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Biological Journal of fhe Linnean SocieQ (1993), 49: 185-194. With 2 figures Population differentiation in Crepis tectorurn (Asteraceae):patterns of correlation among characters STEFAN ANDERSSON Department of Systematic Botany, University S-223 61 Lund, Sweden Lund, 0. Vallgatan 18-20, of Received 3 October I S S I , accepled f o r publicafion 20 March I992 This study examines the relationship among traits distinguishing populations of C. fecforum and the extent to which existing trait associations reflect underlying (genetic) tradeoffs. Highly consistent trait associations were found in a comparison of 52 populations representing the western part of the geographical range of the species. In addition to a tight integration of traits reflecting plant stature and inflorescence development, there were consistent links between vegetative and reproductive traits; populations characterized by individuals with large leaves and tall stems with terminal branches usually had larger heads, flowers and fruits (achenes) than those whose individuals had small leaves and a short stem branched from the base. There was a weak negative relationship between the extent of leaf dissection and plant stature; short and compact plants had more deeply lobed leaves than tall plants with terminal branches. Few of these associations were present among families representing a single population of C. fecforum,but there was remarkable similarity between the correlations at the between-population level and those obtained in two segregating F2 progenies of crosses between contrasting populations. Hence, provided that the F2 correlations have a strong genetic basis, it appears that the course of population divergence has been constrained by the underlying correlation structure, although some trait associations may also be a result of selection operating in a correlated fashion on functionally related traits, perhaps leading to linkage disequilibrium of parental traits in the first segregating generation of a cross between ecologically differentiated populations. ADDITIONAL KEY WORDS:-Genetic adaptation - pleiotropy. correlations - morphology correlated response ~ - CONTENTS Introduction . . . . . . . . . Materials and methods . . . . . . . . . . . . . . The plant Comparative studies . . . . . . Genetic analyses . . . . . . . Results . . . . . . . . . . Matrix comparison . . . . . . Trait associations. . . . . . . Genetic correlations . . . . . . Discussion . . . . . . . . . Trait associations and their genetic basis . Adaptive changes and correlated responses Acknowledgements . . . . . . . References . . . . . . . . . 0024-4066/93/060185+ 10 s08.00/0 . . . . 185 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 . . . . . . 186 186 186 187 187 188 188 188 189 191 191 193 193 194 1993 The Linnean Society of London 186 S. ANDERSSON INTRODUCTION Whether a trait will evolve in response to natural selection depends on its heritability and the extent to which different traits are affected by the same genes (pleiotropy) or by different genes in linkage disequilibrium (Falconer, 1981). For instance, the origin of a new trait combination may be retarded if the sign of the genetic correlation is opposite to the requirement of the habitat (Antonovics, 1976; Lande, 1982), a common observation in plant populations (Silander, 1985; Roach, 1986; Weis, Hollenbach & Abrahamson, 1987; Wolff & Delden, 1987; Geber, 1990; Dorn & Mitchell-Olds, 1991; but see Mitchell-Olds, 1986; Mazer, 1989; Schwaegerle & Levin, 1991). Genetic correlations may also lead to a non-adaptive change in a trait as a result of selection on another trait, or facilitate changes in suites of traits that are correlated and selected in the same direction (Sokal, 1978; Venable & Blirquez, 1990; Armbruster, 1991). Hence, whether further micro-evolution will be retarded or enhanced depends on the relationship between trait and fitness and the sign of the genetic correlation. While reliable estimates of genetic correlations can improve predictions of the short-term response to selection (Falconer, 198 1; Mitchell-Olds & Rutledge, 1986), it is more difficult to assess the role of genetic tradeoffs in previous generations. An indirect approach is to search for trait associations at more than one taxonomic level. For instance, correlations that are manifest both within and between populations imply that genetic tradeoffs have constrained large-scale patterns of variation (Sokal, 1978; Venable & Blirquez, 1990; Armbruster, 1991). Evidence for genetic constraints would also support the use of allometric relationships as null models against which to test adaptation (Midgley, Cowling & Lamont, 1991) and argue against comparisons based on overall similarity in morphology; the genetic distance between two taxa may be overestimated if there are strong genetic correlations among traits used to distinguish the taxa (Grant, 1975). Previous work has revealed a number of possible constraints in Crepis tectorum, including a presumably ‘energetic’ tradeoff between offspring size and the number of flowers (Andersson, 1989b) and a positive correlation between overall plant size and the size of the floral structures (Andersson, 1989d). However, no effort has yet been made to elucidate the genetic basis of these correlations in order to rule out the possibility that associated traits evolved independently of each other. In the present study, I describe the pattern of correlation among a wide range of vegetative and reproductive traits and test the hypothesis that the underlying correlation structure has constrained large-scale patterns of variation in C. tectorum. Two approaches were used to reveal a genetic basis of existing trait associations. The first approach was based on data obtained in a previous quantitative genetic analysis of a natural population (Andersson, 199la), while the second was based on patterns of character segregation in the F2 generation of two crosses between widely different populations. MATERIALS AND METHODS The plant Crepis lectorurn L (Asteraceae) is a diploid (2n = 8) weedy plant native to Eurasia and introduced to North America. The species has undergone PATlERNS OF CORRELATION IN C. TEC’TORUM 187 considerable morphological divergence, particularly in areas where the weed type grades into more specialized forms adapted to rocky outcrops (Babcock, 1947; Anderson, 1989a-d, 1990, 1991a, b). Most plants behave as winter annuals, germinating in the autumn, overwintering as leaf rosettes and flowering in the following summer. Each inflorescence (‘head’) contains a large number of ligulate flowers surrounded by protective involucral bracts. Fertilized flowers develop into one-seeded indehiscent fruits (‘achenes’) as in all composites. Comparative studies Morphometric data from individuals from different populations raised in a greenhouse were obtained with the aim of detecting consistent trait associations in C. tectorum. I compared 52 populations from the western part of the geographical range of the species (Europe and Canada) on the basis of the median for each of 11 quantitative traits (Table I ) , all of which showed significant among-population variation (P< 0.00 1 ; Kruskal-Wallis tests). I used the medians rather than the means to reduce the influence of single outliers (unpublished). Trait associations were expressed in terms of the Spearman rank correlation coefficient ( rs) instead of the conventional product-moment correlation coefficient, since transformations sometimes failed to improve normality (unpublished). For some traits, I extended the analysis to the amongspecies level by using data from an extensive monograph on the genus (Babcock, 1947). Genetic analyseJ If genetic tradeoffs have constrained trait combinations in the course of population divergence, one would expect to find similar associations among genotypes within populations (Sokal, 1978; Venable & Burquez, 1990; Armbruster, 1991). This idea was tested by calculating the rank correlations based on the means of 40 greenhouse-grown families representing a single population; each family consisted of 20-25 offspring derived from a maternal TABLE 1. The traits measured in this study. All analyses were based on one measurement per plant, the only exception being achene length which represents an average of two achenes ~~~ Trait Leaf traits: Leaf dissection Leaf length Plant slature: Plant height Side branches Basal branches Head traits: Peduncle length Involucre length Flowers per head Ligule length Head width Achene length Description (unit of measurement) 1 -(the smallest width between lobes/the maximum leaf width) (mm) (mm) The proportion of nodes with side branches The number of basal branches Length of third side branch from the top (mm) (mm) (mm) (mm) (mm) S. ANDERSSON 188 TABLE 2. Information on the parents (P) used in the F2 analyses ~~ ~ Locality Habitat Cross no. I P, Filehajdar, island of Gotland, Sweden P, Bordeaux, France Cross no. 2 P, Vickleby, island of Oland, Sweden P, Algutsboda, Srnkland, Sweden Outcrop Ruderal weed Outcrop Weed in arable field plant (Andersson, 1991a). Family-mean correlations yield rough estimates of the extent to which genes have direct effects on more than one trait or are in linkage disequilibrium with other genes (Falconer, 1981; Mazer, 1989). Patterns of segregation among F2 progeny of crosses between populations provided additional information on the extent to which existing trait associations have a genetic basis (Grant, 1975; Dijk, 1984; Soltis, 1986). Seeds of two segregating F2 generations were obtained by selfing F1 plants of crosses between some of the populations used in the comparative study (Table 2). I raised a large number of F2 plants in the greenhouse, scored the plants for traits that distinguished the parents (P< 0.001, Mann-Whitney U-test) and calculated the rank correlation coefficient among traits that segregated in the F2 generation (as indicated by greater variance in the F2 than in the F1 generation). To examine whether F2 correlations could reflect environmental responses in the same or in the opposite direction, I performed a similar analysis of the non-segregating F1 generation to reveal correlations with a strong environmental component. RESULTS Matrix comparisons The largest absolute values of the correlations were found between populations of C. lectorurn (average r, = 0.391) and among species of Crepis (average r, = 0.323), while trait correlations were weaker among maternal families in a population of C. tectorum (average r, = 0.248) and among individual plants in two segregating F2 progenies (average r, = 0.180 and 0.272, for cross nos 1 and 2, respectively). A comparison based on the individual correlations revealed great similarity between the two F2 correlation matrices ( r , = 0.929) and between these and the among-population correlation matrix (r, > 0.80; Fig. 1). The within-population correlation matrix was moderately correlated to the among-population and F2 matrices (rs = 0.390-0.555), while only a weak correspondence was found between the among-species matrix and the other matrices (rs < 0.277). N o attempt was made to test the significance of these correlations (the data not being independent). Trait associations Many significant correlations were detected in the between-population matrix (Fig. 2A), although some of these may be an artefact of the large number of tests performed ( = 55 pairwise correlations). Hence, I focus on correlations that were PArTERNS OF CORRELATION IN C. TECTORUM . ‘I 0. I89 0 0 0 a P -1 0 1 F2 correlation Figure I . The relationship between trait associations at the between-population level and in two segregating F2 populations. The Spearman correlation coefficient was 0.802 and 0.977 for cross no. I and 2, respectively. significant at conservative levels of significance (P< 0.01 or 0.001). Traits directly related to the development of the inflorescence and the individual flowers (‘head traits’) were highly integrated, as shown by strong positive correlations between the length of the branch supporting the head (peduncle length), the length of the involucral bracts surrounding the flowers, the number of flowers per head, the size of the flowers (ligule length), the diameter of the flowering heads, and the size of the fruit associated with each flower (achene length). Another group of related traits (‘plant stature’) distinguished populations with a tall stem and terminal branches from those with a short stem with side branches that have become ‘basal’ branches due to a secondary shortening of the lowermost internodes (Fig. 2A). Strong associations were also found between traits in different developmental categories; populations characterized by the tallest stem and the largest leaves usually had the largest involucral bracts, the widest heads, the largest flowers and the largest achenes. The extent of leaf dissection varied independently of leaf length but showed a weak relationship with plant stature; short and compact plants had more deeply lobed leaves than tall plants with terminal branches. There was no tradeoff between achene size and the number of flowers per head (Fig. 2A). Most of the trait associations in C. tectorum were also present at a higher taxonomic level; large-leaved species of Crepis had a taller stem and higher values of all head traits than those with smaller leaves; species usually exhibited large (or small) values for several head traits simultaneously (Fig. 2B). However, plant height was only weakly related to the head traits, contrasting with patterns observed among populations of C. tectorum (Fig. 2A). Genetic correlations Attempts to relate trait associations within and between populations were hampered by the few significant correlations detected at the within-population S. A N D E R S O N 190 BETWEEN-POPULATIONCORRELATIONS (N = 62) Leaf branch- F2 CORRELATIONS (CROSS NO 1; N = 186 - 197) h f Aehene length Achnnr hngh bmnchm A D BETWEEN-SPECIES CORRELATIONS (N = 121 .193) F2 CORRELATIONS (CROSS NO 2; N 31 175 - 185) Ache- Iad khsne P l ~ t heisht Plowon perherd branch- E B - BETWEEN-FAMILY CORRELATIONS (N 40) Led dismction lengh Achane width - P co.001 - Plant height Pc0.01 P < 0.05 Bad bnnchm Peduncle hngh C Figure 2. Diagram showing the pattern of correlation among characters. Positive correlations are shown with solid lines and negative correlations are shown with dashed lines (only for correlations significant at the 5% level). Asterisks denote F2 correlations that were statistically significant in the genetically uniform F1 generation. level. The major trait association distinguished between families with a tall stem, wide flower heads and large achenes and those with the opposite features (Fig. 2C) and corresponds to a similar relationship among populations (Fig. 2A). PATTERNS OF CORRELATION IN C. TECTORUM 191 Analysis of segregating F2 progenies revealed a larger number of significant correlations (Fig. 2D, E), most of which parallel those obtained at the amongpopulation level (Fig. 1). The segregation pattern not only confirms a strong genetic correlation among traits in the same developmental category, but also provides evidence for a genetic basis of the numerous links between plant stature and head traits. The extent of leaf dissection and plant architecture were also related; F2 plants with deeply lobed leaves were more compact and of shorter stature than those with weakly lobed leaves. Achene length and the number of flowers per head showed independent segregation in the F2 generation (Fig. 2E). Similar results were obtained when flower number was expressed on a per-plant basis (r, = -0.08, P = 0.27). Few of the F2 correlations were significant in the F1 generations (marked with asterisks in Fig. 2D, E), indicating that trait associations in the F2 generation were largely genetic. However, the result may also reflect differences in statistical power; the F1 correlations were based on smaller samples ( N = 39-40) than the F2 correlations ( N = 175-197). DISCUSSION Trait associations and their genetic basis The present study demonstrates consistent correlations among traits distinguishing populations of C. tectorum. As expected, there were strong positive correlations among developmentally related traits, particularly those reflecting plant stature or the size of structures associated with the flower head (see also Berg, 1960; Armbruster, 1991, but see Venable & Bfirquez, 1990). In addition to a tight integration of traits related by development, there was a tendency for populations to have either large or small values of traits reflecting the size of the leaves, the main stem and the flower heads. Most of these associations also occurred at the between-species level, the only exception being plant height which varied more or less independently of the other traits. The link between vegetative and reproductive features complements similar findings by other authors (Primack, 1987; Giles & Bengtsson, 1988; Bond & Midgley, 1988; Midgley & Bond, 1989; Thompson & Rabinowitz, 1989; Venable & Bh-quez, 1990). A weaker trend distinguished between populations with a prostrate growth form and deeply lobed leaves from those with a tall stature and weakly lobed leaves; this correlation largely reflects the unusually short stem and the finely divided leaves of subsp. pumila on the Baltic island of Oland (Andersson, 1989a, c). More data are needed to examine whether similar associations exist in other species. Although comparative studies sometimes provide insights into genetic constraints on character evolution, only a genetic analysis can exclude the possibility that associated traits have been integrated due to selection rather than genetic tradeoffs. Yet, surprisingly few studies have examined the genetic basis of trait associations above the population level (e.g. Venable & Bh-quez, 1990; Grant, 1975; Dijk, 1984; Soltis, 1986). In the following, I examine the hypothesis that underlying (genetic) correlations have constrained large-scale patterns of variation in C. tectorum. I92 S. A N D E R S O N Population divergence should be easiest along the major axis of variation, since there is a lack of variation for the alternative trait combinations. Hence, provided that the genetic correlation structure has been stable, one would expect that the pattern of correlation among a set of populations will be similar to the pattern of correlation within these populations (Sokal, 1978; Venable & Bfirquez, 1990; Armbruster, 1991). Genetic data from a single population of C. tectorum (Andersson, 1991a) lend some support to this idea, but only for some trait combinations; there were highly significant positive correlations between plant height and head width and between head width and achene length. Apart from these conservative associations, there was only a weak relationship between the two correlation matrices (see also Venable & Blirquez, 1990), a reflection of population divergence in quantitative genetic parameters (Mitchell-Olds, 1986) or the use of maternal families; family-mean correlations may be biased for various reasons (non-additive genetic effects, etc.; Falconer, 1981; Mitchell-Olds & Rutledge, 1986). In contrast, there was remarkable similarity between the between-population correlation matrix and the two F2 correlation matrices; trait associations found between populations were also detected in segregating progenies of crosses between some of these populations. This is consistent with results of a detailed analysis of leaf shape; both comparative and gene& evidence indicated a major trend distinguishing between plants with lanceolate, wide and deeply lobed leaves and those with the opposite features (Andersson, 1991b), Hence, provided that differences among F2 plants have a strong genetic basis and that linkage due to selection can be ruled out (see below), it seems that the underlying correlation structure has indeed influenced the present-day pattern of variation in C. tectorum. Pleiotropy is usually regarded as the major cause of genetic correlations among traits (Falconer, 1981), but linkage disequilibrium may also contribute to trait associations (Grant, 1975; van Dijk, 1984; Soltis, 1986). Genetic linkage seems likely in C. tectorum, considering the low chromosome number of this species (2n = 8). The effect of linkage disequilibrium can be substantial in the first segregating generation(s) of population crosses, particularly if the parental differences involve a large number of adaptive characters. Hence, the following discussion contains the caveat that the between-population and F2 correlations could both arise from selection (see below). Analyses of further generations are needed to distinguish between these hypotheses and to confirm the idea that most F2 correlations have a strong genetic component (as suggested by F1 data). That correlations among populations were of lower magnitude (though still significant) in the F2 generations may reflect slight differences in growth conditions; quantitative genetic parameters are valid only for the environment in which they have been measured (Falconer, 1981). The observation that large-scale patterns of differentiation can be predicted from patterns of variation at a lower organizational level also applies to other kinds of variation. Firstly, morphometric data of C. tectorum raised in a series of differing environments showed that population divergence in the overall mean has been greatest in the phenotypically most plastic traits and lowest in the least plastic traits (Andersson, 1989~).Secondly, there appears to be a positive correlation between the extent of divergence and the extent of variation among family groups derived from a single population (Andersson, 1991a). Similarities PATTERNS OF CORRELATION IN C. TECTORUM 193 in patterns of variation across taxonomic levels combined with evidence that selection still operates on traits characterizing subsp. pumilu (Andersson, 1992) support the view that processes acting within populations are sufficient to account for diversity at higher levels (Sokal, 1978; Davis & Gilmartin, 1985). Adaptive changes and correlated responses The major evolutionary trend in C. lectorum reflects adaptation to shallow soil on rocky outcrops, particularly in the Baltic region, where populations have undergone considerable reduction in plant size compared with populations in more mesic habitats (Andersson, 1989a-d, 1991a). While the short stem probably evolved as a selective response to stressful conditions in the outcrop habitat (drought, nutrient deficiency and wind exposure; Witte, 1906; Andersson, 1989c, 1991a), it is more difficult to explain the small flower heads of some of these populations. For instance, plants of the largely self-sterile subsp. pumilu have less conspicuous heads than more or less autogamous plants of the taller weed type (Andersson, 1989d), a surprising result considering the usual observation that insect-pollinated plants have more attractive structures than plants capable of automatic self-pollination (Ornduff, 1969). O n the basis of genetic data (Fig. 2C-E), I suggest that some of the floral reduction in the outcrop habitat may be a correlated response to selection for a smaller stature. The small achenes of the decumbent outcrop plants may have a similar ‘non adaptive’ explanation, although a functional explanation is also possible; small size of propagules may increase the dispersability or the probability of becoming incorporated in the seed bank (Andersson, 1990). Another presumably adaptive association relates leaf shape and plant stature. The extremely dry and exposed conditions on the Baltic island of Oland favour combinations of traits which reduce transpiration and overheating, e.g. deeply lobed leaves and a short stature, two distinctive features of subsp. pumilu (Andersson, 1989a, c). Hence, the direction of selection and the sign of the genetic correlation (Fig. 2D, E) probably converge for these traits. Previous work on C. tectorum in southern Sweden indicated a possible tradeoff between achene size and the number of flowers (Andersson, 1989b, c). However, the present study indicates sufficiently independent control of these traits to allow selection (or drift) to operate independently on them. There was no correlation between achene size and the number of flowers when a large number of populations was studied (Fig. 2A), nor was there any tradeoff between these variables when genetic data were considered (Fig. 2C-E). This illustrates the importance of genetic analyses or large comparative studies before any broad generalizations are made regarding life-history ‘tradeoff?. ACKNOWLEDGEMENTS I would like to thank K. Ryde for linguistic advice, the staff of various botanical gardens for providing seed material and M. Lawrence and an anonymous reviewer for their comments. Technical assistance by Monica Christiansson and Helena Persson is also acknowledged. I94 S. 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