Chrysoporthe puriensis, a sibling species of the well-known Eucalyptus canker pathogen Chr. cubensis, has recently been described from Brazil. Both species are thought to be native to South America, but previous population genetic analyses were conducted prior to the ready availability of robust markers such as microsatellites to test this hypothesis. The objective of this investigation was to analyse the structure and genetic variability of Chr. cubensis and Chr. puriensis populations from non-native Myrtaceae and native Melastomataceae hosts in Brazil, using microsatellite markers developed for this purpose. The fungal isolates were obtained from Eucalyptus species, Corymbia citriodora and Tibouchina species in different regions of the country. Isolates were separated into sub-populations based on host families (Melastomataceae and Myrtaceae) and on region of origin. There was high genetic variability in all sub-populations with the highest levels detected within, rather than among sub-populations. Gene and genotypic diversities were higher for the isolates from the Melastomataceae than the Myrtaceae isolates. High levels of gene flow were found between sub-populations based on host and geographic distribution of the sub-populations. The presence of genetically diverse Chr. cubensis and Chr. puriensis populations on native hosts in Brazil supports a Latin American centre of origin for the two pathogens. Both undergo sexual and asexual reproduction and have a high potential for gene flow.

ABSTRACT. Analyses carried out with fluorescence in situ hybridization (FISH) in C-metaphases of the Lolium-Festuca complex have shown the occurrence of spontaneous fragile sites (FSs) in 45S rDNA regions. FSs are expressed as gaps but they do not result in breaks or chromosomal fragments in these species. These gaps have high DNA condensation observed as thin chromatin fibers that connect the apparent segments of the fragile chromosome, allowing for genomic stability. Assessing the behavior of these regions in the cell cycle of Lolium and Festuca species may lead to a better understanding of the dynamics that preserve stability during cell division. Furthermore, it is interesting to track the dynamics of chromosomes bearing 45S rDNA sites in the cell cycle as well as to observe the expression of FSs with no effect of the mitotic block. We observed variation in both the number and size of 45S FISH signals from the S/G2 phases of interphase and from prophase to anaphase where gaps in 45S rDNA sites also were observed. The change in the degree of condensation of the 45S site begins in the S/G2 phase and appears to be related to the transcriptional demand. Taking into account that the number of 45S rDNA sites tends to be reestablished when cells reach telophase, we suggest that the chromatin fiber goes back to the normal condensation level to the anaphase (after segregation), allowing for the approximation of chromosome segments and ensuring dynamics that favor the genomic stability of these species.

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