Fitness effects of mutation: testing genetic redundancy in Arabidopsis thaliana.

Screens of organisms with disruptive mutations in a single gene often fail to detect phenotypic consequences for the majority of mutants. One explanation for this phenomenon is that the presence of paralogous loci provides genetic redundancy. However, it is also possible that the assayed traits are affected by few loci, that effects could be subtle or that phenotypic effects are restricted to certain environments. We assayed a set of T-DNA insertion mutant lines of Arabidopsis thaliana to determine the frequency with which mutation affected fitness-related phenotypes. We found that between 8% and 42% of the assayed lines had altered fitness from the wild type. Furthermore, many of these lines exhibited fitness greater than the wild type. In a second experiment, we grew a subset of the lines in multiple environments and found whether a T-DNA insert increased or decreased fitness traits depended on the assay environment. Overall, our evidence contradicts the hypothesis that genetic redundancy is a common phenomenon in A. thaliana for fitness traits. Evidence for redundancy from prior screens of knockout mutants may often be an artefact of the design of the phenotypic assays which have focused on less complex phenotypes than fitness and have used single environments. Finally, our study adds to evidence that beneficial mutations may represent a significant component of the mutational spectrum of A. thaliana.

First Report of White Rust Caused by Albugo bliti on Seabeach Amaranth in the United States.

Seabeach amaranth (Amaranthus pumilus Raf.), a threatened annual marine plant, is a primary colonizer of the windward side of Atlantic coastal dunes. It serves an important ecological role in dune accumulation and stabilization. Because Hurricane Floyd eliminated all native seabeach amaranth in South Carolina in 1999, experimental reestablishment plantings have been attempted. In August 2000, seabeach amaranth on Dewees and Cape Island in Charleston County, Huntington Beach in Georgetown County, and Otter Island in Colleton County, South Carolina were stunted and senesced prematurely. Leaves on affected plants were only one-half of the normal size and internodes were shortened. Most plants (>90%) at each location were affected. Diseased leaves had small, pale green-to-tan spots above hypophyllous pustules that contained numerous, dry, hyaline, subglobose conidia. Conidia measured 13.5 (10 to 17) × 15.0 (11 to 18) µm. Based on morphological characters and the host, the pathogen was identified as Albugo bliti (Biv.-Bern.) Kuntze (1,2). No oospores were observed. Diseased plants were collected from Dewees and Otter Islands and kept frozen for use as a source of inoculum. Six A. pumilus plants each of six Plant Introductions (PI), 553080 through 553085, that had been grown from seed were sprayed with a suspension of 4.7 × 105 conidia per ml. One plant of each PI was sprayed with sterile distilled water as a noninoculated control. All plants were placed in a humidity chamber for 48 h and then moved to a greenhouse bench. Thirteen days after inoculation, all inoculated plants had pustules of white rust. Diseased plants had a mean of 42 pustules per plant and PI's did not differ in susceptibility. Five of six noninoculated plants also had white rust pustules, but only a mean of 2.3 (range 1 to 5) pustules each. White rust likely appeared on noninoculated plants because plants were spaced closely together in the chamber. Pustules and conidia on inoculated plants were identical to those on plants collected originally. Albugo bliti has been reported on 19 other Amaranthus species (1), but to our knowledge, this is the first report of white rust on seabeach amaranth in the United States. White rust reduced the biomass of infected plants and, hence, their ability to trap sand. White rust was not observed on subsequent plantings in 2001 and 2002 at any location. References: (1) D. F. Farr et al. Fungal Databases. Systematic Botany and Mycology Laboratory, On-line publication. ARS USDA, 2002. (2) G. W. Wilson. Bull. Torrey Bot. Club 34:61, 1907.

Long-distance seed dispersal in plant populations.

Long-distance seed dispersal influences many key aspects of the biology of plants, including spread of invasive species, metapopulation dynamics, and diversity and dynamics in plant communities. However, because long-distance seed dispersal is inherently hard to measure, there are few data sets that characterize the tails of seed dispersal curves. This paper is structured around two lines of argument. First, we argue that long-distance seed dispersal is of critical importance and, hence, that we must collect better data from the tails of seed dispersal curves. To make the case for the importance of long-distance seed dispersal, we review existing data and models of long-distance seed dispersal, focusing on situations in which seeds that travel long distances have a critical impact (colonization of islands, Holocene migrations, response to global change, metapopulation biology). Second, we argue that genetic methods provide a broadly applicable way to monitor long-distance seed dispersal; to place this argument in context, we review genetic estimates of plant migration rates. At present, several promising genetic approaches for estimating long-distance seed dispersal are under active development, including assignment methods, likelihood methods, genealogical methods, and genealogical/demographic methods. We close the paper by discussing important but as yet largely unexplored areas for future research.

Nuclear DNA-based markers for plant evolutionary biology.

While DNA-based markers can provide a wealth of information for the study of plant evolutionary biology, progress is limited by the lack of primers available for PCR. To overcome this limitation, we outline a protocol for developing oligonucleotide primers targeting regions of low copy-number nuclear genes. This protocol is intended to lead to universally useful primer sets. To test our approach, we designed eight primer sets and tested their abilities to amplify targets from representatives of each dicot and one monocot subclass. Five of the eight primer sets amplified targets from at least five of the seven taxa and thus exhibited broad taxonomic usefulness; the remaining primers were rather specific, however, and amplified targets from at most three taxa. In only one primer-taxon combination was a complex multiple-banded amplification produced. Overall, the protocol outlined proved quite useful at identifying broadly applicable primers targeted to low copy-number nuclear genes. Wider application of this approach should be effective at greatly increasing the amount of genetic information available for a diversity of plant nuclear genomes.