Polymorphic floral traits in Linaria canadensis (Scrophulariaceae).
Animal-pollinated flowers contain structures designed to attract and reward pollinators in order to disperse and receive pollen. While some of these structures may be highly canalized in appearance, others may be quite variable (Waser, 1983; Campbell, 1989; Herrera, 1996). For example, flower size (Galen and Kevan, 1980; Andersson, 1994; Meagher and Costich, 1994; Wolfe, 1995; Delph, 1996) and flower color (Ennos and Clegg, 1983; Wolfe, 1993; Stanton et al., 1986) are known to vary within some species. Any variability in floral structure or color may affect parameters of the pollination system, and hence, reproductive success (Campbell, 1989; Murcia, 1990; Stanton, 1987; Stanton et al., 1991). Yet, identifying the cause of variable reproductive success among individuals in a population becomes more difficult when multiple traits vary simultaneously. In several species variation in flower color is associated with other variable characters that may be vegetative (Simms and Bucher, 1996) or reproductive (Sobrevila et al., 1989). For example, in Echium plantagineum (Burden et al., 1983) and Hydrophylum appendiculatum (Wolfe, 1992) rare white-flowered individuals had lower levels of seed production than the wild type, blue-flowered plants. However, in these systems differential fitness was not a direct consequence of pollination. Rather, poorer competitive ability or lower flower production were genetically associated (e.g., pleiotropy) with white flower color.
We examined various aspects of a previously unreported flower color polymorphism in the annual plant, Linaria canadensis (Blue Toadflax, Scrophulariaceae). Our preliminary observations indicated that individual plants in this species produce flowers that are either dark purple or very pale blue in color. We were particularly interested in determining if flower color variation was associated with other variable characters. Linaria canadensis is widespread in disturbed areas and cultivated fields throughout the eastern U.S. (Britton and Brown, 1970). Individual flowers arise from slender racemes and have relatively long nectar spurs. The specific objectives of the study were to: (1) quantitatively describe the variation in flower color in a number of populations; (2) determine if the flower color morphs differ with respect to any phenotypic characteristics such as flower size, several estimates of plant size, or number of fruit per plant, and (3) determine if seeds produced by different color morphs have different germination probabilities.
TABLE 1. - The proportion of flower color morphs in six populations of Linaria canadensis. Chi-square tests for deviation from 1:1 ratios. ns = not significant. * P [less than] 0.0001 Light Dark Number of morph morph plants sampled Chi square King Road 0.53 0.47 377 2.2(ns) Pecan Orchard 0.10 0.90 150 102.4(*) Clito Road 0.37 0.63 1308 20.3(*) Hwy 25 0.97 0.30 102 90.3(*) Claxton 0.45 0.55 206 1.94(ns) Statesboro 0.94 0.60 101 80.0(*)
MATERIALS AND METHODS
The frequency of the petal-color morphs was determined in six natural populations in southeastern Georgia (five in Bulloch County, one in Evans County; see Table 1) in March-April, 1995. The study populations were separated by a minimum of 12 km and occurred in roadsides, fields and pastures. Two petal-color morphs were identified in Linaria canadensis: the 'dark' morph had a deep purple color and the 'light' morph was a light blue or lavender color. To determine the frequency of color morphs, 3-5 line transects were established through each population and we recorded the flower color of all plants growing within 50 cm on either side of the transect. A minimum of 100 plants was sampled in each population.
Measurements were conducted in the laboratory on 96 field-collected plants to determine if the dark and light morphs differed in vegetative and reproductive traits. Forty-eight plants (24/morph) were collected randomly in both Highway 25 and Pecan Orchard populations. For each plant we recorded (1) flower color; (2) number of stems per plant; (3) total stem length (mm); (4) number of open flowers per plant; (5) flower length (mm), and (6) flower diameter (mm). Randomly chosen flowers (two flowers per plant) were measured by pressing a single flower under a microscope slide and measuring the diameter and length with vernier calipers to the nearest 0.01 mm.
Mature fruit collected from 20 randomly chosen plants in two populations (Hwy. 25 and Pecan Orchard) were used to determine mean individual fruit weight. All the fruits from each plant were placed in a paper bag. Unfortunately, the fruits dehisced while in the bag and therefore it was not possible to count the number of seeds per fruit. Empty fruits were oven-dried for 3 days before weighing. Since individual Linaria canadensis fruits are small, three fruits were weighed at a time.
Germination trials were conducted on seeds from the two color morphs. Seeds were placed in a cold treatment for 30 days before planting. Twenty seeds from each of 20 maternal families were placed on top of moist soil in individual pots in a constant temperature (7 C), dark refrigerator in January 1996. After 1 mo the pots were removed from the refrigerator, placed in the greenhouse and watered every 2 days. The pots were monitored for seedling emergence 1 mo later and no further emergence was seen after this date.
[TABULAR DATA FOR TABLE 2 OMITTED]
Flower morph frequency in different populations. - The light and dark morphs were present in all six populations of Linaria canadensis (Table 1). We also found two completely white-flowered individuals: these albinos were growing in the Clito Road and Statesboro populations. The frequency of the color morphs varied dramatically among populations. In two populations, the morphs were equally common (King Road and Claxton). In two populations, the dark morph was significantly more abundant than the light morph (Pecan Orchard and Clito Road) while the light morph was significantly more common than the dark morph in Statesboro and Hwy. 25 populations. Combining all populations, the frequency of the two flower color morphs was not significantly different from unity (light = 619, dark = 632; chi square = 2.0, P [greater than] 0.10).
Comparison of phenotypic traits between flower color morphs. - All flowers on an individual plant were the same color and there was no change in color with age. There was no significant difference in any trait measured between Hwy. 25 and Pecan Orchard so the data were pooled for all analyses. When all characters were considered together, there was a highly significant difference between the light and dark color flower morphs (MANOVA Wilks [Lambda] = 0.482, P [less than] 0.0001). Considering the characters singly, there were no significant differences between the color morphs in the number of stems, stem length, number of flowers and fruit (Table 2). However, flower size was extremely variable, ranging from 624 mm in length [ILLUSTRATION FOR FIGURE 1 OMITTED]. The distribution was bimodal, with the two peaks representing the average of the dark and light morphs, respectively [ILLUSTRATION FOR FIGURE 1 OMITTED]. Flowers of the light morph were approximately 40% larger (length and diameter) than those of the dark morph (Table 2). Petal length was correlated with petal diameter (Pearson's product-moment correlation r = 0.72, P [less than] 0.01). A nested analysis of variance revealed significant variation in flower size among individual plants within each color morph (F = 17.3, P [less than] 0.0001). Thus, flower size was relatively consistent within an individual. Neither estimate of flower size was correlated with plant size (r = 0.10; P [greater than] 0.05).
There were other reproductive differences associated with the flower color polymorphism. Plants that produced light-colored flowers produced significantly heavier fruits (mean [+ or -] SE of three fruits) than plants with dark-colored flowers (8.2 [+ or -] 1.9 mg vs. 4.3 [+ or -] 1.5 mg, respectively; t = 12.97, P [less than] 0.0001). While approximately 35% of the planted Linaria canadensis seeds germinated, seed germination was independent of flower color (Table 2).
Flower color polymorphism is a consistent feature of the Linaria canadensis populations we sampled in southeastern Georgia. However, the frequency of different color morphs varies extensively among populations. Only two of the sites had morph ratios close to 1:1 while the remaining four sites had skewed morph ratios. Among-population variation did not seem to be associated with any obvious environmental feature. It is possible that this patchwork of flower color patterns at the interpopulation level is the result of previous selection driven by pollinator preference. This would require that the pollinator species vary among sites to drive the observed variability in colors. Yet our observations indicate that same pollinators [(relatively small generalist bees (i.e., honeybees, sweat bees)] service all these sites. An alternative explanation to account for among-site variation in morph frequency is that differences are not due to biotic selection, but to chance events accompanying the founding of new populations.
A clear finding of this study was that flower color polymorphism was associated with flower size: light-colored flowers were larger than dark-colored flowers. The difference in flower size between the two color morphs is evidence that this trait is associated in some manner (e.g., linkage, pleiotropy) with the locus controlling flower color. At this point it is not known to what extent this variation is due to genetic or environmental factors.
The finding that light-colored flowers produce heavier fruits than dark-colored flowers may be a direct consequence of variation in flower size. Anatomical studies on other species have shown that the size of plant organs is dependent upon the amount of vascular tissue supplying them (Carlquist, 1969; Housley and Peterson, 1982). Given the developmental continuity between flowers and fruits (Kang and Primack, 1991; Wolfe, 1992), it is not surprising that large flowers should develop into large fruits (Wolfe, 1995). The consequence of this asymmetry in fruit weight is that the larger fruit of the light morph would either contain more seeds or heavier seeds than the smaller fruit of the dark morph. Thus it is possible that the light-colored morph has higher levels of reproductive success due to female function.
Acknowledgments. - We thank Katrina Motes for weighing fruits. Janet Burns, Ray Chandler, Maureen Stanton and an anonymous reviewer kindly provided helpful comments on an earlier draft of this paper.
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|Author:||Wolfe, Lorne M.; Sellers, Susan E.|
|Publication:||The American Midland Naturalist|
|Date:||Jul 1, 1997|
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