Sex ratio and phenotypic differences of Silene dioica in Ces

Report for the Field Course Evolutionary Biology in Ces, 6th – 10th June 2011Institute of Integrative Biology, ETH Zurich & EAWAG Dübendorf

Sex ratio and phenotypic differences of Silene dioica in Ces

Gattiker Etienne, Ostertag AnicETH Zürich, 6th August 2011

Supervision: Prof. Dr. Jukka Jokela1, Dr. Ben Sadd2, Dr. Seth Barribeau2, Caroline Baumgartner3, Silvana Käser3

1Institute of Integrative Biology, ETH Zurich & EAWAG, Dübendorf, 2Institute of Integrative Biology, ETH Zurich, 3 Aquatic Ecology, EAWAG Dübendorf

Abstract

Dioeciousness is widespread across plant families but rare within most families and leads of-ten to sexual dimorphism. The principle of «sexual selection» can be applicable by dioecious plants.

We studied Silene dioica populations around Ces in the Swiss alps to address two central questions: «Does the sex ratio of S. dioica differ from 1:1?» and «Are there phenotypic dif-ferences between sexes and areas of S. dioica?»

Of the 350 individuals, examined in Ces, the sex ratio was significantly biased towards males. Female plants were on average taller than the males, and males had on average significantly (2.85 times) more flowers than females. The ratio of open to total number of flowers showed no difference between the sexes. S. dioica in different groups varied in height but not in total number of flowers.

To understand the differences between the sexes the «bateman principle» is useful. There are phenotypic differences between the sexes because of «sexual selection» and different con-straints for reproduction. The population in Ces is weak male-biased. here are some driving forces contrary to a male-biased population. But if the living conditions for Silene dioica are not optimal here can be selection for male-biased population, since males are cheaper and need less resources than females.

Report for the Field Course Evolutionary Biology in Ces

1 Introduction

The motivation to investigate Silene dioica was its dioeciousness. Dioeciousness is wide-spread over the plant families but rare within most of the families with some exceptions, i.e. the family salicaceae. It leads often to sexual dimorphism. Because dioecious plants have dis-tinct sexes, they are subject to sexual selection. Sexual selection theory suggests that in the S. dioica reproductive success for males is limited by the amount of insect pollinators whereas females are limited by resources. Consequently the males should invest more in their perianth and females more in their reproductive apparatus. The investment in males' perianth should attract more insect pollinators which disperse the pollen, trying to increase the reproductive success. Bateman's principle describes the different investment of the sexes in their reproduc-tive apparatus. Males have many but small gametes (sperms) on the opposite the females have only few but big gametes (eggs). Consequently males can fecundate a lot of different females, whereas females only have limited number of eggs to be fertilized. Therefore males show a bigger variation in reproductive success than females. To be evolutionary stable it is important to hold an sex ratio of 1:1.

On the alp Ces S. dioica is not widespread (see Fig.1). There are only a few populations around and among the buildings. We suggested that the growth conditions for S. dioica corre-spond not to its optimum. Because males do not need as much resources to reproduce as fe-males and therefore can grow in worse conditions, more males than females are predicted around Ces. To test this suggestion we recorded the sex ratio of S.diocia and phenotypic dif-traits of male and female plants to determine whether the sex ration differed and whether the phenotypic differences between the sexes and patches of S.diocia.

• Does the sex ratio of S. dioica differ from 1:1?

• Are there phenotypic differences between sexes and areas of S. dioica?

Plant description

S. dioica is a perennial of the family caryophyllaceae. It is a pest and ruderal plant and nor-mally lives as hemicryptophyte – under good conditions even a potential therophyte. As hemicryptophyte S. dioica lives for two years and hibernates with a basal whorl, called rosette (Lauber and Wagner, 2007). The rosette leaves are green during the winter. Together with the central root-stock, the rosette is regarded as an important element for the retention of re-sources during the winter (Baker, 1947). Seeds are dispersed in August, and germination take place in early June the following year (Elmqvist and Gardfjell, 1988).

S.dioica naturally occur in megaphorbia1 and marsh areas. It occurs generally in collin, mon-tane and sub-alpine levels, but grows well in montane environment. S. dioica is a moisture, nutrient and half-light indicator, grows in a pH range of 4.5-7.5 and occurs in both types of climate, namely oceanic and continental climate (Lauber and Wagner, 2007).

1 Zones with high and hygrophile herbaceous vegetation. Source: http://proceedings.esri.com/library/userconf/proc06/papers/papers/pap_1214.pdf, visited 19.06.2011

Gattiker Etienne, Ostertag Anic ETH Zürich, 08/25/2011


2 Material and Methods

Ten areas, where S. dioica was growing, were chosen in the village of Ces (see Pic. 1). In each area we recorded the sex ratio of up to 40 individuals, as well as the height, total number of flowers, and number of open flowers of ten individuals of each sex. The pH was measured by «AgroQuant® 14606».

For the statistical analysis «JMP® Statistical Discovery Software» was used. The sex ratio was tested by the « χ2-test », the hypotheses about the phenotypic differences were tested by «ANOVA2» and «normal correlation».

3 Results

3.1 Sex ratio

For the total of 350 individuals, found in Ces, the sex ratio of the S. dioica populations showed a significant bias for the males (χ2

1 = 4.5814, p-value 0.0323). 155 (44.29%) females

and 195 (55.71%) males were found (see Fig. 2).

Fig. 2: Sex ratio of the Silene dioica populations in Ces. N = 350. m: males 55.71%, f: females 44.29%.

2 ANOVA is the symbol for «Analysis of Variance»


Fig. 1: Aerial view of the alp Ces, ticino Switzerland. The red triangles mark the ten investigated areas of totally 13 areas. Source: «Google Earth».

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3.2 Phenotypic differences

The height correlated positively with the total number of flowers (R2 = 0.1075, p < 0.0001) where as the pH did not correlate with height (R2

1 = 0.0304, F Ratio = 0.2509, p-value = 0.6299) or with the total number of flowers (R2

1 = 0.1199, F Ratio = 1.0899, p-value = 0.3270). The pH range is 4.5 – 7.5 among the areas.

3.2.1 Between the sexes

The comparison of the sexes in S. dioica showed a significant difference in the mean height (ANOVA, F-value = 4.4772, p-value = 0.0360) as can be seen in figure 3 below. The female plants were on average higher (43.52 cm) than the male plants (39.85 cm). Although the males had on average significantly (2.85 times) more flowers than the females (ANOVA, F-value = 84.2834, p-value < 0.0001; fig. 4). There was no difference between the sexes in the ratio of the open to the total number of flowers (ANOVA, F-value = 0.1019, p-value = 0.7500; fig. 5).

Fig. 3: The mean height of S. dioica in cm and standard error for each sex. N = 172. m: males. f: females.

Fig. 4: The mean of the total number of flowers of S. dioica and standard error for each sex. N = 172. m: males. f: females.

Fig. 5: The mean of the ratio of the open to the total number of flowers of S. dioica and standard error for each sex. N = 172. f/m: fe-/male

3.2.2 Between the areas

There was a significant difference in the mean height of the S. dioica between areas (ANOVA, F-value = 9.1937, p-value < 0.0001; fig. 5). However the mean number of total flowers did not differ between the areas (ANOVA, F-value = 1.8674, p-value = 0.0603) as can be seen in figure 7.


Fig. 6: The mean height in cm and standard error of the S. dioica for the ten different areas studied in Ces. N = 172.

Fig. 7: The mean of total flowers and standard error of the S. dioica for the ten different areas studied in Ces. N = 172.


4 Discussion

4.1.1 Sex ratio

Sex ratio was in favour of males (see chapter 3.1). The genetical probability of the gametes to producing a male (or female) would normally be q = 0.5. The results agree to the Bateman's principle, because female invest more in reproduction. Consequently females are a limited re-source, because it is cheaper to produce a male (less investment in reproducing) and easier for males to grow (they need less resources), especially under bad conditions. Considering the fact that a population without a sex ratio of 1:1 is not evolutionary stable it is suggested that the population of Silene dioica in Ces could become extinct. But there are driving forces into the direction of an equilibrated sex ratio. Males rosette leaves are preferred by herbivores than those of females and males have a lower ability to replace lost biomass needed for reproduc-tion (Elmqvist and Gardfjell, 1988). This is because females have a greater under-ground (root-stock) than an above-ground (rosette) resources storage compared to males (Stanfield 1937, 1944). Additionally males show a higher mortality under defoliation (Elmqvist and Gardfjell, 1988). Therefore it is possible that the male biased population is more evolutionary stable than supposed.

Furthermore bumble bees, a pollinator, show a preference for male flowers (Kay et al., 1984). This situation increases the probability of female flowers to be pollinated, because of the higher probability that a bumble bee has recently visited a male flower (reducing pollen limi-tation). On the other hand there can be reduced frequency of visits to female flowers (enhanc-ing pollen limitation) or an overall higher visitation frequency to populations with a high male-density (reducing pollen limitation). Therefore it can be better for a population of Silene dioica to have more male than female flowers. Considering the fact that the number of total flowers per individual is nearby three times higher in males than in females (see chapter 3.1), a sex ratio of 1:1 would be sufficient for a flower sex ration of 3:1 (75% to 25%). A male-bi-assed floral sex ratio of 75 % male flowers is normal in populations of Silene dioica (Carlsson-Granér et al., 1998). A weakly male-biased population still can be healthy. But the more males in a population the higher the risk of «pollen limitation» because of a insufficient frequency of visits to female flowers by pollinators.

4.1.2 Phenotypic differences – between sexes

Males of Silene dioica are smaller than females (Elmqvist and Gardfjell, 1988). Females were on average 3.67 cm taller than males (about 10%). A reason for that is to better expose the fe-male flowers to potential pollinators, because the female flowers are less preferred by pollina-tors and would be hard to find in a floral male-biased surrounding area. The height correlated significantly with the total number of flowers but it explained only ten percent of the results (see chapter 3.2). Maybe the correlation would be better if the sexes are calculated separately, because of the significant difference in height and the heavy difference in the total number of flowers between sexes.

The number of total flowers per individual was about three times higher in males than in fe-males. The fact that a population of Silene dioica is naturally floral male-biased has been de-scribed by Carlsson-Granér et al. (1998). The influence of the unequal floral sex ratio on the population of Silene dioica is discussed in chapter 4.1.1.

The equal “open to total flower ratio” suggest that male and female have the same state of de-



velopment. But this isn't right, because in Fig.5 the total number of flowers contains the flow-ers already flowered, which was only available for females. Recalculation without the flow-ered female flowers shows a significant difference in the averaged open/total flowers ratio (calculated with «Microsoft Excel®»). Males have a ratio of 0.27, whereas females have one of 0.59 (and not 0.26 like visible in Fig.5). A reason for that can be that males start flowering earlier than females in the season and also flower for a longer period of time (Elmqvist and Gardfjell, 1988).

4.1.3 Phenotypic differences – between areas

The sampled plants grew in a pH range of 4.5-7.5 as recorded by Lauber and Wagner, 2007 (see chapter 1).

The height of the plants differed significantly between the areas, especially area «5» and «6» (see Fig.6). pH probably does not affect total number of flowers (see chapter 3.2) but other parameters likely differ between the areas. Living conditions like light, nutrient availability, temperature, moisture, soil composition, external interferences, etc. are supposed to affect height but no data is on hand. High disturbance by pedestrians and the bad location between path and house wall is probably responsible for the low height in area «5». Furthermore, the soil very dense and hard so that the soil composition is not ideal to root. Area «6» is in the meadow besides the teleferic (Fig.1, the red triangle top right). High disturbance by grazing and cutting probably leads to the low height. Nutrient poorness, too much light or the soil composition can also be liable for the low height in this area.

The total number of flowers did not differ significantly between the areas. Accordingly it is assumed that there are not big differences in living conditions which affect the total number of flowers. Only area «1», «6» and «10» were – not significant – evident different to the other areas (see Fig.7).

Area «9» was the only female-biassed area with 55% females (22 individuals) and 45% males (n=40; m: 18, f: 22). Considering the fact that females have few flowers compared to males this area seems to be very healthy regarding the total number of flowers (Fig.7).

4.1.4 Supplementary observations

Males were more branched as females which agree with the higher above-ground biomass mentioned by Carlsson-Granér et al. (1998) and had more flowers. Males bore a paracorolla3, it imitates the female styles. This agrees – together with the high number of flowers - with the Bateman's principle that males should more invest in their perianth (see chapter 1).The para-corolla is – beside nectar and sugar production – supposed to be important for the bumble bees preference for male flowers (Carlsson-Granér et al., 1998). Females had thicker leaves and are highly hirsute compared to males. A possible ecological explanation is a higher pro-tection against herbivores (freeze or uv radiation).

5 Conclusion

Bateman's principle is applicable for the dioecious plant Silene dioica. Females have fewer flowers and no paracorolla but are bigger than males. There are other phenotypic differences,

3 A secondary or inner corolla; a corona, as of the Narcissus. Source: http://www.thefreedictionary.com/corolla, visited 19.06.2010



like the height, degree of branching, etc. because of sexual selection and different constraints for reproduction.

The population in Ces is weakly male-biased but to be ecological stable, a sex ratio of 1:1 is favored. There are some driving forces against a male-biassed population. But if the living conditions for Silene dioica are not optimal it is suggested that the population turns more and more into a stronger male-biassed population, since males are cheaper and need less resources to grow than females.

BibliographyBaker H.G. (1947): Biological flora of the British Isles. Melandrium (Roehling em.), Fries.

J. Ecol. (1947) 25, 271-292

Carlsson-Granér U., Elmqvist T., Agren J., Gardfjell H. and Ingvarsson P. (1998): Floral sex ratios, disease and seed set in dioecious Silene dioica, Journal of Ecology (1998) 86, 79-91

Elmqvist T. and Gardfjell H. (1988): Differences in response to defoliation between males and females of Silene dioica, Oecologia (1988) 77, 225-230

Kay Q.O.N., Lack A.J., Bamber F.C. and Davies C.R. (1984): Differences between sexes in floral morphology, nectar production and insect visit in a dioecious species, Silene dioica, New Phytol. (1984) 98, 515-529

Lauber K. and Wagner G. (2007): Flora Helvetica, 2007 (4 th edition), Haupt Verlag Bern-S-tuttgart-Wien, S.246

Stanfield J.F. (1937): Certain physico-chemical aspects of secual differentiation in Lychnis dioioca, Am. J. Bot. (1937) 24, 710-719

Stanfield J.F. (1944): Chemical composition of roots and tops of dioecious Lychnis in veg-etative and flowering phases of growth, Plant Physiol. (1944) 19, 377-383


Sex ratio and phenotypic differences of Silene dioica in Ces

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