pollen donors and recipients in this experiment.
The enzyme extraction methodology and extraction buffer were identical to those used by Gottlieb and Greve (1981)
. Ten microlitres of extraction solution were placed in each well of the CAGE application plate for application to the gel. Twelve seedlings were scored per gel. Each gel measured 76 x 76 mm and was soaked in running buffer for a minimum of 30 min prior to electrophoresis. We used a tris-glycine (pH = 8.3) buffer solution (Hebert and Beaton, 1994
). The gels were run for 20 min at 200 V. Gel staining was accomplished with agar overlays. Stain recipes were those of Hebert and Beaton (1994)
.
Twenty-eight homozygous plants (14 BB and 14 CC genotypes) were identified for use in this experiment, the design of which consisted of 14 pairs of plants. Each pair included one pollen recipient and one outcross donor. We established seven pairs of parental plants with BB recipients and CC donors, and seven pairs with CC recipients and BB donors (the two plants in each pair were homozygous for different alleles at the Pgi locus). All 28 pollen donors and recipients represented different field-collected maternal sibships.
Mixed pollinations of flowers produced by the fourteen maternal plants were performed with variable amounts of pollen in the pollen load. By varying the number of pollen grains in mixed pollen loads, we sought to distinguish between differential pollen performance and differential abortion of embryos as the cause of observed differences in siring success between donors (Bertin, 1990
; Cruzan and Barrett, 1996
). If outcross pollen donors sire more seeds than self-pollen donors due exclusively to superior pollen performance (faster pollen germination or pollen-tube growth), then the proportion of seeds per fruit sired by the outcross donor following mixed pollinations of self and outcross pollen should increase with increasing pollen load. If outcross pollen sires more seeds per fruit than self pollen solely as a result of differential abortion of embryos sired by self donors, the siring success of outcross donors should not increase with pollen load (Bertin, 1990
; Cruzan and Barrett, 1996
).
Mixed pollinations were conducted using one of three pollen loads: high, medium, or low. Each pollen load treatment was replicated three times in each maternal plant, with each treatment applied in a cyclical manner among consecutive flowers. In all pollen load treatments, the ratio of self to outcross pollen was 1:1. In the high and medium pollen load treatments, we maintained equal proportions of self and outcross pollen by stripping the pollen from equal lengths of self and outcross anther sacs with a dissecting needle. There is a strong positive relationship between anther sac length and the number of pollen grains contained within that length of anther sac (y = 180.5x - 23.4, df = 27, R2 = 0.955, P < 0.001; S. E. Travers, unpublished data). High pollen loads consisted of the pollen from one 4-mm section of anther sac from each of the competing donors. Medium pollen loads contained pollen from 1-mm sections of outcross and self anther sacs. In the low pollen load treatment, pollen grains were counted visually. Each low pollen load consisted of 40 self-pollen grains and 40 outcross-pollen grains. Given that C. unguiculata flowers contain 40–120 ovules (S. E. Travers, personal observation), the low pollen load treatment generally represents an absence of gametophytic competition. In all three pollen load treatments, pollen collected from the self and outcross donor in each pair was placed on a glass microscope slide and mixed with a dissecting needle for 30 s.
To determine the exact size of the pollen load applied to each stigma, we collected the pollinated stigmas 24 h after pollination and stored them in alcohol. The stigmas were then stained with analine blue (0.2 g analine blue and 0.02 g ethidium bromide in 200 mL of 0.1 mol/L K3PO4) for 24 h and squashed on a glass slide for observation. The number of pollen grains per stigma was counted using bright-field microscopy on an Olympus binocular microscope at 100x magnification.
In addition to mixed pollinations, we also conducted two types of single-donor pollinations per plant pair. Self- and outcross-pollen grains were each applied to separate stigmas on the same plant. These types of pollinations are referred to as "self" and "outcross" pollinations, respectively. In these pollinations, stigmas were saturated with pollen by brushing dehiscent anthers across the stigma surface. One self and one outcross single-donor pollination were conducted on each maternal plant.
The ovaries of recipient flowers in all single-donor and mixed pollinations were collected once they began to turn from green to tan, but before they began to dehisce. We counted the number of viable, aborted, and undeveloped seeds in each fruit. Viable seeds (mean mass = 4.7 x 10-4 g) weigh more than aborted (1.1 x 10-4 g) or undeveloped seeds (0.054 x 10-4 g) and have a dark seed coat. Aborted seeds have dark seed coats but lack a swollen endosperm. Undeveloped seeds are yellow and flat relative to aborted and viable seeds. In addition to counting the numbers of seeds per fruit, we measured the mass of ten viable seeds from each cross.
We determined the siring success of self and outcross donors following each mixed pollination by scoring the Pgi genotypes of a sample of 20 seeds from each fruit collected. Seeds were taken from both basal and distal halves of each fruit to eliminate potential biases due to position effects on siring success. If the total number of viable seeds per fruit was fewer than 20, all viable seeds in that fruit were scored. Fruits with fewer than five viable seeds were considered abnormal and excluded from further analysis. The siring success of outcross donors (SSO) was calculated as the percentage of seeds per fruit that were scored for the Pgi-c1 gene and that were sired by the outcross donors.
Cellulose acetate gel electrophoresis was used to score the Pgi genotype of seeds. The seeds were first hydrated on wet filter paper in petri dishes in the dark at 4°C for 24 h. We then extracted proteins from each of the seeds by crushing them in 10 µL of extraction buffer. Twelve seeds were run per gel according to the protocol described above for the parental plants.
Analyses
Because the data were not normally distributed, we used the nonparametric Kruskal-Wallis test to compare the mean number of aborted seeds per fruit among three categories of pollinations: single-donor self, single-donor outcross, and mixed pollinations. Similar analyses were conducted with the number of viable seeds per fruit and mean mass per seed as dependent variables. Only data from the medium and high pollen load treatments were used for the mixed pollinations. The analyses did not include data from fruits produced by low pollen load pollinations because seed sets were much lower in these fruits than in those produced by the higher pollen loads. All statistical analyses were performed with the JMP statistical package (SAS, 1994
).
We also conducted a linear regression of the siring success of outcross donors (SSO) against pollen load per fruit. SSO values were arcsine-square root transformed prior to analysis (Zar, 1984
). The best-fit line was determined using least squares regression.
RESULTS
Comparisons of the numbers of viable and aborted seeds per fruit indicated that the seed set of fruits produced from self, outcross and mixed pollinations were similar regardless of the type of pollen applied (Table 1). The mean number of viable seeds per fruit ranged from 35 to 40 across all pollination types, excluding mixed pollinations with low pollen loads. The analysis comparing the mean number of viable seeds per fruit for self, outcross, and mixed pollinations with medium and high pollen loads indicated that there were no significant differences among the means (
2 = 1.21, P = 0.546, df = 79). There were also no significant differences among the pollination types in the mean number of aborted seeds per fruit (2 = 0.577, P = 0.749, df = 79) or mean individual seed mass (2 = 1.82, P = 0.404, df = 70).
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In contrast to previous studies, the results of the mixed pollinations in this experiment do not support the hypothesis that CSI is operating in C. unguiculata. Based on the outcome of 91 pollinations representing a total of 26 individual plants, both self and outcross pollen donors sired roughly half of the seeds per fruit, on average, when equal amounts of pollen from both types of donor were applied to receptive stigmas. Overall, the mean percentages of seeds sired by self and outcross donors did not differ significantly. This result suggests that outcross donors do not have a siring success advantage over self donors, at least under the environmental conditions studied here. Seed characteristics did not differ between self and outcross donors for seeds produced from single-donor pollinations. There were no statistically significant differences in any of the variables measured on experimental fruits among self, outcross, and mixed pollinations. Based on these results, we conclude that self pollen was as successful as outcross pollen in competing for fertilizations and siring viable seeds.
Why were the measures of outcross siring success (SSO) observed in this study so much lower than those found in previous studies of cryptic self-incompatibility in C. unguiculata (mean = 85%; Bowman, 1987
) and C. gracilis (77.4%; Jones, 1994
)? CSI may vary in intensity among populations of Clarkia species. It is possible that in contrast to other populations, CSI has simply not evolved in the population from which our seeds were sampled. Our source population was relatively large and commonly visited by pollinators (S. E. Travers, personal observation). These conditions of apparently high outcross pollen availability may not select for CSI if self pollen rarely represents a sizable portion of the pollen load deposited on stigmas. Another possible explanation for the absence of CSI in this population is that selection against selfed seeds and seedlings was relatively low or nonexistent. Consistent with this explanation are the results presented here, which suggest that there is no inbreeding depression expressed during seed development. It would be informative to correlate the degree of outcross pollen availability, selection against selfed seeds, and prevalence of CSI in wild Clarkia populations. However, regardless of the evolutionary explanation for the absence of CSI, these results lead us to question the ability to generalize about CSI from the results of a single population.
Alternatively, Clarkia unguiculata may simply not be cryptically self-incompatible. The difference between the results of this study and those of previous studies may be due to the kinds of markers used to detect differential siring success of self vs. outcross pollen. Previous studies used phenotypic markers with Mendelian inheritance (C. unguiculata: petal albinism; C. gracilis: petal spot) rather than allozymes to determine paternity following mixed pollinations. In both cases, only individuals that had the recessive phenotype were used as pollen recipients in order to simplify paternity determination of the seeds. This means that self-pollinations were always conducted by pollen bearing the recessive allele. If this allele conferred any disadvantage to the male gametophytes bearing it, this could account for the results observed. In other words, the high siring success of outcross donors relative to self donors in these studies may have been due to differences in pollen performance between pollen donors with different petal phenotypes rather than differences between self and outcross pollen. The recessive allele associated with white petals in Clarkia unguiculata may have deleterious effects on fitness as suggested by the observation that individual plants with pure white petals are almost never found in wild populations of C. unguiculata (personal observation). However, in a separate study, Jones (1996)
found no differences in siring success between sires with the two different petal phenotypes of C. gracilis following competitive crosses.
Both phenotypic and genetic markers have been used to examine CSI in a single genus, Amsinckia. Casper, Sayigh, and Lee (1988)
compared the siring success of self and outcross pollen by crossing plants with different homozygous genotypes at the Pgi locus. The results from this study of A. douglasiana corroborate the conclusions from a previous study of CSI in A. grandiflora in which a phenotypic marker (style length) was used to determine siring success of self and outcross pollen (Weller and Ornduff, 1977
). In both species of Amsinckia, self pollen sired fewer seeds than did outcross pollen following mixed pollination.
To date, the majority of examples of CSI have come from studies of heterostylous species. In these taxa, single-donor pollinations have been observed to fruits with equivalent seed sets regardless of whether they resulted from intramorph (self pollen or pollen from flowers with the same morphology as the recipient) or intermorph pollinations. Siring success following mixed pollinations, however, was found to be lower for intramorph donors relative to intermorph donors (Weller and Ornduff, 1977
; Glover and Barrett, 1986
; Eckert and Barrett, 1994
; Cruzan and Barrett, 1996
). In these studies, differential abortion of seeds sired by the two types of pollen was ruled out as a possible mechanism for the observed siring success differences, supporting the contention that these species are cryptically self-incompatible (Weller and Ornduff, 1989
; Cruzan and Barrett, 1996
; Eckert and Allen, 1997
).
The best demonstration of CSI in a nonheterostylous species is the original study by Bateman (1956)
on Cheiranthus cheiri. In this species, over 90% of the seeds per fruit were sired by the outcross donor following pollinations with equal amounts of self and outcross pollen, while there was no difference in seed set between single-donor self and single-donor outcross pollinations. Additional studies that compare the siring success of self- and outcross-pollen donors with a variety of self- and outcross-donor genotypes will allow us better to estimate the frequency of CSI in species that are not heterostylous.
The question remains as to whether CSI is in fact a mechanism that has evolved to avoid inbreeding. In C. unguiculata, there is still much to be learned about inbreeding depression, as current evidence is equivocal. The possibility that this species is subject to inbreeding depression is suggested by the fact that it has evolved two clear mechanisms to reduce selfing. Male and female organs in C. unguiculata are separated both temporally (protandry) and spatially (herkogamy), suggesting that there is a fitness benefit to outcrossing (Lewis and Lewis, 1955
; Vasek, 1964, 1977
). On the other hand, Holtsford and Ellstrand (1990)
detected no evidence for strong inbreeding depression in C. unguiculata. In addition, the failure of the current study to detect CSI in C. unguiculata suggests at least that CSI has not evolved in all populations as a response to inbreeding depression in this species. From a more general perspective, whether CSI is indeed a phenomenon that has evolved to reduce inbreeding depression is a puzzle that will require coordinated studies to detect both inbreeding depression and CSI in a wide range of self-compatible homomorphic species.
FOOTNOTES
1 The authors thank Bernard Godelle, Scott Hodges, and Bob Warner for their critiques of early versions of this manuscript. Funding was provided by grants from the California Native Plant Society and the National Science Foundation (DEB 95–20611). 
2 Author for correspondence, current address: Department of Biology, Amherst College, Amherst, Massachusetts 01002 USA.
LITERATURE CITED
Aizen, M. A., K. B. Searcy, and D. L. Mulcahy. 1990 Among- and within-flower comparisons of pollen tube growth following self- and cross-pollinations in Dianthus chinensis (Caryophyllaceae). American Journal of Botany 77: 671–676. [CrossRef][ISI]
Bateman, A. J. 1956 Cryptic self-incompatibility in the wallflower: Cheiranthus cheiri L. Heredity 10: 257–261.
Bertin, R. I. 1990 Paternal success following mixed pollinations of Campsis radicans. American Midland Naturalist 124: 153–163.
Bowman, R. N. 1987 Cryptic self-incompatibility and the breeding system of Clarkia unguiculata (Onagraceae). American Journal of Botany 74: 471–476. [CrossRef][ISI]
Casper, B. B., L. S. Sayigh, and S. S. Lee. 1988 Demonstration of cryptic incompatibility in distylous Amsinckia douglasiana. Evolution 42: 248–253.
Cruzan, M. B. 1989 Pollen tube attrition in Erythronium grandiflorum. American Journal of Botany 76: 562–570.
———, and S. C. H. Barrett. 1993 Contribution of cryptic incompatibility to the mating system of Eichhornia paniculata (Ponte-deriaceae). Evolution 47: 925–934. [CrossRef][ISI]
———, and ———. 1996 Post-pollination mechanisms influencing mating patterns and fecundity: an example from Eichhornia paniculata. American Naturalist 147: 576–598.
Eckert, C. G., and M. Allen. 1997 Cryptic self-incompatibility in tristylous Decodon verticillatus (Lythraceae). American Journal of Botany 84: 1391–1397. [Abstract]
———, and S. C. H. Barrett. 1994 Post-pollination mechanisms and the maintenance of outcrossing in self-compatible Decodon verticillatus. Heredity 72: 396–411.
Glover, D. E., and S. C. H. Barrett. 1986 Variation in the mating system of Eichornia paniculata (Spreng.) Solms. (Pontederiaceae). Evolution 40: 1122–1131. [CrossRef][ISI]
Gottlieb, L. D., and V. S. Ford. 1996 Phylogenetic relationships among the sections of Clarkia (Onagraceae) inferred from the nucleotide sequences of PgiC. Systematic Botany 21: 45–62.
———, and L. C. Greve. 1981 Biochemical properties of duplicated isozymes of Phosphoglucose Isomerase in the plant Clarkia xantiana. Biochemical Genetics 19: 155–172.
Hebert, P. D., and M. J. Beaton. 1994 Methodologies for allozyme analysis using cellulose acetate electrophoresis. Helena Laboratories, Beaumont, Texas, USA.
Holtsford, T. P., and N. C. Ellstrand. 1990 Inbreeding effects in Clarkia tembloriensis (Onagraceae) populations with different natural outcrossing rates. Evolution 44: 2031–2046. [CrossRef][ISI]
Jones, K. N. 1994 Nonrandom mating in Clarkia gracilis (Onagraceae): a case of cryptic self-incompatibility. American Journal of Botany 81: 195–198.
———. 1996 Pollinator behavior and postpollination reproductive success in alternative flora phenotypes of Clarkia gracilis (Onagraceae). International Journal of Plant Sciences 157: 733–738. [CrossRef][ISI]
Lewis, H., and M. E. Lewis. 1955 The genus Clarkia. University of California Publications in Botany 20: 241–392
MacSwain, J. W., P. H. Raven, and R. W. Thorp. 1973 Comparative behavior of bees and Onagraceae. IV. Clarkia bees of the western United States, vol. 33. University of California Press, Berkeley, California, USA.
SAS. 1994 JMP Statistics and graphics manual. SAS Institute, Cary, North Carolina, USA.
Vasek, F. C. 1964 The evolution of Clarkia unguiculata derivatives adapted to relatively xeric environments. Evolution 18: 26–42. [CrossRef][ISI]
———. 1965 Outcrossing in natural populations. II. Clarkia unguiculata. Evolution 19: 152–156.
———. 1977 Phenotypic variation and adaptation in Clarkia section Phaestoma. Systematic Botany 2: 251–279. [CrossRef][ISI]
Waser, N. M., M. V. Price, A. M. Montalvo, and R. N. Gray. 1987 Female mate choice in a perennial herbaceous wildflower, Delphinium nelsonii. Evolutionary Trends in Plants 1: 29–33.
Weller, S. G., and R. Ornduff. 1977 Cryptic self-incompatibility in Amsinckia grandiflora. Evolution 31: 47–51.
———, and ———. 1989 Incompatibility in Amsinckia grandiflora (Boraginaceae): distribution of callose plugs and pollen tubes following inter- and intramorph crosses. American Journal of Botany 76: 277–282. [CrossRef][ISI]
Zar, J. 1984 Biostatistical analysis, 2nd ed. Prentice Hall, Englewood Cliffs, New Jersey, USA.
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