AOBPreview originally published online on July 26, 2004
Annals of Botany 2004 94(3):419-426; doi:10.1093/aob/mch159
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Annals of Botany 94/3, © Annals of Botany Company 2004; all rights reserved
Genetic structure and outcrossing rates in Flourensia cernua (Asteraceae) growing at different densities in the South-western Chihuahuan Desert
1 Instituto de Ecología A. C., Ap. Postal 63, 91070 Xalapa, Veracruz, México, 2 Departamento de Ecología Evolutiva, Instituto de Ecología, UNAM, Apartado Postal 70-275, 04510 México D. F., México and 3 Instituto de Ecología A. C., Ap. Postal 63, 91070 Xalapa, Veracruz, México
* For correspondence. E-mail mferrer{at}miranda.ecologia.unam.mx
Received: 4 March 2004 Returned for revision: 13 April 2004 Accepted: 25 May 2004 Published electronically: 26 July 2004
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXACKNOWLEDGEMENTSLITERATURE CITED |
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• Backgrounds and aims Flourensia cernua is a partially self-incompatible, wind-pollinated shrub that grows in two scrub types of contrasting densities. It was anticipated that differences in plant density would affect the amount of genotype availability, and thus higher outcrossing rates and less genetic differentiation would be found at high-density sites.
• Methods At five high-density sites and at five low-density sites, 11 allozyme loci were analysed in adults. Outcrossing rates were estimated using five allozyme loci sampled from eight families from each scrub type.
• Key results High levels of genetic variation were found at all sites (ranging from P = 82–100 %, He = 0·33–0·45, and Ho = 0·4–0·59). Heterozygotes were found in excess (FIS = –0·15 ± 0·06 s.d.), suggesting that natural selection favours heterozygosity, and there was little differentiation between sites (FST = 0·08 ± 0·02 s.d.). Life history attributes, such as long-lived habit and wide geographic distribution, as well as the presence of a self-incompatibility system may explain these results. Outcrossing rates did not differ from 1·0 in both scrub types, and there was no genetic differentiation between scrub types (FST = –0·01 ± 0·004 s.d.).
• Conclusions The high rate of outcrossing favoured by partial incompatibility may generate unrestricted gene flow between scrub types and thus may explain the lack of differentiation between them. High heterozygosity could be expected in long-lived plants of arid zones as they confront a variable and stressing environment.
Key words: Density, outcrossing rates, genetic structure, genetic variability, heterozygosity excess, self-incompatibility, Flourensia cernua
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXACKNOWLEDGEMENTSLITERATURE CITED |
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The genetic structure (i.e. the amount of genetic variability within and between populations) has an impact in the ecological adaptation and evolution of plant species (Loveless and Hamrick, 1984
; Booy et al., 2000). For instance, it has been stated that high genetic variability at the population level is important in situations of stress and also favourable when individuals grow in heterogeneous environments (Hedrick et al., 1976; Ewing, 1979; Gillespie and Turelli, 1989). This kind of reasoning made Nevo and Beiles (1989) propose that organisms inhabiting arid and semi-arid zones should display greater levels of genetic variability at the population level than those inhabiting other habitats. On the other hand, demographic factors such as density and population size influence the genetic structure of plant species (Loveless and Hamrick, 1984; Levin, 1988; Brown and Schoen, 1992). Life-history traits such as mating system and seed dispersal mechanisms also have an influence on the genetic structure of plant species (Hamrick et al., 1992; Hamrick and Godt, 1996).
Variability in outcrossing rates as a function of plant density is one of the most important factors that determine genetic structure (Farris and Mitton, 1984; Levin, 1988; Eguiarte et al., 1992). In particular, in anemophyllous species outcrossing rates in general are higher at higher plant densities (Farris and Mitton, 1984; Knowles et al., 1987; Vaquero et al., 1988), and this may be due to the proportionally smaller representation of own pollen with respect to alien pollen (Farris and Mitton, 1984). Vaquero et al. (1988) also found that decreased outcrossing rates in less dense sites were partially due to a breakdown of the incompatibility system in rye.
The genetic incompatibility system is a pre-zygotic mechanism that ensures strict outcrossing by preventing selfing and biparental inbreeding (Charlesworth and Charlesworth, 1987). The presence of this system is documented within 50 % of all angiosperms (de Nettancourt, 2001). The Asteraceae family presents a sporophytic self-incompatibility system, although a partial breakdown of the incompatibility system has been found in several species (Lane, 1996; de Nettancourt, 2001). This breakdown allows some plants to produce seeds through selfing, and leads to variation in outcrossing rates in Centaurea solstitialis (Sun and Ritland, 2000).
The presence of the genetic incompatibility system and an entomophyllous syndrome of pollinization in Asteraceae are thought to be ancestral traits (Stebbins, 1970; Lane, 1996). However, in arid, windy and/or cold habitats, lack of pollinators could lead to a shift from an entomophylous pollinization syndrome to an anemophylous one (Stebbins, 1970; Levin, 2000). In Espeletia (an Andean genus) this shift in pollination syndrome is related to a breakdown in the self-incompatibility system (Berry and Calvo, 1989). In this sense, Asteraceae species from arid zones are interesting systems to test hypotheses about effects of wind pollination and incompatibility systems, and the effects of both on outcrossing rates and on genetic structure.
In addition to outcrossing rates and the general mating system, seed dispersion life-history traits can also have a strong influence on the genetic structure of plant populations (Loveless and Hamrick, 1984; Hamrick and Godt, 1996; Booy et al., 2000). For instance, it is well known that genetic differentiation can be very high if outcrossing rates are low and seeds are only locally dispersed (Allard et al., 1968; Brown and Schoen, 1992; Hamrick and Godt, 1996). On the other hand, even moderate rates of outcrossing may prevent population subdivision, especially if pollen and seeds can be transported for large distances (Schaal, 1980; Slatkin, 1985). Pollen and seed flow are highly leptokurtic, and therefore a greater gene flow is expected when plants grow in a more clumped habit (Levin and Kester, 1974; Levin, 1988).
Flourensia cernua (Asteraceae: Heliantheae), a long-lived shrub characteristic of the Chihuahuan Desert (MacMahon, 1989), is distributed in the southwestern portion of its range among two scrub types of contrasting densities: high-density scrubs (824 individuals ha–1 ± 187 s.e.) and low-density scrubs (11 individuals ha–1 ± 2·4 s.e.; M. Ferrer, L. Eguiarte and C. Montaña, unpublished data). It is a wind-pollinated species, with hermaphroditic protandrous flowers (the first two days as male and three last days as female). The species presents a self-incompatibility system, although some individuals show a breakdown of the system and are self-compatible at different levels (M. Ferrer et al., unpubl. data). The production of viable seeds after selfing is low (2·4 % of flowers produce a viable seed); but after cross-pollination treatments the production increases to 20 % of pollinated flowers (M. Ferrer et al., unpubl. data). In this sense F. cernua is a pseudo-self-fertilizated species (sensu Levin, 1996); that is, some individuals can produce fruits after self-fertilization, but offspring derived from cross-pollination have a larger fitness than those derived from self-pollination. The seeds are primarily dispersed by gravity and subsequently by water flow. Germination rates are low and recruitment is sporadic (Mauchamp et al., 1993).
In this study, the effects of population density on genetic variation, genetic structure and outcrossing rates were evaluated in this shrub. It was predicted that this species would follow the patterns found in most wind-pollinated species: in less dense sites a decreased outcrossing rate would be found due either to a lower proportion of exogamous pollen or to a local breakdown of the incompatibility system. As a consequence, it was also expected that genetic diversity would be lower at low-density sites. Thus fewer genotypes and alleles would be found in the low-density scrub, and this would be expressed as genetic differentiation between scrub types.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXACKNOWLEDGEMENTSLITERATURE CITED |
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Study species
This study was done on the Mapimí Biosphere Reserve, Durango, Mexico (26°40'N and 103°40'W; 1100 m above sea level, 264 mm of precipitation per year, 72 % of which falls between June and September; Montaña et al., 1990
). High-density scrubs are found on lower bajadas, sloping less than 1·5 %, whereas low-density scrubs occur on upper bajadas sloping more than 3 % (Montaña, 1990). These two types of scrub do not differ in general climatic or edaphic characteristics, except for a slightly higher clay content in dense scrub soils and higher water availability due to run-off rainwater redistribution (Cornet et al., 1992; Galle et al., 2001). In the high-density scrubs, F. cernua is the dominant shrub, along with Hilaria mutica and Prosopis glandulosa var. torreyana (Montaña et al., 1990). In the low-density scrubs the dominant species are Opuntia rastrera and Larrea tridentata (Montaña, 1990).
Sampling
Sampling was carried out at five high-density sites and at five low-density sites. Sites were randomly selected within an area of 20 km2 (7 km2 of high-density scrubs and 13 km2 of low-density scrubs). Distance between the centres of the sites ranged from 0·41 km to 5·74 km, with an average of 3 km (Fig. 1). Sites were divided into 10 x 10 m quadrants. Within each quadrant, a single adult individual was chosen in a central position. Foliar tissue was collected from these individuals in October 1998 for genetic analyses. Following this scheme, foliar tissue was collected from 120 individuals found in high-density scrubs and 125 individuals occurring in low-density scrubs (with an average of 25 individuals per site). Samples were transported to the laboratory in liquid nitrogen, where frozen tissues were maintained and stored in an ultra-cold freezer at –70° C.
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To estimate outcrossing rates, fifteen individuals were selected at random from the high-density scrubs and fifteen from the low-density scrubs. In February 1999, foliar tissue as well as all the seeds produced by each of these individuals were collected and treated as 30 different genetic families in electrophoresis analyses from mother tissue and from seedling tissue. The seeds were taken to the laboratory in paper bags and maintained at room temperature, while foliar tissue was transported to the laboratory in liquid nitrogen where it was stored in an ultra-cold freezer at –70° C. During the months of July and August 1999, all seeds were allowed to germinate in environmental chambers on a substrate of moistened cotton. Seedlings were obtained for only eight out of the fifteen families from each scrub type due to low viability of the seeds (see Valencia-Díaz and Montaña, 2003
). Seedlings were maintained under a photoperiod of 12 h light (at 26° C), 12 h darkness (at 16° C), and 60 % humidity until harvesting at three weeks of age.
Electrophoresis technique
Leaves from adults were macerated with liquid nitrogen and then 75 mg of the tissue was mixed with 375 mL of extraction buffer. Seedlings were macerated with 100 mL of extraction buffer. The buffer consisted of one part Veg II (Cheliak and Pitel, 1984) and three parts YO solution (Yeh and O'Malley, 1980). The suspension was centrifuged at 210 g for 3 min at 4° C. The supernatant was absorbed using filter paper wicks (25 mm wide x 15 mm long; Whatman, number 17) and then the wicks were stored in an ultra-cold freezer at –70° C until needed.
Allozyme analysis was undertaken on horizontal starch gels (11 %; Sigma Chemical Co., St. Louis, MO). The buffer systems used were Poulik pH 8·0 and pH 7·6 (gel and electrode, respectively; modified from Piñero and Eguiarte, 1988), and L-histidine pH 6·3 and pH 7·0 (gel and electrode, respectively; also modified from Piñero and Eguiarte, 1988). The Poulik gel was allowed to run at 270 V for 12 min, after which time the wicks were removed, and the gels allowed to continue running until the front had reached 8 cm (after approximately 6 h). The L-histidine gels ran at 30 V for 15 min, after which time the wicks were removed and the gels allowed to run for 12 h until the front attained 10 cm. In the Poulik system, the following enzymes were stained: menadion reductase (Mnr, EC 1.6.99), leucyl aminopeptidase (Lap, EC 3.4.11.1), isocitrate dehydrogenase (Idh, EC 1.1.4.2), phosphoglucose isomerase (Pgi, EC 5.3.1.9), superoxide dismutase (Sod, EC 3.1.3.2), acid phosphatase (Acph, EC 3.1.3.2), and esterase locus 1 (Est, EC 3.1.1.1). In the L-histidine system, the Est (locus 2), peptidase (Pep, EC 3.4.11) and peroxidase (Apx, EC 1.11.1.7) enzymes were stained. All 11 enzyme-staining protocols were modified from Soltis et al. (1983), Vallejos (1983) and Piñero and Eguiarte (1988). In adults, all enzymes showed readable banding patterns. However, in seedlings a consistently interpretable band was only achieved with the Mnr, Acph, Apx (loci 1 and 2) and Est (locus 1) enzymes. The loci and alleles that migrated most rapidly were designated as ‘1’.
Genetic variability
For the adults at each site, estimates were made of the proportion of polymorphic loci (P, 95 % criterion), the expected (He) and observed (Ho) levels of heterozygosity, and the average fixation index (F) (Hedrick, 2000). In addition, chi-square tests were used to analyse deviations of observed genotypic frequencies from expected Hardy–Weinberg equilibrium frequencies (Sokal and Rohlf, 1995), as well as to analyse the heterogeneity of allelic frequencies among populations (Workman and Niswander, 1970). To determine if there were differences between high- and low-density scrubs in these parameters of genetic variability in adults, data were subjected to a Kruskal–Wallis non-parametric ANOVA (Sokal and Rholf, 1995). For this test, sites were considered to be repetitions within each of the two scrub types.
Genetic structure
To evaluate the genetic differentiation among adult plants between high-density and low-density scrub types, Wright's F-statistics (Wright, 1965) were estimated according to the method described by Weir and Cockerham (1984). The TFPGA program (Miller, 1996) used to undertake this analysis considered the two scrub types as populations and the individual sites as sub-populations. Standard deviations and 95 % confidence intervals for mean F-statistics were obtained through a jackknife test. The 
2 test of Li and Horvitz (1953) was used to test if FIS and FIT values per locus differed from 0, and the 2 test of Workman and Niswander (1970) was used to test if FST values per locus were different from 0. Additionally, cluster phenograms were obtained and graphically presented using Nei's (1978) genetic distances between subpopulations and UPGMA algorithms (Sokal and Michener, 1958). Finally, the correlations between estimates of Nm (Slatkin, 1993) and values of FST/(1 – FST) (Rousset, 1997) with the geographic distances between sites were also calculated, and statistical significance analysed by a Mantel test after 1000 permutations (Mantel, 1967).
Outcrossing rates
Outcrossing rates were estimated for each of the 16 families and also for each scrub type considering the eight families from each scrub type. Electrophoretic data from mother tissue and from seedling tissue were used for this purpose. A range of 8–15 seedlings was obtained per mother, providing a total of 92 individuals from the high-density scrubs and 100 individuals from the low-density scrubs. The mean outcrossing rates per scrub type for single and multiple loci (ts and tm, respectively) were estimated using the mixed mating model proposed by Ritland and Jain (1981) and the Multilocus Mating System Program (MLT; Ritland, 1990). Ninety-five percent confidence intervals were calculated as the region comprising between the 25th and 975th value of 1000 bootstrapped values of ts and tm mean values (previously sorted by their values), obtained by resampling within the families.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXACKNOWLEDGEMENTSLITERATURE CITED |
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Genetic variability
The levels of genetic diversity of adult Flourensia cernua at each site were estimated as the proportion of polymorphic loci (P), and expected and observed heterozygosity (He and Ho, respectively, Table 1). These estimates indicate high levels of genetic variation within each site, but no differences between the two contrasting plant densities (Kruskal–Wallis ANOVA, P > 0·05, Table 1). Estimates of P ranged from 81·82 to 100 %, while He varied from 0·33 to 0·45 and Ho ranged from 0·41 to 0·59.
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The mean value of the fixation index per loci per scrub, F, for all loci was negative, indicating that there was an excess of heterozygotes among adults (n = 145 individuals, 11 loci; Table 1). Observed levels of heterozygosity in five loci from the high-density scrub sites and in four loci from the low-density scrub sites were larger than those expected by the Hardy–Weinberg model (P < 0·05), varying from one locus to four loci between sites (Table 1).
Genetic structure
Estimates of FIS for adults indicated a general excess of heterozygotes as they were significantly negative for most (six) of the loci (Acph, Est1, Est2, Idh, Mnr and Pep), while only significantly positive for two loci (Apx1 and Apx2, Table 2). Mean FIS was negative and differed significantly from zero (FIS = –0·1463, 95 % confidence interval –0·2530 to –0·0276; Table 2).
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Estimates of FIT in adults were also negative and significantly different from zero (mean FIT = –0·2475, 95 % confidence interval –0·3270 to –0·1412; Table 2). Nine out of eleven loci showed a significant excess of heterozygotes (Acph, Est1, Est2, Idh, Lap, Mnr, Pep, Pgi and Sod1), whereas none of the loci showed a significant excess of homozygotes (Table 2).
Estimates of FST among sites were statistically different from zero (FST = 0·0807, P < 0·05; Table 2), indicating that despite the fact that the genetic differences are low, there is significant differentiation between the ten sites. Seven estimates of FST between the ten sites were significantly different from zero in the loci Acph, Apx1, Apx2, Est1, Est2, Lap, Pgi and Sod1 (Table 2). On the other hand, if only the two scrub types are compared, there is no genetic differentiation (FST = –0·0130, P > 0·05; Table 2); negative values of FST are usually considered a statistical artefact and not different from zero.
As suggested by the low FST values between site values, the Nei's genetic distances (D) for adults estimated between paired sites were relatively small (D = 0·0037; range –0·022 to 0·1366). Average Nm was 3·47 ± 1·47 s.d. for all pairs of sites; in sites within low-density scrubs Nm was 3·27 ± 2·48 s.d.; and in sites within high-density scrubs Nm was 2·48 ± 0·87 s.d. (Table 3). Pairs estimates of Nm (M) and geographic distances correlation was –0·133, but did not differ significantly from zero (Mantel test, original Z = 747·30, mean Z after permutations = 773·91, P = 0·55 and 0·45 upper and low tail probability, respectively, see Table 3), and the correlation analysis using Rousset estimates did not improve the correlation (r = –0·133, Mantel test, original Z = 14·56, mean Z after permutations = 15·08, P = 0·80 and 0·19 upper and low tail probability, respectively). Both results indicate no isolation by distance. High- and low-density scrubs intermingle in the phenogram, suggesting that genetic similarity depends more on geographic distribution than on the scrub type identity (Figs 1 and 2).
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Outcrossing rates
Mean estimates of ts (single-locus outcrossing rate) and tm (multi-locus outcrossing rates) were high in both types of scrub (1·19 ± 0·19 s.d. and 1·19 ± 0·21 s.d. for the high-density scrubs, and 0·83 ± 0·27 and 0·79 ± 0·25 s.d. for the low-density scrubs, respectively). The value of ts was slightly higher than tm in both scrub types, however this difference was not significant. The 95 % confidence intervals for ts and tm comprised a large region, suggesting that progeny may have been derived both from selfing and outcrossing in both scrub types (95 % confidence intervals ranged from 0·66 to 1·49 and from 0·23 to 1·37 for ts, and from 0·67 to 1·47 and from 0·23 to 1·22 for tm for high-density and low-density scrubs, respectively). No differences in tm were detected between the two types of scrub, given that confidence intervals for both types overlapped and neither one differed significantly from 1·00.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXACKNOWLEDGEMENTSLITERATURE CITED |
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Ten sites of F. cernua were studied, and high levels of genetic variation, significant genetic differentiation and an excess of heterozygotes were found. Outcrossing rates were high, and not significantly different from 1, but contrary to our expectations, neither effects of the plant density on levels of genetic variation, genetic structure or outcrossing rates, nor signals of isolation by distance were found. These results may be explained by life-history attributes such as a woody, long-lived habit and wide geographic distribution, as well as by the presence of a self-incompatibility system. We suggest that this pattern is enhanced by natural selection favouring heterozygosity, and that high gene flow between scrub types prevents genetic differentiation.
Variability and genetic structure
Genetic variability in Flourensia cernua is relatively high, while genetic differentiation between sites is low but significant. On the other hand, there are no genetic differences between the two scrub types (i.e. high-density scrub and low-density scrub). Similar patterns (genetic variability within populations and not between them) have been reported in several long-lived, widespread and outcrossing species (see review in Hamrick et al., 1992; Hamrick and Godt, 1996). In fact, our estimates of genetic variability within sites were higher than the averages for long-lived, widespread and outcrossing species of several families, and for 101 Asteraceae species (P = 45·3, He = 0·127; Hamrick and Godt, 1996) and also greater than the values reported for other woody or succulent species occurring in arid zones (Martínez-Palacios et al., 1999, and citations therein). Genetic variability in F. cernua is comparable to that encountered in Prosopis glandulosa var. torreyana, a long-lived and preferentially cross-pollinated species common to the Chihuahuan Desert (Ho = 0·45 ± 0·03 s.d., F = – 0·46 ± 0·1 s.d., tm = 1, Golubov et al., 1999).
Our results clearly indicate a significant excess of heterozygotes in the adults for six out of 11 loci for FIS and in nine out of 11 loci for FIT. A significant excess of heterozygotes among adults may be explained in general by natural selection favouring heterozygosity (Hedrick, 2000), and perhaps for some loci by a linkage between the allozyme loci and the loci that control the self-incompatibility system (Leach, 1988). In the first case, the excess of heterozygosity comprises various loci and increases along the life-cycle development (e.g. Eguiarte et al., 1992; Alvarez-Buylla and Garay, 1994; González-Astorga et al., 2003), while in the second case, only one or two allozyme loci shows this significant excess (e.g. Manganaris and Alston, 1987; O'Leary and Boyle, 1998).
In F. cernua, seed viability in general is low (approx. 9 %), but the viability of progeny derived from outcrossing is higher than that of offspring derived from selfing (M. Ferrer et al., unpubl. results). These results suggest that few self-sired seedlings were able to survive until adult stage due to inbreeding depression. Therefore, adult populations are expected to be composed mainly of outcrossed and heterozygous plants. The high level of heterozygosity in F. cernua may confer certain selective advantages. For example, heterozygosity plays an important role in species that inhabit stressful environments (Rainey et al., 1987; Mopper et al., 1991). Plants in arid zones face great temporal environmental heterogeneity that is related primarily to the unpredictability of precipitation. For this reason, high genetic diversity might be favoured (Nevo and Beiles, 1989). However, we found no differences between the two types of contrasting densities, and the relative performance of heterozygote individuals is not known for life-history stages other than germination in this species. For these reasons, selection favouring heterozygosity must remain a hypothesis yet to be tested.
On the other hand, 1–4 loci per site had a significant excess of heterozygotes and in these cases only the Mnr locus always had a significant heterozygote excess at all sites, suggesting that this locus may be linked with the incompatibility locus (see Appendix). After finding a significant heterozygote excess, O'Leary and Boyle (1998) demonstrated that the Lap-1 locus is linked with the self-incompatibility locus in the Christmas cactus Schlumbergera. More genetic studies are needed to understand linkage between the Mnr locus and self-incompatibility locus in F. cernua.
Genetic structure
Pollen and seed movement may influence the low genetic differentiation between populations of F. cernua. The pollen of this species is dispersed by wind (Mauchamp et al., 1993). Although direct estimates of pollen movement are lacking in this study, other studies of anemophyllous species have found that pollen is capable of being dispersed for distances greater than 3 km (Ratchke and Lacey, 1985; Levin, 1988). Geographic distances between sites in this study ranged from 0·5 km to 6 km (Fig. 1), and for this reason it is doubtful that the physical distances between sites could be a barrier to pollen dispersal.
The seeds of F. cernua disperse primarily by gravity and secondarily by surface water run-off after rare heavy rains (Montaña et al., 1990; Mauchamp et al., 1993). Surface water run-off could easily allow the movement of seeds from low-density to high-density scrub associations, since the latter are always located down-slope. Individuals in low-density scrubs are typically distributed around ephemeral streams (i.e. arroyos), which run from the upper bajadas down to the lower bajadas where high-density F. cernua scrubs are found (Fig. 1). The surface water run-off might have caused a directional flow of seeds (i.e. down-slope) at these sites, and thus may explain the pattern of genetic similarity observed in this study (Figs 1 and 2).
Effect of density on outcrossing rates
No significant differences were found among ts and tm estimates, and there were no deviations from the expectation of the mixed mating model. However, constraints of the mixed mating model could be observed if it is considered that partial negative assortative mating is occurring in F. cernua due to the presence of the self-incompatibility system. Because departures from the mixed mating model have less of an effect on the multilocus outcrossing rates, tm can be regarded as the more reliable estimate (Ritland and Jain, 1981; Ritland, 1990; Eguiarte et al., 1992). The outcrossing rates in F. cernua are high and not different from 1, but the estimated variances are also very large. We suggest that this variance may be due to the fact that, as controlled self-pollinations revealed, most plants are completely self-incompatible, but some of them display a variable degree of self-compatibility that can be very high in some cases (M. Ferrer et al., unpubl. data).
Contrary to our initial expectations, population density did not affect outcrossing rates in F. cernua. The results of this study agree with results for some other species, including Picea abies (a wind-pollinated conifer; Morgante et al., 1991), Solidago sempervivens (an incompatible Asteraceae; Innes and Hermanutz, 1988), and Cakile maritima (an incompatible Brassicaceae; Thrall et al., 2000). Long-distance wind-dispersion of pollen and the presence of a partial self-incompatibility system in F. cernua may be the principal factors determining the lack of statistical association between population densities and outcrossing. Morgante et al. (1991) associated the lack of correlation between outcrossing rates and plant density in P. abies with high viability and dispersion capacity of pollen (dispersion greater than 5 km). The presence of a self-incompatibility system is the reason for the absence of a density effect on outcrossing rates in Solidago sempervivens (Innes and Hermanutz, 1988) and in Cakile maritima (Thrall et al., 2000).
High heterozygosity could be favourable in long-lived plants of arid zones as they confront a stressing environment: thus high outcrossing rates should be favoured despite a high spatial variability in population density. We suggest that high outcrossing rates in long-lived desert plants would result in high gene flow, and little genetic differentiation such as is seen in conifers, as opposed to self-compatible, bee-pollinated species living in spatially restricted ranges.
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APPENDIX |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXACKNOWLEDGEMENTSLITERATURE CITED |
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Allelic frequencies of F. cernua populations in high-density scrubs (High) and low-density scrubs (Low). For locus with two alleles, allelic frequencies for just one of the two alleles are showed, similarly for locus with three alleles, allelic frequencies for two alleles are shown
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXACKNOWLEDGEMENTSLITERATURE CITED |
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We thank E. Vega, K. Herrera, A. Valera, V. Souza, S. Valencia-Díaz, J. Flores, R. Avila, A. Herrera, M. Ortega, A. Ortega, and S. Montiel for help in field and laboratory work, and the staff of the Desert Laboratory of the Instituto de Ecología, A. C. at the Mapimí Biosphere Reserve for logistical support and D. Piñero, Sara V. Good-Avila, Erika Aguirre, C. A. Domínguez, J. A. González-Astorga and two anonymous reviewers for helpful comments on the manuscript. This work was made as partial fulfillment of a M.M. Ferrer PhD degree at the Universidad Nacional Autónoma de México. A CONACyT grant to C. Montaña, and CONACyT, PAEP (UNAM) and DGEP (UNAM) scholarships to M. Ferrer supported the research.
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LITERATURE CITED |
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