Composition and Nature of Heterochromatin in the Electrical Fish (Knifefishes) Gymnotus (Gymnotiformes: Gymnotidae)

Fishes of the genus Gymnotus have been suggested as a good model for biogeographic studies in the South American continent. In relation to heterochromatin, species of this genus have blocks preferably distributed in the centromeric region. The content of these regions has been shown to be variable, with description of transposable elements, pseudogenes of 5S rDNA and satellite sequences. In G. carapo Clade, although geographically separated, species with 2n = 54 chromosomes share the distribution of many 5S rDNA sites, a unique case within the genus. Here, repetitive DNA sequences from G. sylvius (2n = 40) and G. paraguensis (2n = 54) were isolated and mapped to understand their constitution. The chromosome mapping by FISH showed an exclusive association in the centromeres of all chromosomes. However, the cross-FISH did not show positive signs of interspecific hybridization, indicating high levels of heterochromatic sequence specificity. In addition, COI-1 sequences were analyzed in some species of Gymnotus, which revealed a close relationship between species of clade 2n = 54, which have multiple 5S rDNA sites. Possibly, the insertion of retroelements or pseudogenization and dispersion of this sequence occurred before the geographic dispersion of the ancestor of this clade from the Amazon region to the hydrographic systems of Paraná-Paraguay, a synapomorphy for the group.

Observations on the induction of position effect variegation of euchromatic genes in Drosophila melanogaster.

In the T(1;2)dorvar7 translocation, the 1A-2B7-8 segment of the X chromosome is brought to the vicinity of 2R-chromosome heterochromatin resulting in position effect variegation of dor, BR-C and more distal genes, as well as compaction of chromatin in this segment. By irradiation of T(1;2)dorvar7, nine reversions (rev) to a normal phenotype were recovered. In two cases (rev27, rev226), the 1A-2B7-8 section is relocated to the 19A region of the X chromosome, forming free duplications (1A-2B7-8/19A-20F-X-het). Modifiers of position effect do not change the normal expression of the BR-C and dor genes in these duplications. In five reversions (rev3, rev40, rev60, rev167, rev175), free duplications have formed from the 1A-2B7-8 fragment and X chromosome heterochromatin. In these rearrangements, modifiers of position effect (low temperature, removal of Y and 2R-chromosome heterochromatin and a genetic enhancer (E-var(3)201) induce position-effect again. Two reversions (rev45 and rev110) are associated with additional inversions in the original dorvar7 chromosomes. The inversions relocate part of the heterochromatin adjacent to the 1A-2B7-8 section into new positions. In T(1;2)dorrev45, position-effect is seen in the 2B7-8-7A element as compaction spreading from 2B7-8 proximally in some cases as far as the 5D region. Thus, in rev45 the pattern of euchromatin compaction is reciprocal to that of the initial dorvar7 strain. Apparently, it is due to the same variegation-evoking center near the 2R centromere in both cases. In all nine revertants, weakening or complete disappearance of the position-effect is observed despite retention of the 20-kb heterochromatic segment adjacent to the 1A-2B7-8 region. Thus, a 20-kb heterochromatic sequence does not inactivate euchromatin joined to it.

Rye heterochromatin. II. Characterization of a derivative from chromosome 1DS.1RL with a reduced amount of the major repeating sequence

Homoeologous recombination between wheat chromosomes and the long arm of chromosome 1R of rye present in a wheat background as the wheat–rye translocation, 1DS.1RL, has been reported. During the course of this study an unexpected plant (308-17), with a recombinant phenotype, arose from a control population where homoeologous pairing was thought to be suppressed. The putative recombinant chromosome in plant 308-17 carried two seed protein loci located on 1RL (namely, the rye Glu-R1 locus and a recently discovered globulin-like marker) but appeared to lack C-banded heterochromatin. Further investigation of this apparent recombinant chromosome in progeny of 308-17 using a cloned probe for the 350 family of rye heterochromatic sequence indicated that some terminal heterochromatin was still present but in a much reduced amount. Because it appears that only the terminal region has been changed, the modified chromosome in 308-17 most likely did not arise from homoeologous recombination but rather from some form of heterochromatin loss event, possibly involving an unequal sister chromatid exchange within the terminal heterochromatin of 1RL. Sister lines to plant 308-17 carrying Glu-R1 contained a normal amount of 1RL heterochromatin.Key words: rye–wheat, heterochromatin, recombinant, unequal sister chromatid exhange.