Robert Wyatt

The effects of fire regime on legume reproduction in longleaf pine savannas: is a season selective?

Abstract. The biodiversity of fire-dependent ecosystems is increasingly threatened by habitat fragmentation and fire suppression. Reducing species loss requires that salient features of natural fire regimes be incorporated into managed regimes. Lightning-season burns have been emphasized as the critical component of disturbance regimes that maintain native biodiversity within endangered longleaf pine savannas, the most diverse plant community in North America. Over evolutionary time, lightning-season fire is thought to have exerted strong selection pressure on plant pollinator interaction, flower production, and seedling establishment. In this study, season of burn had no effect on pollination activity of native legumes. Contrary to the conventional paradigm, we found a wide range of reproductive responses among dominant legumes in response to the season of burn treatments, suggesting that a variable fire season, rather than a single season of burn, is appropriate to maintain a greater variety of native species. We propose that varying the components of fire regimes, rather than selecting a particular fire regime, is likely to be important to conserve biodiversity in this and other fire-dependent communities.

Patterns of soybean proline-rich protein gene expression.

The expression patterns of three members of a gene family that encodes proline-rich proteins in soybean (SbPRPs) were examined using in situ hybridization experiments. In most instances, the expression of SbPRP genes was intense in a limited number of cell types of a particular organ. SbPRP1 RNA was localized in several cell types of soybean hypocotyls, including cells within the phloem and xylem. SbPRP1 expression increased within epidermal cells in the elongating and mature regions of the hypocotyl; expression was detected also in lignified cells surrounding the hilum of mature seeds. SbPRP2 RNA was present in cortical cells and in the vascular tissue of the hypocotyl, especially cells of the phloem. This gene was expressed also in the inner integuments of the mature seed coat. SbPRP3 RNA was localized specifically to the endodermoid layer of cells surrounding the stele in the elongating region of the hypocotyl, as well as in the epidermal cells of leaves and cotyledons. These data show that members of this gene family exhibit cell-specific expression. The members of the SbPRP gene family are expressed in different types of cells and in some cell types that also express the glycine-rich protein or hydroxyproline-rich glycoprotein classes of genes.

Electrophoretic confirmation of interspecific hybridization in Aesculus (Hippocastanaceae) and the genetic structure of a broad hybrid zone.

Within a broad (>200 km wide) hybrid zone involving three parapatric species of Aesculus, we observed coincident clines in allele frequency for 6 of 14 electrophoretic loci. The cooccurrence of alleles characteristic of A. pavia, A. sylvatica, and A. flava was used to estimate genetic admixtures in 48 populations involving various hybrids between these taxa in the south- eastern United States. High levels of allelic polymorphism (up to 40% greater than the parental taxa) were observed in hybrid populations and also in some populations bordering the hybrid zone. A detailed analysis of a portion of the hybrid zone involving A. pavia and A. sylvatica revealed a highly asymmetrical pattern of gene flow, predominantly from Coastal Plain populations of A. pavia into Piedmont populations of A. sylvatica. Computer simulations were used to generate expected genotypic arrays for parental, F1, and backcross individuals, which were compared with natural populations using a character index scoring system. In these comparisons, hybrid individuals could be distinguished from either parent, but F, and backcross progeny could not be distinguished from each other. Most hybrid populations were found to include hybrids and one of the parental taxa, but never both parents. Three populations appeared to be predominantly hybrids with no identifiable parental individuals. Hybrids occurred commonly at least 150 km beyond the range of A. pavia, but usually not more than 25 km beyond the range of A. sylvatica. Introgression, suggested by genetically hybrid individuals and significant gene admixtures of two or more species in populations lacking morphological evidence of hybridization, may extend the hybrid zone further in both directions. The absence of one or both parental species from hybrid populations implies a selective disadvantage to parentals in the hybrid zone and/or that hybridization has occurred through long-distance gene flow via pollen, primarily from A. pavia into A. sylvatica. Long-distance pollen movement in plants may generate hybrid zones of qualitatively different structure than those observed in animals, where gene flow involves dispersal of individuals.

Expression of the Arabidopsis AtAux2-11 auxin-responsive gene in transgenic plants

Five constructions containing deletions of the promoter from an auxin-inducible gene of Arabidopsis thaliana, AtAux2-11, were fused to the coding region of the reporter gene LacZ, which encodes β-galactosidase, and a polyadenylation 3′-untranslated nopaline synthase sequence from Agrobacterium. These chimeric genes were introduced into Arabidopsis by Agrobacterium tumefaciens-mediated transformation, and expression of the gene was examined by spectrophotometric and histochemical analyses. A 600 bp fragment from the AtAux2-11 promoter conferred histochemical patterns of staining similar to the longest 5′ promoter tested, a 3.0 kb fragment. Localization of AtAux2-11/LacZ activity in the transgenic plants revealed spatial and temporal expression patterns that correlated with tissues and cells undergoing physiological processes modulated by auxin. LacZ activity was expressed in the elongating region of roots, etiolated hypocotyls, and anther filaments. Expression was detected in the vascular cylinder of the root and the vascular tissue, epidermis, and cortex of the hypocotyl, and filament. The AtAux2-11/LacZ gene was preferentially expressed in cells on the elongating side of hypocotyls undergoing gravitropic curvature. Expression of the chimeric gene in the hypocotyls of light-grown seedlings was less than that in etiolated seedling hypcotyls. The AtAux2-11/LacZ gene was active in the root cap, and expression in the root stele increased at sites of lateral root initiation. Staining was evident in cell types that develop lignified cell walls, e.g. trichomes, anther endothecial cells, and especially developing xylem. The chimeric gene was not expressed in primary meristems. While the magnitude of expression increased after application of exogenous auxin (2,4-D), the histochemical localization of AtAux2-11/LacZ remained unchanged.Transgenic plants with a 600 bp promoter construct (−0.6 kb AtAux2-11/LacZ) had higher levels of basal and auxin-inducible expression than plants with a 3.0 kb promoter construct. Transgenic plants with a −500 bp promoter had levels of expression similar to the −3.0 kb construct. The −0.6 kb AtAux2-11/LacZ gene responded maximally to a concentration of 5 × 10−6 to 5 × 10−5 M 2,4-D and was responsive to as little as 5 × 10−8 M. The evidence presented here suggests that this gene may play a role in several auxin-mediated developmental and physiological processes.

Bryophyte Isozymes: Systematic and Evolutionary Implications

Traditionally, bryophytes have been treated as a single division comprising three classes: Musci (mosses), Hepaticae (liverworts), and Anthocerotae (hornworts). The relative sizes of these groups are estimated as 700 genera and 10.000 species of mosses; 330 genera and 8,000 species of liverworts; and four genera and 360 species of hornworts (Schofield, 1985). Estimates of the total number of bryophyte species vary from 16,000 to 22,000 (Bold et al., 1986). Much of the unity of bryophytes as a group derives from their uniform possession of an alternation of generations unique among land plants. The sexual cycle involves a dominant, free-living, haploid gametophyte alternating with a reduced, dependent, diploid sporophyte (Fig. 11.1). Despite the elaborate specialization of its tissues in some groups, the sporophyte remains attached to, and nutritionally dependent on, the gametophyte throughout its lifetime. Bryophytes have successfully exploited a diversity of habitats in which their direct competition with seed plants is minimized (Anderson, 1980). According to Anderson (1980), this may explain why the number of bryophyte species is nearly twice that of pteridophytes. Morphologically and anatomically, bryophytes are considered structurally simple compared to vascular plants, but they have proven to be unexpectedly rich in chemical variation (Giannasi, 1978).

Genetic Variability in Natural Populations of the Moss Atrichum angustatum 1

An electrophoretic survey of the moss Atrichum angustatum (Brid.) B.S.G. showed varying degrees of staining activity for eight of the ten specific enzymes surveyed. Of the eight active systems, four were scoreable: acid phos- phatase, glutamate dehydrogenase, malate dehydrogenase and phosphoglucoisom- erase. Comparisons of 15 populations of A. angustatum utilizing these four enzyme systems indicated significant levels of genetic variability both within and among populations. The traditional view of bryophytes as genetically impoverished or- ganisms with severely limited potential for response to natural selection is chal- lenged by these results.

Evolution of Self-Pollination in Granite Outcrop Species of Arenaria (Caryophyllaceae). III. Reproductive Effort and Pollen-Ovule Ratios

Patterns of resource allocation and pollen-ovule ratios were measured in nine pop- ulations of Arenaria uniflora, a winter annual endemic to rock outcrops in the southeastern United States. Dry weights of plants from cross-pollinating populations were greater than those from self- pollinating populations for both vegetative and reproductive biomass. Reproductive effort (pro- portion of total biomass represented by reproductive structures), however, was greater for selfers. Pollen-ovule ratios showed a close fit to expectations: low pollen-ovule ratios are associated with low rates of outcrossing (assessed in terms of degree of protandry). Number of pollen grains per flower and pollen-ovule ratios show no variation between years and little variation within and among flowers or plants. Number of ovules per ovary is less stable. It is suggested that use of pollen-ovule ratios obscures the evolutionary dynamics of forces influencing pollen grain number and ovule number. Pollen and ovule production should be studied as separate, and potentially

Hybridization and Introgression in Buckeyes (Aesculus: Hippocastanaceae): A Review of the Evidence and a Hypothesis to Explain Long-Distance Gene Flow

Evidence from morphology, distribution patterns, allozyme variation, and meiotic irregularities associated with decreased pollen germinability confirms the existence of a broad hybrid zone involving three parapatric species of Aesculus in the southeastern United States. The overall hybrid zone involving the three species is at least 200 km in width and probably represents the overlap of two hybrid zones: one between A. pavia and A. sylvatica and the other between A. flava and A. sylvatica. Both zones are highly asymmetrical, with hybrid populations occurring primarily in the Piedmont, where A. sylvatica is native. Detailed analyses of the hybrid zone involving A. pavia and A. sylvatica showed that hybrid populations consistently lack one or both of the putative parental species. Morphology and allozyme variation provide similar estimates of the position of the hybrid zone, but allozymes allow the detection of a larger zone than apparent on the basis of morphology. All available evidence is consistent with the hypothesis that extensive introgression has occurred among these species. Nevertheless, allozymic differentiation between these species is insufficient to reject hypotheses other than introgression that could generate the genetic structure observed in hybrid populations. Observations of pollinator activity in populations of A. pavia, A. sylvatica, A. flava, and their hybrids showed that these species share a number of important polli- nators, including several species of bumblebees (Bombus) and the ruby-throated hummingbird (Ar- chilochus colubris). Spring migration patterns of the ruby-throated hummingbird coincide closely with the flowering phenology of each of the Aesculus species, suggesting that hummingbirds could be vectors of long-distance pollen dispersal. The lifespan of pollen under field conditions is sufficient to permit transport of pollen in this fashion over distances of tens or hundreds of kilometers. Directional migration and arrival of hummingbirds after peak flowering would enforce a directional pattern of gene flow and could generate an asymmetrical hybrid zone of unusually great width.

Clonal distribution, fertility and sex ratios of the moss Plagiomnium affine (Bland.) T.Kop. in forests of contrasting age

Abstract Six populations of the clonal forest floor moss Plagiomnium affine from forests of different age were screened for genetic variation at 23 allozyme loci, of which nine were polymorphic. Samples consisting of two adjacent unconnected shoots were taken at regular intervals along one transect from each population. A total of 602 shoots was analysed. Almost 80% of the shoots were sterile (i.e. not expressing male or female gender). Sex remained unknown for only 10% of shoots after identification of genets based on electrophoretic data. We identified a mean number of 3.7 fertile clones per population. The mean length of clones along transects in each population ranged between 2 and 3 m. The size distribution within populations was bimodal, with a few dominant clones and a varying number of much smaller clones. The overall sex ratio was slightly female biased at the ramet level, but balanced at the genet level. Forest age was negatively correlated with percentage of sterile shoots and positively correlated with frequency of sporophytes. In both cases correlations were significant only if population 1, which was subject to extreme soil disturbance by badgers, was excluded. We conclude that the effective population size is larger, and the susceptibility to genetic drift is lower, in old forests.

Reproductive biology of Asclepias exaltata

An analysis of the relationships between plant size and survivorship and reproductive success was carried out by sampling four populations of the herbaceous perennial milkweed Asclepias exaltata in Virginia from 1980 to 1982. The annual survivorship rate (about 65%) is the lowest measured for any species of Asclepias. Survivorship was strongly size-dependent but showed no clear relationship with previous history of fruit production. Non-flowering plants were significantly smaller than flowering plants and showed very strong (r > 0.87) correlations between root dry weight and stem or leaf dry weight. Flowering plants were similar to nonflowering plants in root: shoot ratio (approximately 1:1) but differed in that root dry weight was not strongly correlated with stem or leaf dry weight. Components of inflorescence size were strongly correlated within a given level of comparison (e.g., stems per plant with flowers per plant) but less strongly correlated between levels (e.g., stems per plant with flowers per stem). Number of fruits per plant and percentage fruit-set were positively correlated with every component of inflorescence size. Although overall fruit-set was low (about 2%), fruits that were initiated had a high probability of surviving to maturity. There was no evidence of an early period of high fruit abortion: a relatively constant proportion of fruits aborted between each age class.