Spencer C. H. Barrett

A DNA barcode for land plants

DNA barcoding involves sequencing a standard region of DNA as a tool for species identification. However, there has been no agreement on which region(s) should be used for barcoding land plants. To provide a community recommendation on a standard plant barcode, we have compared the performance of 7 leading candidate plastid DNA regions (atpF–atpH spacer, matK gene, rbcL gene, rpoB gene, rpoC1 gene, psbK–psbI spacer, and trnH–psbA spacer). Based on assessments of recoverability, sequence quality, and levels of species discrimination, we recommend the 2-locus combination of rbcL+matK as the plant barcode. This core 2-locus barcode will provide a universal framework for the routine use of DNA sequence data to identify specimens and contribute toward the discovery of overlooked species of land plants.

The evolution of plant sexual diversity

Charles Darwin recognized that flowering plants have an unrivalled diversity of sexual systems. Determining the ecological and genetic factors that govern sexual diversification in plants is today a central problem in evolutionary biology. The integration of phylogenetic, ecological and population-genetic studies have provided new insights into the selective mechanisms that are responsible for major evolutionary transitions between reproductive modes.

Multiple Multilocus DNA Barcodes from the Plastid Genome Discriminate Plant Species Equally Well

A universal barcode system for land plants would be a valuable resource, with potential utility in fields as diverse as ecology, floristics, law enforcement and industry. However, the application of plant barcoding has been constrained by a lack of consensus regarding the most variable and technically practical DNA region(s). We compared eight candidate plant barcoding regions from the plastome and one from the mitochondrial genome for how well they discriminated the monophyly of 92 species in 32 diverse genera of land plants (N = 251 samples). The plastid markers comprise portions of five coding (rpoB, rpoC1, rbcL, matK and 23S rDNA) and three non-coding (trnH-psbA, atpF–atpH, and psbK–psbI) loci. Our survey included several taxonomically complex groups, and in all cases we examined multiple populations and species. The regions differed in their ability to discriminate species, and in ease of retrieval, in terms of amplification and sequencing success. Single locus resolution ranged from 7% (23S rDNA) to 59% (trnH-psbA) of species with well-supported monophyly. Sequence recovery rates were related primarily to amplification success (85–100% for plastid loci), with matK requiring the greatest effort to achieve reasonable recovery (88% using 10 primer pairs). Several loci (matK, psbK–psbI, trnH-psbA) were problematic for generating fully bidirectional sequences. Setting aside technical issues related to amplification and sequencing, combining the more variable plastid markers provided clear benefits for resolving species, although with diminishing returns, as all combinations assessed using four to seven regions had only marginally different success rates (69–71%; values that were approached by several two- and three-region combinations). This performance plateau may indicate fundamental upper limits on the precision of species discrimination that is possible with DNA barcoding systems that include moderate numbers of plastid markers. Resolution to the contentious debate on plant barcoding should therefore involve increased attention to practical issues related to the ease of sequence recovery, global alignability, and marker redundancy in multilocus plant DNA barcoding systems.

Evolutionary processes in aquatic plant populations

Abstract Aquatic plants exhibit striking taxonomic, morphological and ecological diversity. This variation limits the ability to pose general hypotheses with regards to evolutionary processes in aquatic plants. Here we ask whether the population structure, reproductive systems, gene flow and patterns of genetic differentiation in aquatic plants are likely to differ in any significant way from terrestrial plants. Defining the limits of aquatic plant populations is best attempted using demographic and genetic techniques for estimating effective population size (Ne. Data available for terrestrial species suggest that Ne in many annual aquatics is likely to be small, a fraction of the census number. In highly clonal species, especially those with water-dispersed vegetative fragments, effective population sizes may differ widely from those of related terrestrial taxa. However, measuring Ne in such species will probably require approaches more similar to those used to study vagile parthenogenetic animals than those used in plant populations. Reproductive systems in aquatic plants, though well described, have only begun to receive quantitative study. Levels of inbreeding and other mating-system parameters have been measured in several emergent species but are lacking for floating-leaved, submerged or free-floating taxa. Extensive clonal propagation presents analytical difficulties but also provides experimental opportunities for studying mating-system variation, particularly the relationship between large clone size and self-fertilization. Limited sexual reproduction has been observed in many highly clonal, aquatic species; there has been little attempt, however, to investigate the extent to which sterility can be attributed to genetic and environmental factors, or to explore whether sterility accumulates in clonal lineages. Gene flow in aquatic plants may be greatly affected by the discrete and patchy nature of many aquatic habitats and the directional transport of propagules in running waters. While the extent of gene movement may be influenced by habitat structure, genetic consequences of local and long-distance dispersal are likely to depend on the type of propagule involved. Transport of vegetative fragments may lead more frequently to successful gene establishment than dispersal of seed, and may, in part, explain the extensive geographical ranges of many clonal aquatic species. A survey of electrophoretic variation in 81 aquatic taxa revealed that the distribution of genetic diversity within and among populations of emergent species, as in their terrestrial counterparts, appears to be determined primarily by their breeding systems and life histories. In contrast, data for several submerged groups suggest widespread genetic monomorphism. The data, however, are limited, making interpretation of this pattern difficult, especially in cases where uniformity at isozyme loci appears to be associated with morphological and physiological differentiation. Further microevolutionary studies of aquatic plant populations may help to clarify the apparent conservative macroevolutionary pattern exhibited by certain aquatic plant families.

BAKER'S LAW REVISITED: REPRODUCTIVE ASSURANCE IN A METAPOPULATION

Bakers Law states that it is more likely for self‐compatible than for self‐incompatible individuals to establish sexually reproducing colonies after long‐distance dispersal, because only the former can do so with a single individual. This hypothesis, proposed by H. G. Baker 40 years ago is based largely on the observation that self‐compatibility is particularly frequent among colonists of oceanic islands. Here we argue that the principle of Bakers Law applies equally in the context of a metapopulation in which frequent local extinction is balanced by recolonization of sites by seed dispersal: metapopulation dynamics will select for an ability to self‐fertilize. We review several studies that support this hypothesis and present a metapopulation model in which the seed productivity required by obligate outcrossers for their maintenance in a metapopulation is compared with that of selfers. Our model also estimates the reduction in the advantage of reproductive assurance to selfers as a result of perenniality and seed dormancy. In general, selection for reproductive assurance is greatest when the colony occupancy rate, p, is low and is much reduced when p approaches its maximum. This provides an explanation for the observation that many highly successful colonizers, in which p is often high, are self‐incompatible. The basic model we present also lends itself to comparisons of metapopulation effects between unisexuality and cosexuality and between different modes of self‐incompatibility.

Pollen Dispersal and Mating Patterns in Animal-Pollinated Plants

Immobility complicates mating by angiosperms because the transfer of male gametes between individuals requires pollen vectors. Although abiotic and biotic vectors can transport pollen considerable distances (Bateman, 1941a; Squillace, 1967; Kohn and Casper, 1992; Godt and Hamrick, 1993), the resulting pattern of pollen dispersal does not intrinsically maximize the number and quality of matings. Consequently, floral evolution generally involves two classes of adaptations that promote mating success. The morphological traits that characterize floral design and display modify the actions of pollen vectors so as to enhance fertility (see below). In contrast, physiological traits mitigate unsatisfactory pollen dispersal by rejecting unsuitable male gametophytes (Jones, 1928; de Nettancourt, 1977; Marshall and Ellstrand, 1986; Seavey and Bawa, 1986; Barrett, 1988; Snow and Spira, 1991; Walsh and Charlesworth, 1992) or zygotes (Stephenson, 1981; Casper, 1988; Becerra and Lloyd, 1992; Montalvo, 1992). As a result of postpollination processes, the realized mating pattern does not simply mirror the pattern of pollination (e.g., Campbell, 1991; also see Waser and Price, 1993). However, these processes can only filter the incipient mating pattern established during pollination, so that pollination fundamentally determines the maximum frequency and diversity of mating opportunities. Consequently, the role of pollination in governing the scope for mating inextricably links the evolution of pollination and mating systems.

Crop mimicry in weeds

The selective forces imposed by agricultural practices have resulted in the evolution of agricultural races of weeds or agroecotypes. Some agroecotypes are intimately associated with a specific crop. Such associations can involve a system of mimicry, whereby the weed resembles the crop at specific stages during its life history and, as a result of mistaken identity, evades eradication. Mimetic forms of weeds are most likely to be selected by handweeding of seedlings or by harvesting and seed cleaning procedures. A striking example of morphological and phenological resemblance is found in the cultivated rice mimic,Echinochloa crus-galli var.oryzicola, a native of Asian rice fields but now widely distributed in rice-growing areas of the world. Comparative studies of the growth, devel-opment and patterns of phenotypic variation of cultivated rice,E. crus-galli var.oryzicola andE. crus-galli var.crus-galli demonstrate that the crop mimic is more similar to rice in many attributes than it is to its close relative. It is proposed that intense handweeding practices in Asia constitute the main selective force favoring the evolution of rice mimicry inE. crus-galli var.oryzicola.

Ecology and evolution of plant mating

Plants exhibit complex mating patterns because of their immobility, hermaphroditism and reliance on vectors for pollen transfer. Research on plant mating attempts to determine who mates with whom in plant populations and how and why mating patterns become evolutionarily modified. Most theoretical models of mating-system evolution have focused on the fitness consequences of selling and outcrossing, stimulating considerable empirical work on the ecology and genetics of inbreeding depression. Less attention has been given to how the mechanics of pollen dispersal influence the transmission of self and outcross gametes. Recent work on the relation between pollen dispersal and mating suggests that many features of floral design traditionally interpreted as anti-selling mechanisms may function to reduce the mating costs associated with large floral displays.

A METAPOPULATION PERSPECTIVE IN PLANT POPULATION BIOLOGY

1 A metapopulation approach considers the ecology and genetics of populations as a product of local dynamics and the regional processes of migration, extinction and colonization. While conventional metapopulation theory involves species with frequent population turnover, limited migration and random extinction, it is likely that metapopulation dynamics, broadly defined as the product of local population dynamics and dispersal, is a feature of all species. 2 Theoretical metapopulation models of single species make three critical insights. First, metapopulations will consist of a shifting mosaic of local populations linked through migration with only a fraction of the available habitat patches occupied at one time. Secondly, there is a threshold number of habitats available, below which the species cannot persist because extinction exceeds colonization. Thirdly, the antagonism between selective forces acting during recolonization and population growth can influence the evolution of phenotypic traits. Unfortunately, little empirical data is available to evaluate these ideas for plants or to address the broader issue of whether processes at a regional scale add anything to our understanding of population dynamics. 3 Plants may seem particularly appropriate for metapopulation analyses as a result of their immobility, strong spatial structure and restricted dispersal. However, a review of the literature revealed a paucity of studies that explicitly adopted a metapopulation approach, particularly in terms of testing theoretical models. We argue that this is because of the difficulty of measuring parameters such as extinction, colonization and migration that are central to most metapopulation models. 4 Plants possess a number of special features that present both challenges and opportunities for the development of new insights into the biology of metapopulations. Three particular characteristics, seed dormancy, restricted dispersal and local adaptation, need to be incorporated into existing theoretical models so they more accurately reflect the dynamics of plant metapopulations. Finally, more effort is needed to incorporate the explicit spatial structure of individuals within metapopulations and to investigate the effect that dispersion has on their growth and reproduction.

Floral biology: studies on floral evolution in animal-pollinated plants.

Part One : Historical perspective: Discovery of the secret of nature in the structure and fertilization of flowers-- C. K. Sprengel (translated by P. Haase) Christian Konrad Sprengels theory of the flower: the cradle of floral ecology-- S. Vogel Part Two: Conceptual issues: Floral traits and plant adaptation to insect pollinators: a devils advocate approach-- C.M. Herrera How do flowers diverge-- P. Wilson and J.D. Thomson Floral longevity: fitness consequences and resource costs-- T.L. Ashman and D.J. Schoen Pollen dispersal and mating patterns in animal-pollinated plants-- L.D. Harder and S.C.H. Barrett The ecology of geitonogamous pollination-- A.A. Snow, T.P. Spira, R. Simpson, and R.A. Klips Flower size dimorphism in plants with unisexual flowers-- L.F. Delph Part Three: Model systems: Evolution of floral morphology and function: an integrative approach to adaptation, constraint and compromise in Dalechampia (Euphorbiaceae)-- W.S. Armbruster The evolution of floral form: insights from an Alpine wildflower, Polemonium viscosum (Polemoniaceae)-- C. Galen Deceit pollination in the monoecious, neotropical herb Begonia oaxacana (Begoniaceae)-- D.W. Schemske, J. Agren, and J. Le Corff Reproductive success and gender variation in deceit-pollinated orchids-- A.L. Fritz and L.A. Nilsson Stylar polymorphisms and the evolution of heterostyly in Narcissus (Amaryllidaceae)-- S.C.H. Barrett, D.G. Lloyd, and J. Arroyo Evolution of Campanula flowers in relation to insect pollinators on islands-- K. Inoue, M. Maki, and M. Masuda Index.