Outcrossing Plant Species Research Articles

Parallel Declines in Pollinators and Insect-Pollinated Plants in Britain and the Netherlands

Despite widespread concern about declines in pollination services, little is known about the patterns of change in most pollinator assemblages. By studying bee and hoverfly assemblages in Britain and the Netherlands, we found evidence of declines (pre-versus post-1980) in local bee diversity in both countries; however, divergent trends were observed in hoverflies. Depending on the assemblage and location, pollinator declines were most frequent in habitat and flower specialists, in univoltine species, and/or in nonmigrants. In conjunction with this evidence, outcrossing plant species that are reliant on the declining pollinators have themselves declined relative to other plant species. Taken together, these findings strongly suggest a causal connection between local extinctions of functionally linked plant and pollinator species.

Hamilton's rule, imprinting and parent–offspring conflict over seed mass in partially selfing plants

When genes in the offspring control the provisioning of the seed, the optimal seed size can be calculated exactly by applying Hamilton's rule. When seed size is a compromise between mother and offspring, we predict that outcrossing plant species produce larger seeds than selfers. This trend was found in the British flora and in a number of well-studied plant families. The analysis was extended to imprinting, a conditional strategy in which a gene in the offspring takes more resources when derived from the father than from the mother. The conditions for imprinting to be selected were rather restrictive. The analysis is relevant for the current debate about the evolution of imprinting in Arabidopsis thaliana.

Simultaneous Maximum Likelihood Estimation of Linkage and Linkage Phases in Outcrossing Species

With the advent of new molecular marker technologies, it is now feasible to initiate genome projects for outcrossing plant species, which have not received much attention in genetic research, despite their great agricultural and environmental value. Because outcrossing species typically have heterogeneous genomes, data structure for molecular markers representing an entire genome is complex: some markers may have more alleles than others, some markers are codominant whereas others are dominant, and some markers are heterozygous in one parent but fixed in the other parent whereas the opposite can be true for other markers. A major difficulty in analyzing these different types of marker at the same time arises from uncertainty about parental linkage phases over markers. In this paper, we present a general maximum-likelihood-based algorithm for simultaneously estimating linkage and linkage phases for a mixed set of different marker types containing fully informative markers (segregating 1:1:1:1) and partially informative markers (or missing markers, segregating 1:2:1, 3:1, and 1:1) in a full-sib family derived from two outbred parent plants. The characterization of linkage phases is based on the posterior probability distribution of the assignment of alternative alleles at given markers to two homologous chromosomes of each parent, conditional on the observed phenotypes of the markers. Two- and multi-point analyses are performed to estimate the recombination fraction and determine the most likely linkage phase between different types of markers. A numerical example is presented to demonstrate the statistical properties of the model for characterizing the linkage phase between markers.

Population genetic structure in the dioecious pioneer plant species Hippophae rhamnoides investigated by random amplified polymorphic DNA (RAPD) markers

Hippophae rhamnoides is an outcrossing pioneer plant species with a severely fragmented distribution. Random amplified polymorphic DNA (RAPD) marker variation was analysed in 10 populations of ssp. rhamnoides and in one population of ssp. mongolica to estimate the amount and distribution of genetic variability. No less than 89.7% of the scorable markers were polymorphic, but few of these were fixed and populations consequently differed mainly by frequency variation of individual markers. Within‐population gene diversity was somewhat low for an outcrossing plant species: 0.192 or 0.159 for ssp. rhamnoides, depending on whether it was based on all 156 polymorphic RAPDs or on only those 63 RAPDs that fulfilled the 3/N criterion. Analysis of molecular variance applied to the ssp. rhamnoides showed only 15% between‐population variability, indicating a relatively restricted population differentiation as expected in outcrossing species and shown in several other AMOVA studies. The tendency for island populations to be somewhat more differentiated, and to have less within‐population diversity than mainland populations, may indicate an effect of population fragmentation. Genetic distance estimates among populations, obtained with and without pruning of RAPD loci on the basis of the 3/N criterion, were generally in very good agreement. Cluster analyses and principal coordinate analyses showed populations of ssp. rhamnoides to be rather close, but quite isolated from the single ssp. mongolica population. Genetic and geographical distances between the ssp. rhamnoides populations were not associated, indicating that large‐scale geographical and ecotypic differentiation was not reflected in the RAPD profiles.

FLUCTUATING ASYMMETRY IN CENTRAL AND MARGINAL POPULATIONS OF LYCHNIS VISCARIA IN RELATION TO GENETIC AND ENVIRONMENTAL FACTORS.

Developmental instability in the form of increased fluctuating asymmetry can be caused by either genetic or environmental stress. Because extinctions can be attributed broadly to these factors, fluctuating asymmetry may provide a sensitive tool for detecting such stresses. We studied the level of fluctuating asymmetry of flowers of a perennial outcrossing plant species, Lychnis viscaria, both in natural and common-garden populations. The degree of flower asymmetry was higher in small, isolated, and marginal populations of the species range. These marginal populations also were the most homozygous. In the core area of the species' range, flowers were more symmetrical The level of asymmetry was correlated with both population size and heterozygosity. However, a partial correlation analysis revealed that when the impact of population size was controlled for, there was a negative relationship between fluctuating asymmetry and heterozygosity, whereas when controlling for heterozygosity, no relationship between population size and fluctuating asymmetry was found. This indicates that genetic consequences of small population size probably underlie the relationship between the level of asymmetry and population size. Results from a transplantation experiment showed that individuals subjected to a higher environmental stress had an increased level of asymmetry compared to control plants. In the common-garden conditions the level of fluctuating asymmetry did not differ between the central and marginal populations. This suggests that presumably both genetic and environmental factors affected to the higher level of asymmetry among marginal populations compared to central ones. In all we conclude that even though fluctuating asymmetry seems to be a sensitive tool for detecting stresses, results from studies focusing on only one factor should be interpreted with caution.

Genetic Diversity and Geographic Differentiation in Pawpaw [Asimina triloba (L.) Dunal] Populations from Nine States as Revealed by Allozyme Analysis

As a new National Clonal Germplasm Repository for Asimina species at Kentucky State University (KSU), of major concern to us is the genetic variation within our germplasm collection. The present study investigated the extent of genetic diversity for the pawpaw germplasm in our collection and the geographical pattern of genetic diversity among populations using isozyme markers. Allozyme diversity was high in Asimina triloba (L.) Dunal (Annonaceae) collected from all nine different states, as is typical for temperate woody perennial, widespread and outcrossing plant species. Averaged across populations, mean number of alleles per locus (A), percent polymorphic loci (P), effective number of alleles per locus (Ae), and expected heterozygosity (He) were 1.54, 43.5, 1.209, and 0.172, respectively. Significant deviations from Hardy-Weinberg equilibrium were found in nine populations at an average of 4.8 loci. Observed heterozygosity was higher than expected. Partitioning of genetic diversity showed that 88.2% resided within populations. The proportion of genetic diversity among populations (Gst = 0.118; FST = 0.085) was either lower than or within the range of those species with similar ecological and life-history traits. The mean genetic identity among populations was high (I = 0.988). An analysis using UPGMA clustered most populations as one major group, with the southernmost (Georgia) and the westernmost (Illinois) populations readily separated from the main group. The relationships discovered by principal component analysis (PCA) were similar to those revealed by UPGMA. In addition, PCA separated the northernmost population (New York) from the major group. Sampling strategies for future germplasm collection of A. triloba are also discussed.

Clonal Diversity and Allozyme Variation in Populations of the Arctic Sedge Carex Bigelowii (Cyperaceae)

1 A study of allozyme variation in vegetatively propagating populations of the rhizomatous sedge Carex bigelowii, revealed high levels of clonal diversity (genet diversity) within populations. The structure of allelic variation within the populations suggests that sexual reproduction has played a significant role in these C. bigelowii populations, despite present lack of seedling recruitment. The study was carried out in moss-heath communities on Icelandic lava-fields. Two adjacent populations were studied at one site, while a third population was studied at a second site, 35 km away from the first two populations. 2 The number of genets detected in each population, among 85-88 analysed ramets, ranged from 41 to 55 (minimum estimate). Samples were taken every 4 m along transects in the populations. Ramets with the same allozyme genotype were often spatially aggregated. No seedlings have been observed in the populations during five years of demographic studies. 3 All the populations studied showed a diploid expression of allozymes and high levels of allelic variation, with on average 491.77 alleles per locus (A) and an allelic diversity (HS) of 0.167. Similar levels of within-population variability are found in many wind-pollinated and outcrossing plant species. 4 The difference between observed and expected heterozygosity was small in all populations, suggesting high levels of outbreeding. 5 Comparisons with other Carex taxa show that the levels of and structuring of allozyme diversity in C. bigelowii is similar to that in other outbreeding species (usually rhizomatous), and much higher than in inbreeding species (which are usually caespitose). 6 Only 5the total allelic diversity was explained by differences between the two study sites (G ST = 0.055), suggesting extensive recent or historic gene-flow. (Less)

Bumblebee preference for symmetrical flowers.

Fluctuating asymmetry, which represents small random deviations from otherwise bilateral symmetry, is a measure of the phenotypic quality of individuals indicating the ability of controlled development under given environmental and genetic conditions. I tested whether floral symmetry reliably reflects phenotypic quality measured in terms of pollinator rewards and whether pollinators respond to floral symmetry in a series of observations and experiments on Epilobium angustifolium (Onagraceae). Lower petal asymmetry was negatively related to mean lower petal length, whereas asymmetry in leaf width was positively related to mean leaf width. Flowers visited by bumblebees were larger and more symmetrical than the nearest neighboring flower. This relationship between pollinator preference for large and symmetrical flowers was demonstrated to be causal in experiments in which the lower petals were manipulated symmetrically or asymmetrically. Nectar production was larger in symmetrical flowers, and this may explain the bumblebee preference for flower symmetry. Floral symmetry therefore reliably reflects nectar production and hence enhances pollen transport. Extensive embryo abortion has been documented in E. angustifolium and other outcrossing plant species. Floral fluctuating asymmetry, which reflects general developmental homeostasis, may explain such developmental selection in these plants.

The origin and function of self-incompatibility in flowering plants

The evolutionary significance of self-incompatibility (SI) traditionally has been linked to reduced inbreeding through enforced outcrossing. This view is founded on the premise that outcrossing reduces inbreeding. It is important, when considering the evolutionary significance of any genetic system, to try to distinguish those factors related to the evolution of, from those related to the maintenance of, the system in question. Three factors are considered important for the maintenance of SI: (1) phylogenetic constraint in species descended from SI ancestors, (2) reduced inbreeding in populations, and (3) fitness benefits to individuals resulting from the avoidance of selfing. I suggest that the first two factors should be rejected when considering the origin of SI (whether one or more origins are hypothesized) and that the increase in individual fitness resulting from the avoidance of self-fertilization among individuals that are heterozygous for deleterious alleles may be sufficient to account for the origin of SI. Self-fertilization in plants (except in species that predominantly self-fertilize) generally results in a reduction in fitness of some individuals due to the increased expression of deleterious or lethal recessive alleles, regardless of the degree of inbreeding in the population or the frequency of the allele in question. Inbreeding is a consequence of population structure in many outcrossing plant species. Complex (multi-locus and multi-allelic) systems of SI exist that reduce inbreeding. However, it is argued that these are derived either from simpler systems of SI that may have very little or no effect on inbreeding, in which case any effect on level of inbreeding is secondary, or are not true self-incompatibility systems and are part of a regulatory system that serves to balance the level of inbreeding and outbreeding. Multi-locus and multi-allelic systems of SI and heteromorphic systems of SI are discussed in terms of derived versus ancestral characteristics. A reassessment of the role of breeding systems in the development of a population structure promoting inbreeding is suggested, which may have been of crucial importance in the success and diversification of angiosperms.

Maximizing gain per effort by using clonal replicates in genetic tests

Models for predicting cummulative genetic gain from recurrent selection applicable to predominantly outcrossing plant species are derived to include the effect of observations on clonal replicates (ramets) in addition to observations on individuals and family means. Such models are discussed with special reference to forest trees. The consequence of redistributing effort from individuals to ramets is investigated for several conditions with a fixed number of families and fixed total test size. Factors that affect the distribution of variance among sources and factors that affect individual selection intensity are the primary determinants of the optimum distribution of effort. The optimum number of ramets ranged from 1 to 6 for the conditions tested and the efficiency of redistribution (ratio of gain for the optimum distribution to the gain for the single-ramet, or non-clonal case) ranged from 1.00 to 1.20. Using clonal replicates in genetic tests usually results in increased cummulative genetic gain relative to non-clonal tests, without an increase in test effort.

Homozygosity and patch structure in plant populations as a result of nearest-neighbor pollination

The population genetic consequences of nearest-neighbor pollination in an outcrossing plant species were investigated through computer simulations. The genetic system consisted of two alleles at a single locus in a self-incompatible plant that mates by random pollen transfer from a neighboring individual. Beginning with a random distribution of genotypes, restricted pollen and seed dispersal were applied each generation to 10,000 individuals spaced uniformly on a square grid. This restricted gene flow caused inbreeding, a rapid increase in homozygosity, and striking microgeographic differentiation of the populations. Patches of homozygotes bordered by heterozygotes formed quickly and persisted for many generations. Thus, high levels of inbreeding, homozygosity, and patchiness in the spatial distribution of genotypes are expected in plant populations with breeding systems based on nearest-neighbor pollination, and such observations require no explanation by natural selection or other deterministic forces.