Repetitive DNA in the automictic fungus <Emphasis Type="Italic">Microbotryum violaceum</Emphasis>

The small genomes of fungi are expected to have little repetitive content other than rDNA genes. Moreover, among asexual or highly selfing lineages, the diversity of repetitive elements is also expected to be very low. However, in the automictic fungus Microbotryum violaceum, a very large proportion of random DNA fragments from the autosomes and the fungal sex chromosomes are repetitive in nature, either as retrotransposon or helicase sequences. Among the retrotransposon sequences, examples were found from each major kind of elements, including copia, gypsy, and non-LTR sequences. The most numerous were copia-like elements, which are believed to be rare in fungi, particularly among basidiomycetes. The many helicase sequences appear to belong to the recently discovered Helitron type of transposable elements. Also, sequences that could not be identified as a known type of gene were also very repetitive within the database of random fragments from M. violaceum. The differentiated pair of fungal sex chromosomes and suppression of recombination may be the major forces determining the highly repetitive content in the small genome of M. violaceum.     

ANTHER-SMUT DISEASE IN SILENE DIOICA: VARIATION IN SUSCEPTIBILITY AMONG GENOTYPES AND POPULATIONS,AND PATTERNS OF DISEASE WITHIN POPULATIONS

In an archipelago in northern Sweden, populations of the perennial, dioecious, and insect-pollinated herb Silene dioica are commonly infected by the sterilising anther-smut fungus Microbotryum violaceum. The results from transplantation and inoculation experiments in this study show that variation between populations in the incidence of disease may partly be due to variation in resistance among populations. In the transplantation experiment in which plants were naturally exposed to the fungus, disease levels varied greatly among transplants from three healthy populations when transplanted to three diseased populations in the archipelago. Clear genotypic differences in susceptibility among 25 replicated genotypes of the host plants were found when inoculated manually with two different isolates. Susceptibility varied between 0-90%, but the two isolates used did not differ in inoculation success. The results also suggest a geographical structuring in resistance of the host and virulence of the fungus. First, disease levels among experimental plants from two of the disease-free populations of S. dioica (originating from inner and outer archipelago, respectively), were high when transplanted to a diseased population nearby, but low when transplanted far away. Second, regardless of origin, plants from all healthy populations became diseased in the diseased experimental populations located in the middle part of the archipelago. Due to isostatic land upheaval in the studied archipelago, there is a vertical age-axis within islands such that the highest point on an island is the oldest. Since this may affect the demography of the host, disease spread, and the dynamics of disease, spatial patterns in adult and seedling densities, and disease and spore deposition along the age-axis were studied within three diseased populations. A low incidence of disease was found in the young, low and old, high parts of the populations and a high incidence in the vertical, middle parts of the populations. The higher disease incidence in the middle part compared with the lower part of the population may reflect less disturbance and an increased probability of disease with age.     

Anther-smut infection of Silene alba caused by Ustilago violacea: factors determining fungal reproduction

Summary The anther-smut fungus Ustilago violacea sporulates in flowers of the dioecious host plant Silene alba. Growth chamber comparisons of healthy and diseased plants, with the genetic background of host and pathogen controlled, revealed that fungal infection increases the number of flowers produced per plant and decreases the size of individual flowers. There were few consistent effects of plant genotype or fungal isolate on diseased flower traits, but differences between the plant sexes were apparent. Stimulation of flower production is proportionally greater in females than males: thus, although healthy male plants produce many more flowers than healthy females, sexual differences in diseased flower number are reduced. Sexual differences in diseased flower size also exist, with male flowers smaller than females. A field inoculation study confirmed dimorphism in diseased flower size and demonstrated that spore production per flower was greater for males than females for all flower size classes.