Norman C. Ellstrand

What Can Molecular and Morphological Markers Tell Us About Plant Hybridization

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Gene flow by pollen: implications for plant conservation genetics

The absence of gene flow, genetic isolation, is frequently emphasized in conservation genetics. However, the presence of gene flow can play an equally important role in determining the genetic fate of populations. Here, I first review what is known of patterns of gene flow by pollen. Gene flow by pollen is often substantial among plant populations. I next review the expectations for gene flow patterns in the small populations typical of endangered species. Then, I consider what role gene flow can play in plant conservation genetics. Depending on the specific situation, such gene flow could be either beneficial or detrimental. Geographically disjunct populations might not always be as reproductively isolated as previously thought, and thereby less vulnerable to detrimental drift-based processes (...)

EXPERIMENTAL STUDIES OF THE EVOLUTIONARY SIGNIFICANCE OF SEXUAL REPRODUCTION. I. A TEST OF THE FREQUENCY‐DEPENDENT SELECTION HYPOTHESIS

This study tests the hypothesis that one evolutionary advantage of sexual reproduction is that it produces genetically variable progeny with a density-dependent advantage mediated by resource partitioning or pest pressure. Our experimental approach involved planting separate plots of sexually-derived and asexually-derived tillers of the grass Anthoxanthum odoratum in density gradients at the two natural sites from which the source material was taken. The sexual progeny displayed a significant fitness advantage compared to the asexual progeny. But, in contrast to the expectations of the density-dependent selection hypothesis, the advantage of the sexually produced progeny is most marked at lower densities. Thus, the results of this experiment and our previous report (Antonovics and Ellstrand, 1984) seem to best support the frequency-dependent selection hypothesis for the advantage of sexual reproduction.

Hybridization as an avenue of escape for engineered genes.

T| r he most likely opportunity for the escape of a crops engineered genes is through cropweed hybridization. The likelihood of such hybridization depends first on the presence of compatible relatives. If these congeners are present in the natural flora, then steps must be taken to reduce the risk of escape. Even before the days of genetic engineering, a few cases of crop-weed hybridization are known to have led to the evolution of aggressively weedy crop mimics (reviewed by Barrett 1983). These weeds are difficult to control because they share so many traits with the crop. For example, a common noxious weed of pearl millet (Pennisetum americanum) in Africa is a race of the same species. The weed evolved through hybridization of pearl millet with a nonweedy race of P. americanum (Brunken et al. 1977). Therefore, any crop-weed hybridization carries a risk of increased aggressiveness in the weed. To a great extent, the danger depends on how well the crop genes fare in the hybrid.

Hybridization and the evolution of invasiveness in plants and other organisms

Less than a decade ago, we proposed that hybridization could serve as a stimulus for the evolution of invasiveness in plants (Ellstrand and Schierenbeck Proc Nat Acad Sci USA 97:7043–7050, 2000). A substantial amount of research has taken place on that topic since the publication of that paper, stimulating the symposium that makes up this special issue. Here we present an update of this emergent field, based both on the papers in this volume and on the relevant literature. We reevaluate the lists that we presented in our earlier paper of reports in which hybridization has preceded the evolution of invasiveness. We discard a few cases that were found to be in error, published only as abstracts, or based on personal communication. Then we augment the list from examples in this volume and a supplementary literature search. Despite the omissions, the total number of cases has increased. Many have been strengthened. We add a list of cases in which there has been evidence that intra-taxon hybridization has preceded the evolution of invasiveness. We also provide a number of examples from organisms other than plants. We consider how our examples suggest mechanisms whereby hybridization may act to stimulate the evolution of invasiveness. Hybridization does not represent the only evolutionary pathway to invasiveness, but it is one that can explain why the appearance of invasiveness often involves a long lag time and/or multiple introductions of exotics.

Nonlocal transplantation and outbreeding depression in the subshrub Lotus scoparius (Fabaceae)

The genetic background of transplants used to create or augment wild populations may affect the long-term success of restored populations. If seed sources are from differently adapted populations, then the relative performance of progeny from crosses among populations may decrease with an increase in genetic differences of parents and in the differences of parental environments to the transplant location. We evaluated the potential for such outbreeding depression by hybridizing individuals from six different populations of Lotus scoparius var. scoparius and L. s. var. brevialatus. We used allozyme data to calculate genetic distances between source populations, and compiled climatic data and measured soil traits to estimate environmental distances between source populations. We found significant outbreeding depression following controlled crosses. In the greenhouse, the success of crosses (seeds/flower × seedlings/seed) decreased with increasing genetic distance between populations revealing genetically based outbreeding depression unrelated to local adaptation. After outplanting to one native site (in situ common garden), field cumulative fitness of progeny (survival × fruit production) decreased significantly with mean environmental distance of the parental populations to the transplant site, but not with genetic distance between the crossed populations. This result is consistent with a disruption of local adaptation. At the second, ecologically contrasting common garden, where low survival reduced statistical power, field cumulative fitness (survival × progeny height) did not decrease significantly with either environmental distance or genetic distance. Overall, intervariety crosses were 40 and 50% as fit (seeds/flower × seedlings/seed × survival × fruits at the first garden or × height at the second) as intravariety crosses. These results suggest that the cumulative outbreeding depression was caused by a combination of genetically based ecological differences among populations and other genomic coadaptation. We conclude that mixing genetically differentiated seed sources of Lotus scoparius may significantly lower the fitness of augmented or restored populations. Genetic and environmental similarities of source populations relative to the transplant site should be considered when choosing source materials, a practice recommended by recent seed transfer policies. Geographic separation was not a good surrogate for either of these measures.

Genetic and environmental variation in floral traits affecting outcrossing rate in Clarkia tembloriensis (onagraceae)

Clarkia tembloriensis exhibits a wide range of variation among its natural populations in outcrossing rate and in separation of male and female function in space (anther‐stigma separation or herkogamy) and in time (protandry). Here we show that outcrossing rate is highly correlated with protandry and anther‐stigma separation. Both genetic and environmental variation contribute to inter‐ and intrapopulation variation in protandry and anther‐stigma separation. Interpopulation differentiation for protandry and anther‐stigma separation was found to be polygenic. Genetic variation for protandry and anther‐stigma separation within populations was demonstrated by a significant among‐family variance in two populations with contrasting breeding systems. Environmental effects on the expression of mating system traits were manifested in two ways. First, significant variation among lathhouse benches suggests that small‐scale environmental heterogeneity may affect the development of floral traits. Second, protandry was shortened under hot summer conditions. Hence, hotter and drier habitats, typical of the more self‐pollinating populations of C. tembloriensis, can promote self‐pollination purely through environmental effects.

CONSEQUENCES OF FLORAL VARIATION FOR MALE AND FEMALE REPRODUCTION IN EXPERIMENTAL POPULATIONS OF WILD RADISH, RAPHANUS SATIVUS L

We documented effects of floral variation on seed paternity and maternal fecundity in a series of small experimental populations of wild radish, R. sativus. Each population was composed of two competing pollen donor groups with contrasting floral morphologies and several designated maternal plants. Progeny testing with electrophoretic markers allowed us to measure paternal success. Realized fecundity by each maternal plant and the fraction of those seeds attributable to each pollen donor group were used as outcome variables in path analysis to explore relationships between floral characters (petal size, pollen grain number per flower, and modal pollen grain size), pollinator visitation patterns, and reproductive success. A wide range of pollinator taxa visited the experimental populations, and patterns of discrimination appeared to vary among them. The impact of visitation on male and female reproduction also varied among taxa; visits of small native bees significantly increased paternal success, while those of honey bees reduced male fitness. Only visits by large native bees had discernible effects on recipient fecundity, and, overall, fecundity was not limited by visitation. Maternal plants bearing large‐petalled flowers produced fewer flowers during the experiment, reducing their total seed production. In these small populations, postpollination processes (at least in part, compatibility) significantly influenced male and female reproductive success. Variation in pollinator pools occurring on both spatial and temporal scales may act to preserve genetic variation for floral traits in this species.

INBREEDING EFFECTS IN CLARKIA TEMBLORIENSIS (ONAGRACEAE) POPULATIONS WITH DIFFERENT NATURAL OUTCROSSING RATES

Inbreeding depression is commonly observed in natural populations. The deleterious effects of forced inbreeding are often thought to be less pronounced in populations with self‐pollinating mating systems than in primarily outcrossing populations. We tested this hypothesis by comparing the performance of plants produced by artificial self‐ and cross‐pollination from three populations whose outcrossing rate estimates were 0.03, 0.26, and 0.58. Outcrossing rates and inbreeding coefficients were estimated using isozyme polymorphisms as genetic markers. Analysis of F statistics suggests that biparental inbreeding as well as self‐fertilization contribute to the level of homozygosity in the seed crop. Biparental inbreeding will reduce the heterozygosity of progeny produced by outcrossing, relative to random outcrossing expectations, and hence will reduce the effects of outcrossing versus self‐fertilization. Heterotic selection may increase the average heterozygosity during the life history. Selfed and outcrossed seeds from all three populations were equally likely to germinate and survive to reproduce. However, inbreeding depression was observed in fecundity traits of plants surviving to reproduction in all three populations. Even in the population whose natural self‐fertilization rate was 97%, plants grown from seed produced by self‐pollination produced fewer fruits and less total seed weight than plants grown from outcrossed seed. There was no detectable inbreeding depression in estimated lifetime fitness. Inbreeding effects for all reproductive yield characters were most severe in the accession from the most outcrossing population and least severe in the accession from the most self‐fertilizing population.

EFFECTIVE MATE CHOICE IN WILD RADISH: EVIDENCE FOR SELECTIVE SEED ABORTION AND ITS MECHANISM

Maternal plants that can affect the paternity of their seeds may influence the quality of their offspring. However, mate choice in plants has proved difficult to demonstrate because it is usually confounded by the effects of pollen competition and because its mechanisms are not well understood. We show here that stressed wild radish plants selectively abort seeds sired by particular donors. This selection occurs by position-dependent seed abortion and position-dependent resource allocation within fruits, coupled with position-dependent fertilization of ovules by pollen donors. The result is that stressed plants select for pollen donors that sire seeds in the basal and middle, rather than the stylar, thirds of fruits. Thus, in wild radish, the position of seeds fathered by pollen donors may act as a selective arena. Because of this position-dependent abortion, successful fathers sire the seeds most likely to mature and to be larger than average. However, siring these high-quality seeds may reduce the quantity of seeds sired by a donor after mixed pollination because pollen tubes must grow past fertilizable stylar ovules to reach middle and basal ovules, allowing pollen tubes from other donors to sire many seeds in the less favored positions. Since multiple paternity is common in wild radish fruits in the field, selective seed abortion probably occurs there as well. Finally, the position-dependent seed abortion necessary to this effective maternal choice occurs in other plants; thus, the mechanism suggested here may be a general mechanism of effective mate choice in plants that selects for position of fertilization, not just speed of pollen-tube growth.