Chromosome Evolution in Cyperales

Karyotypic evolution is a prominent feature in the diversification of many plants and animals, yet the role that chromosomal changes play in the process of diversification is still debated. At the diploid level, chromosome fission and/or fusion are necessary components of chromosomal structural change associated with diversification. Yet the genomic features required for these events remain unknown. Here we present an overview of what is known about genomic structure in Cyperales, with particular focus on the current level of understanding of chromosome number and genome size and their impact in a phylogenetic context. We outline ongoing projects exploring genomic structure in the order using modern genomics techniques coupled with traditional data sets. Additionally, we explore the questions to which this approach might be best applied, and in particular, detail a project exploring the nature of genomic structural change at the diploid level in genus Carex, a group in which chromosome fission/fusion events are common and associated with diversification of many of its 2000 species. A hypothesized mechanism for chromosome number change in this genus is agmatoploidy, denoting changes in chromosome number without change in DNA amount through fission/fusion of holocentric chromosomes (chromosomes without localized centromeres). This project includes the creation of bacterial artificial chromosome (BAC) and expressed sequence tagged (EST) libraries to be used in physical and genetic linkage mapping studies in order to reveal the patterns of genome structural variation associated with agmatoploidy in Carex, and to explore the sequence and genic characteristics of chromosomal break points in the genome

Toward a Unified Genetic Map of Higher Plants, Transcending the Monocot-Dicot Divergence

Closely related (confamilial) genera often retain large chromosomal tracts in which gene order is colinear, punctuated by structural mutations such as inversions and translocations 1. To explore the possibility that conservation of gene order might extrapolate to more distantly related taxa, we first estimated an average structural mutation rate. Nine pairs of taxa, for which there exist both comparative genetic maps and plausible estimates of divergence time, showed an average of0.14 (±0.06) structural mutations per chromosome per million years of divergence (Myr; Table 1). This value is offered as a first approximation, acknowledging that refined comparative data and/or divergence estimates may impel revision

Susceptibility to Phytophthora ramorum in a key infectious host: Landscape variation in host genotype, host phenotype, and environmental factors

• Sudden oak death is an emerging forest disease caused by the invasive pathogen Phytophthora ramorum. Genetic and environmental factors affecting susceptibility to P. ramorum in the key inoculum-producing host tree Umbellularia californica (bay laurel) were examined across a heterogeneous landscape in California, USA. • Laboratory susceptibility trials were conducted on detached leaves and assessed field disease levels for 97 host trees from 12 225-m2 plots. Genotype and phenotype characteristics were assessed for each tree. Effects of plot-level environmental conditions (understory microclimate, amount of solar radiation and topographic moisture potential) on disease expression were also evaluated. • Susceptibility varied significantly among U. californica trees, with a fivefold difference in leaf lesion size. Lesion size was positively related to leaf area, but not to other phenotypic traits or to field disease level. Genetic diversity was structured at three spatial scales, but primarily among individuals within plots. Lesion size was significantly related to amplified fragment length polymorphism (AFLP) markers, but local environment explained most variation in field disease level. • Thus, substantial genetic variation in susceptibility to P. ramorum occurs in its principal foliar host U. californica, but local environment mediates expression of susceptibility in nature

Editors’ Report for Volume 56

Volume: 56Start Page: 297End Page: 29