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1 Department of Biology, University of Oulu, P.O. Box 3000, FIN-90401 Oulu, Finland
Received for publication August 20, 1998. Accepted for publication April 29, 1999.
ABSTRACT
Developmental instability shown by increased fluctuating asymmetry can be caused by either genetic or environmental stress. Genomic coadaptation and heterozygosity are the genetic factors that are commonly assumed to increase the level of developmental stability. Therefore, in hybrid populations the level of fluctuating asymmetry (FA) can be lower due to higher heterozygosity or higher due to disruption of coadapted gene complexes, depending on the degree of divergence between hybridizing taxa. Here I present data on FA in petals from hybrids between Lychnis viscaria (Caryophyllaceae) and Lychnis alpina and from parental species grown in a common garden environment. Petal asymmetry of hybrids was clearly higher than that of either parental species grown in common environment. Between the two parental species petal asymmetry did not differ. The mean size of the petals in hybrids was about the same as in L. viscaria, thus indicating no heterotic effect. Therefore, it seems that hybrids between L. viscaria and L. alpina do suffer from the disruption of coadapted gene complexes as indicated by higher developmental instability.
Key Words: Caryophyllaceae • developmental instability • fluctuating asymmetry • hybridization • Lychnis viscaria • Lychnis alpina
Developmental stability is one of the components of developmental homeostasis (for review, see Zakharov, 1992
) through which organisms reduce phenotypic variation resulting from developmental accidents. Fluctuating asymmetry (FA) is perhaps the most frequently used measure of developmental stability. FA describes small random deviations from perfect bilateral symmetry, which are random with respect to side and normally distributed.
Levels of FA are known to be associated with several environmental and genetic stress factors (e.g., Palmer and Strobeck, 1986
; Leary and Allendorf, 1989
). Two genetic factors that are commonly assumed to increase the level of developmental stability are genomic coadaptation and heterozygosity. Therefore, in hybrid populations the level of FA can be lower due to higher heterozygosity or higher due to disruption of coadapted gene complexes, depending on the degree of divergence between hybridizing taxa (Vrijenhoek and Lerman, 1982
; Graham, 1992
). If the positive association between heterozygosity and developmental stability is due to heterozygosity per se, then an increased heterozygosity among interspecific hybrids should result in increased developmental stability. Even though there have been numerous studies showing that individuals that are more heterozygous at enzyme loci are usually less asymmetric than homozygous ones, this seems not to be a rule (for review, see Møller and Swaddle, 1997
). Moreover, it commonly is assumed that the breakdown of coadapted gene complexes in naturally hybridizing taxa is more important for developmental stability than heterotic effects (Vrijenhoek and Lerman, 1982
; Graham, 1992
; Clarke, 1993
). Part of the studies on FA of hybrid or introgressed populations have shown reduced developmental stability among hybrids (e.g., Zakharov, 1981
; Graham and Felley, 1985
: Leary, Allendorf, and Knudsen, 1985
), whereas others have found opposite results (e.g., Vrijenhoek and Lerman, 1982
; Alibert et al., 1997
). This discrepancy could be at least partly due to the fact that developmental stability of hybrids is not always measured in common environments with parental species. Because the level of FA is clearly affected by environmental stress factors (Valentine and Soulé, 1973
; Siikamäki and Lammi, 1998
), it is crucial to control for the environmental effects when determining genetic effects on FA.
Because studies on developmental stability have involved mainly animals, bilateral symmetry has received special attention. However, given that all symmetries represent potential developmental instability, they may be used analogously to bilateral asymmetry (Freeman, Graham, and Emlen, 1993
; Graham, Freeman, and Emlen, 1993
). Eriksson (1996)
has recently analyzed and discussed the definition and measurements on radially symmetric flowers.
Here I present results on fluctuating asymmetry in flowers from hybrids between Lychnis viscaria L. (Caryophyllaceae) and Lychnis alpina var. alpina L. (Caryophyllaceae) and from both parental species grown in a common garden environment. The aim of the study was to examine the contribution of heterosis and breakdown in coadaptation to the level of petal asymmetry among hybrids.
Both L. viscaria and L. alpina are perennial, nonclonal herbs that grow in open, sunny habitats. Both species are mainly outcrossing, but self-pollination is also possible (Böcher, 1977
). The chromosome number of both species is 2n = 24. Lychnis viscaria is found in most of central Europe, and its main distribution area extends in northern Scandinavia to 62° latitude, but some isolated populations are found up to 68° latitude (Hultén, 1971
; Wilson et al., 1995
). In the surroundings of Jyväskylä, in central Finland (62°15' N, 25°45' E), L. viscaria occurs in its northern range in rather small and very isolated patches in rocky cliffs. The average number of inflorescences per plant ranges from one up to 50 with ~20–25 flowers on each inflorescence (Jennersten, 1991
; Wilson et al., 1995
). The flowers are protandrous with ten anthers and five stigmas and normally carry five petals. Flowers are visited by a wide range of insects, among which bumble bees and butterflies are the most frequent (Jennersten, 1988
).
Lychnis alpina is an amphi-Atlantic subarctic-alpine species. In Europe its distribution area includes northwest Europe and extends southwards to central Spain and eastwards to the northern Urals (Hultén, 1971
; Hultén, and Fries, 1986
). Main habitats of L. alpina are graminid and lichen heaths, rich solifluction slopes, riversides, and lichen rocky habitats. Furthermore, it occurs in habitats with high heavy metal concentrations and ultra-alkaline soils (e.g., Rune, 1953
).
Hybrids between L. viscaria and L. alpina have rarely been recorded from Sweden and Finland where the two parent species are sympatric (Böcher, 1977
). In central Finland habitats of L. viscaria and L. alpina are quite similar lichen rocky habitats, but there is no location where both of the species are known to occur together, and no hybrids are recorded from this area. According to Böcher (1977)
, hybrids originating from artificial crossing were sterile and the stamens had bad or degenerating pollen. Flowering specimens of these hybrids were morphologically intermediate between the parent species.
Crosses and seed material
Before the flowering season in 1995, 33 specimens of L. alpina were transplanted from their original location (Sorvavuori, Konnevesi, central Finland 62°47' N, 26°05' E) close to one L. viscaria population (Kanavuori, Jyväskylä, central Finland 62°16' N, 25°55' E). Transplanted specimens of L. alpina were enclosed in veil bags to prevent a natural pollination by bees and butterflies. During the flowering period the parental species were crossed by gathering fresh stamens of L. viscaria and by hand-pollinating the female-phase flowers of L. alpina. At pollination, fresh anthers of L. viscaria were brushed across stigmas of L. alpina until they were covered with pollen.
Fruits of hand-pollinated L. alpina and fruits of parental specimens of both parental species were collected at maturation and coldstratified for 3 mo at +3°C. After coldstratification interspecific seeds were germinated in petri dishes on paper. Hybrid seedlings were easy to identify morphologically because they were intermediate between seedlings of parents. The correct identification of hybrid seedlings was assured during the common garden experiment by their morphology and by the fact that all flowering specimens of seedlings that were identified earlier as hybrids did not set seeds, whereas all other specimens did. Three hybrid seedlings from each maternal plant were moved to grow in 10 x 10 cm plastic pots and grown in growth chambers at +22°C. They were watered when needed and fertilized once during the seedling period. In the 1996 growing season one random hybrid seedling per maternal plant was moved to the experimental garden in the Konnevesi Research Station (62°37' N, 26°20' E).
Seeds for the parental specimens of L. viscaria and L. alpina were collected from naturally pollinated, randomly selected maternal plants from Kanavuori and Sorvavuori, respectively. Seedlings of the parental species were grown under standard greenhouse conditions in the Laukaa Research and Elite Plant Station. Four seeds were sown per each pot, and if more than one seed germinated extra seedlings were removed after 2 wk. Otherwise, seedlings of the parental species were grown just as hybrid seedlings. They were planted in the Konnevesi experimental garden at the same time as the hybrid seedlings. Possible minor differences in the early seedling environments are very unlikely to affect the petal asymmetry as the environment in which the adult plants complete vegetative growth and commence flowering is the most critical phase with respect to petal asymmetry (Siikamäki and Lammi, 1998
).
Petal asymmetry
Petal asymmetry was determined in 1997 during the common flowering period from the hybrids and specimens of the parental species growing in the common garden conditions. Fresh, just-opened flowers from individuals of parental species and hybrids were collected randomly and measured in the laboratory to reach a better accuracy in estimates of FA. Petal asymmetry was measured whenever suitable flowers were available so that, in total, one randomly selected flower was measured from each of 27, 30, and 39 individuals of hybrid, L. alpina, and L. viscaria plants, respectively. Each petal was separated from the corolla, and the petals were flattened gently between two glass plates. The length of the visible part of a petal was measured to the nearest 0.01 mm using a digital caliper. Mean flower size was calculated as the mean length of all petals on 26, 28, and 38 hybrid, L. alpina, and L. viscaria plants, respectively. Only one flower per individual was measured since earlier studies in L. viscaria have shown that petal asymmetry is quite consistent within an individual (Eriksson, 1996
; Siikamäki and Lammi, 1998
). Consequently, it can be assumed that measurements on one, randomly selected flower per individual collected in the same phase of flowering can give an appropriate estimate of individual FA.
An estimate of absolute flower asymmetry was obtained by calculating the difference between the longest and the shortest petal (Pl - Ps) in each individual flower (Eriksson, 1996
). Because absolute asymmetry was correlated with the mean flower size (r = 0.34, N = 94, P < 0.01), relative FA ({FA/mean petal size} x 100) was used as a measure of FA (Palmer and Strobeck, 1986
). FA between randomly chosen petals was normally distributed (Kolmogorov-Smirnov test, NS) and did not deviate from zero (t = 8.7, df = 95, P = 0.38).
Measurement errors were estimated by measuring petals of ten L. viscaria individuals from one natural population and then remeasuring the same individuals again. The petals were remeasured in a random order in the repeat set of measurements without reference to earlier measurements. The correlation between petal length was rs = 0.96, P < 0.001, and between absolute asymmetry rs = 0.94, P < 0.001, N = 10 for both cases. Data were analyzed by performing a two-way mixed model ANOVA where sides (= the shortest and the longest petals) were the fixed factor and individuals were the random factor (Sokal and Rolf, 1981
; Palmer and Strobeck, 1986
; for discussion, see Merilä and Björklund, 1995
). Measurement error accounted for 1.3% and FA for 28% of the total variance between the shortest and the longest petals (MSW = 0.02, df = 20; MSSI = 0.58, df = 9). The measurement error in estimates of FA was thus low in relation to the variation explained by FA of the total variance.
Nonparametric statistics were used to analyze the differences in petal asymmetry because the assumptions of parametric tests were not met. The data were ranked, and the one-way ANOVA was performed on the ranks by using the three groups as fixed factors (Zar, 1996
). The test value H (= SSSOURCE/MSTOTAL) follows asymptotically the chi-square distribution with dfSOURCE. Nonparametric multiple comparisons were applied as described by Zar (1996)
. Statistical analyses were performed with SPSS for Windows (Noru
is, 1992
).
Mean petal size of hybrids was higher than that of L. alpina but did not differ from that of L. viscaria (ANOVA: F2,93 = 45,1, P < 0.001, pairwise comparisons at the 0.05 significance level with Tukey test; Fig. 1a). Because there were significant differences in petal asymmetry variance among hybrid and parental species (Bartlett's test of homogeneity of variances: F = 3.6, P = 0.027), nonparametric statistics was applied. The relative fluctuating asymmetry (RFA) of petals differed among the groups of plants (H = 15.5, P < 0.001; Fig. 1b.). Nonparametric multiple comparisons revealed that the RFA of hybrids was higher than that of L. alpina or L. viscaria, whereas the RFA did not differ between the parental species (Q = 3.9 and P < 0.001, Q = 2.6 and P < 0.05, Q = 1.6 and ns, respectively).
|
The RFA of hybrids was clearly higher than that of either parental species grown in the common environment. Maternal effects on seeds might also have some impact on the level of RFA. However, one might predict that this kind of maternal effects due to poor maternal environment should cause smaller overall plant size and lower mean petal size. Here I found that overall plant size of hybrids (personal observation) and the mean petal size were about intermediate between the parental species (Fig. 1a), implying that there were neither adverse parental effects nor heterotic effects. This implies that developmental stability of hybrids was lower due to genetic stress imposed by hydridization. The breakdown of coadapted gene complexes is the most probable explanation for the lower developmental stability of hybrids. This is the most common outcome from studies examining the developmental stability in between-species hybrids (e.g., Graham and Felley, 1985
; Leary, Allendorf, and Knudsen, 1985
; for reviews, see Graham, 1992
, and Møller and Swaddle, 1997
). Graham (1992)
reviewed studies on developmental stability among hybrids and found that in eight of 15 studies developmental stability was decreased among hybrids, whereas no difference in developmental stability between hybrids and parental species was found in seven studies. For instance, F1 generation hybrids and backcrosses between black and red spruce (Picea mariana and P. rubens) in eastern Canada show developmental instability, poor growth, and depressed photosynthesis (Manley and Leading, 1979
). Disruption of coadapted gene complexes probably alters the regulation of physiological processes and thus the development of the phenotype, so that the phenotype of an organism is not merely a linear product of the genotype (Freeman, Graham, and Emlen, 1993
, and references therein).
These results suggest that the commonly observed association between heterozygosity and FA (e.g., Soulé, 1979
; Kat, 1982
; Leary, Allendorf, and Knudsen, 1984
; for reviews, see Palmer and Strobeck, 1986
; Leary and Allendorf, 1989
; Møller and Swaddle, 1997
) is apparently not due to heterozygosity per se. The relationship between heterozygosity and fluctuating asymmetry does not appear to be always straightforward and clear-cut (see, e.g., Sherry and Lord, 1996a, b
), and some studies have reported controversial results (for a review, see Møller and Swaddle, 1997
). In any case, the positive association between heterozygosity and developmental stability may exist only within genomes that have the same evolutionary history, as proposed by Dobzhansky (1970)
. Therefore it could be predicted that the effect of hybridization on developmental stability is dependent on the degree of divergence between hybridizing taxa. Lychnis viscaria and L. alpina are probably genetically quite widely separated as indicated, for instance, by the fact that the hybrids between L. viscaria and L. alpina are sterile. This interpretation of the negative effect of the disruption of coadaptation within the genome on FA is corroborated by the results on hybrids between subspecies showing that hybrids are developmentally as stable as parental species or show even enhanced stability (Freeman et al., 1995
; Alibert et al., 1997
). There are several possible genetic mechanisms that can cause a higher developmental instability of hybrids: genes can be expressed at inappropriate stages of development, regulatory loci coding for enzymes in the major amino acid synthesis pathways may be altered by hybridization, and the likelihood of mutations can be increased (reviewed by Møller and Swaddle, 1997
).
FOOTNOTES
1 The author thanks P. Aronta, I. Eriksson, M. Hovi, A. Lammi, K. Mustajärvi, T. Savolainen, and J. Tissari for their assistance in the field, and H. Callahan, M. Hovi, R. Jackson and S.Parri for their comments on the manuscript, and Konnevesi Research Station and Laukaa Research and Elite Plant Station for all kinds of support. This study was financially supported by the Academy of Finland. 
LITERATURE CITED
Alibert, P., F. Fel-Clair, K. Manolakou, J. Britton-Davidian, and J.-B. Auffray. 1997 Developmental stability, fitness, and trait size in laboratory hybrids between European subspecies of the house mouse. Evolution 51: 1284–1295. [CrossRef][ISI]
Böcher, T. W. 1977 Experimental and cytological studies on plant species. -XIV: Artificial hybridizations in Viscaria. Botaniska Tidskrift 72: 31–44.
Clarke, G. M. 1993 The genetic basis of developmental stability. I. Relationships between stability, heterozygosity and genomic coadaptation. Genetica 89: 15–23. [CrossRef][ISI]
Dobzhansky, T. 1970 Genetics of the evolutionary process. Columbia University Press, New York, NY.
Eriksson, M. 1996 Consequences for plant reproduction of pollinator preference for symmetric flowers. M. Sc. thesis, Department of Zoology, Uppsala University, Uppsala.
Freeman, D. C., J. H. Graham, and J. M. Emlen. 1993 Developmental stability in plants: symmetries, stress and epigenesis. Genetica 89: 97–119. [CrossRef][ISI]
———, ———, D. W. Byrd, D. E. McArthur, and W. A. Turner. 1995 Narrow hybrid zone between two subspecies of big sagebrush, Artemisia tridentata (Asteraceae). III. Developmental instability. American Journal of Botany 82: 1144–1152. [CrossRef][ISI]
Graham, J. H. 1992 Genomic coadaptation and developmental stability in hybrid zones. Acta Zoologica Fennica 191: 121–131.
———, and J. D. Felley. 1985 Genomic coadaptation and developmental stability within introgressed populations of Enneacanthus gloriosus and E. obesus (Pisces, Centrarchidae). Evolution 39: 104–114. [CrossRef][ISI]
———, D. C. Freeman, and J. M. Emlen. 1993 Antisymmetry, directional asymmetry and chaotic morphogenesis. Genetica 89: 121–137. [CrossRef][ISI]
Hultén, E. 1971 Atlas of the distribution of vascular plants in northwestern Europe. Generalstabens litografiska anstalts förlag, Stockholm.
———, and M. Fries. 1986 Atlas of the North European vascular plants north of the Tropic of Cancer. Koeltz, Köningstein.
Jennersten, O. 1988 Pollination of Viscaria vulgaris (Caryophyllaceae): the contributions of diurnal and nocturnal insects to seed set and seed predation. Oikos 52: 319–327. [CrossRef][ISI]
———. 1991 Costs of reproduction in Viscaria vulgaris (Caryophyllaceae): a field experiment. Oikos 61: 197–204.
Kat, P. 1982 The relationship between heterozygosity for enzyme loci and developmental homeostasis in peripheral populations of aquatic bivalves (Unionifae). American Naturalist 119: 824–832. [CrossRef][ISI]
Leary, R. F., and F. W. Allendorf. 1989 Fluctuating asymmetry as an indicator of stress: Implications for conservation biology. Trends in Ecology and Evolution 4: 214–217. [CrossRef]
———, ———, and K. L. Knudsen. 1984 Superior developmental stability of heterozygotes at enzyme loci in salmonid fishes. American Naturalist 124: 540–551. [CrossRef][ISI]
———, ———, and ———. 1985 Developmental stability and high meristic counts in interspecific hybrids of salmonid fishes. Evolution 39: 1318–1326. [CrossRef][ISI]
Manley, S. A. M., and F. T. Leading. 1979 Photosynthesis in black and red spruce and their hybrid derivates: ecological isolation and hybrid adaptive inferiority. Canadian Journal of Botany 57: 305–314.
Merilä, J., and M. Björklund. 1995 Fluctuating asymmetry and measurement error. Systematic Biology 44: 97–101. [CrossRef]
Møller, A. P., and J. P. Swaddle. 1997 Asymmetry, developmental stability, and evolution. Oxford University Press, Oxford.
Noruis, M. J./SPSS Inc. 1992 SPSS for Windows, Advanced statistics release 5. Chicago, IL.
Palmer, A. R., and C. Strobeck. 1986 Fluctuating asymmetry: Measurement, analysis, patterns. Annual Review of Ecology and Systematics 17: 391–421.
Rune, O. 1953 Plant life on serpentines and related rocks in the north of Sweden. Acta Phytogeographica Suecica 31: 1–139.
Sherry, R. A., and E. M. Lord. 1996a Developmental stability in flowers of Clarkia tembloriensis (Onagraceae). Journal of Evolutionary Biology 9: 911–930. [CrossRef][ISI]
———, and ———. 1996b Developmental stability in leaves of Clarkia tembloriensis (Onagraceae) as related to population outcrossing rates and heterozygosity. Evolution 50: 80–91. [CrossRef][ISI]
Siikamäki, P., and A. Lammi. 1998 Fluctuating asymmetry in central and marginal populations of Lychnis viscaria in relation to genetic and environmental factors. Evolution 52: 1285–1292. [CrossRef][ISI]
Sokal, R. R., and F. J. Rohlf. 1981 Biometry, 2nd ed. W. H. Freeman, New York, NY.
Soulé, M. E. 1979 Heterozygosity and developmental stability: another look. American Naturalist 33: 396–401.
Valentine, D. W., and M. E. Soulé. 1973 Effects of p.p'–DDT on developmental stability of pectoral fin rays in the grunion, Leuresthes tenuis. U.S. Marine Fisheries Service Fisheries Bulletin 71: 921–926.
Vrijenhoek, R. C., and S. Lerman. 1982 Heterozygosity and developmental stability under sexual and asexual breeding systems. Evolution 36: 768–776. [CrossRef][ISI]
Wilson, G. B., J. Wright, P. Lusby, W. J. Whittington, and R. N. Humphries. 1995 Lychnis viscaria L. (Viscaria vulgaris Bernh.). Journal of Ecology 83: 1039–1051. [CrossRef]
Zakharov, V. M. 1981 Fluctuating asymmetry as an index of developmental homeostasis. Genetica (Pol.) 13: 241–256.
———. 1992 Population phenogenetics: analysis of developmental stability in natural populations. Acta Zoologica Fennica 191: 7–30.
Zar, J. H. 1996 Biostatistical analysis, 3rd ed. Prentice Hall International Editions, London.
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