A number of aberrant morphological phenotypes were noted during propagation of the Arabidopsis thaliana DNA hypomethylation mutant, ddm1, by repeated self-pollination. Onset of a spectrum of morphological abnormalities, including defects in leaf structure, flowering time, and flower structure, was strictly associated with the ddm1 mutations. The morphological phenotypes arose at a high frequency in selfed ddm1 mutant lines and some phenotypes became progressively more severe in advancing generations. The transmission of two common morphological trait syndromes in genetic crosses demonstrated that the phenotypes are caused by heritable lesions that develop in ddm1 mutant backgrounds. Loss of cytosine methylation in specific genomic sequences during the selfing regime was noted in the ddm1 mutants. Potential mechanisms for formation of the lesions underlying the morphological abnormalities are discussed.

The genetic positions of the five Arabidopsis thaliana centromere regions have been identified by mapping size polymorphisms in the centromeric 180-bp repeat arrays. Structural and genetic analysis indicates that 180-bp repeat arrays of up to 1000 kb are found in the centromere region of each chromosome. The genetic behavior of the centromeric arrays suggests that recombination within the arrays is suppressed. These results indicate that the centromere regions of A. thaliana resemble human centromeres in size and genomic organization.Genetic mapping of centromeres is essential for the integration of cytological and genetic maps, and marks an important step toward the molecular characterization of centromeric DNA. Although the centromere is one of the most conspicuous markers on the cytological map, determination of the location of centromeres on genetic maps is frequently difficult to achieve. This is especially true in higher animals and plants where the genetic tools for centromere mapping are limited.We have been pursuing the characterization of centromere regions of the model angiosperm Arabidopsis thaliana. This plant's small genome (∼100 Mb/haploid) (Meyerowitz 1994) and relatively low abundance of repetitive DNA [∼10% of total (Leutwiler et al. 1984)] make it well suited for molecular chromosome studies. Aiding in this analysis is the availability of dense genetic maps that exist for each of A. thaliana's five chromosomes (e.g., Hauge et al. 1993;Lister and Dean 1993). In addition, physical maps are being developed for all A. thaliana chromosomes in the form of overlapping cloned genomic fragments (Schmidt et al. 1995;Zachgo et al. 1996).Considering the generally advanced molecular characterization of the A. thaliana genome, the genomic organization and genetic location of centromeres in this species remain poorly characterized. The A. thaliana centromere regions are heterochromatic (Schweizer et al. 1987) and contain tandem arrays of related repeats (exhibiting ജ80% similarity) that are ∼180 bp in length (Martinez-Zapater et al. 1986;Simoens et al. 1988;Maluszynska and Heslop-Harrison 1991), a genomic organization that resembles the ∼170-bp alphoid repeat arrays at primate centromeres (Willard 1990;Pluta et al. 1995). It is not clear whether the 180-bp repeat arrays flank or span A. thaliana centromeres, but data from mammalian systems suggest that the alphoid repeats are intimately associated with the centromere and are likely to play a role in centromere function (Heartlein et al. 1988;Haaf et al. 1992;Tyler-Smith et al. 1993;Brown et al. 1994;Larin et al. 1994;Harrington et al. 1997). A number of other middlerepetitive sequence elements have been found to be associated with the 180-bp repeats in genomic clones (Richards et al. 1991;Schmidt et al. 1995;Pelissier et al. 1996; Thompson et al. 1996a,b), suggesting that islands of more complex sequence arrangement are located in the A. thaliana centromeric regions.An approximate genetic location of three of the five A. thaliana centromeres (on chromosomes ...

We describe the isolation and characterization of two missense mutations in the cytosine-DNA-methyl-transferase gene, MET1, from the flowering plant Arabidopsis thaliana. Both missense mutations, which affect the catalytic domain of the protein, led to a global reduction of cytosine methylation throughout the genome. Surprisingly, the met1-2 allele, with the weaker DNA hypomethylation phenotype, alters a well-conserved residue in methyltransferase signature motif I. The stronger met1-1 allele caused late flowering and a heterochronic delay in the juvenile-to-adult rosette leaf transition. The distribution of late-flowering phenotypes in a mapping population segregating met1-1 indicates that the flowering-time phenotype is caused by the accumulation of inherited defects at loci unlinked to the met1 mutation. The delay in flowering time is due in part to the formation and inheritance of hypomethylated fwa epialleles, but inherited defects at other loci are likely to contribute as well. Centromeric repeat arrays hypomethylated in met1-1 mutants are partially remethylated when introduced into a wild-type background, in contrast to genomic sequences hypomethylated in ddm1 mutants. ddm1 met1 double mutants were constructed to further our understanding of the mechanism of DDM1 action and the interaction between two major genetic loci affecting global cytosine methylation levels in Arabidopsis.

Purpose: This article includes a study that sheds light on the implications of the Workforce Innovation and Opportunities Act (WIOA) mandated VR knowledge and skills requirements for VR counselors in the state VR program. It is also a commentary on current training, accreditation and intraprofessional challenges in the larger counseling field. Our study investigated VR directors’ perceptions of the importance and counselor preparedness in specific WIOA mandated knowledge areas. Findings revealed that VR directors consider WIOA mandated knowledge areas (e.g., employment-related and case management) to be very important, and they see room for improvement in the preparation of VR counselors to execute these functions. Our commentary provides readers with information related rehabilitation counseling history, intraprofessional practice, and guidelines for the future of rehabilitation counseling and the larger counseling profession.

Longitudinal mixed-methods examination of two consecutive field-based environmental biology research apprenticeship programs for precollege youth indicates that the two-stage apprenticeship structure influenced social cognitive career variables necessary for persistence in pursuit of an environmental research career pathway.