Touch-and-Go Pairing in Chromosomes

Composition and epigenetic markers of heterochromatin in the aphid Aphis nerii (Hemiptera: Aphididae)

A detailed karyotype analysis of the oleander aphid Aphis nerii focusing on the distribution, molecular composition and epigenetic modifications of heterochromatin was done in order to better understand the structure and evolution of holocentric/holokinetic chromosomes in aphids. The female karyotype (2n = 8) consisted of 3 pairs of autosomes and a pair of X chromosomes that were the longest elements in the karyotype and carried a single, terminally located nucleolar organizer region. Males showed 2n = 7 chromosomes due to the presence of a single X chromosome. Heterochromatin was located in the X chromosomes only and consisted of 4 satellite DNAs that have been identified. A. nerii constitutive heterochromatin was enriched in mono-, di- and tri-methylated H3 histones and HP1 proteins but, interestingly, it lacked DNA methylation that was widespread in euchromatic chromosomal regions. These results suggest that aphid heterochromatin is assembled and condensed without any involvement of DNA methylation.

Cytogenetic and molecular analysis of the holocentric chromosomes of the potato aphid Macrosiphum euphorbiae (Thomas, 1878)

Cytogenetic and molecular investigations on the holocentric chromosomes of the aphid Macrosiphum euphorbiae (Thomas, 1878)have been carried out using silver staining and C-banding (followed by chromomycin A3 and DAPI staining) in order to improve our knowledge about the structure of aphid chromosomes. The C-banding pattern is peculiar since only the two X chromosomes and a single pair of autosomes presented heterochromatic bands. Silver staining and FISH with the 28S rDNA probe localized the rDNA genes on one telomere of each X chromosome that were also brightly fluorescent after chromomycin A3 staining of C-banded chromosomes, whereas all other heterochromatic bands were DAPI positive. Interestingly, a remarkable nucleolar organizing region (NOR) heteromorphism was present making the two X chromosomes easily distinguishable. Southern blotting and FISH assessed the presence of the (TTAGG)n repeat at the ends of all the Macrosiphum euphorbiae chromosomes. Karyotype analysis showed that all males possessed the X chromosome with the larger amount of rDNA suggesting a non-Mendelian inheritance of the two X chromosomes.

Statistical analysis of chromosome distribution to the poles in interspecific hybrids with variable chromosome pairing

A probability distribution of chromosome separation to the poles was developed to test the randomness of movement of univalents in asynaptic material where a variable amount of meiotic pairing occurs. Two tables were calculated, one for 24 chromosomes which can be used for any even number equal or less than 24, and the other for 19 chromosomes which can be used for any odd number equal or less than 19.

ThreeNicotianahybrids, namelyN. glutinosa×N. otophora, N. glutinosa, N. sylvestris, andN. tabacum×N. glutinosa, and one polyhaploid were investigated. All hybrids had a tendency towards an equal distribution of unpaired chromosomes to the poles. The polyhaploidN. tabacum(with a substitutedN. plumbaginifoliachromosome) had an opposite tendency, i.e. towards a flatter distribution than expected from random distribution of univalents. A short discussion of the problem is given.

The role of rDNA genes in X chromosome association in the aphid Acyrthosiphon pisum

Silver staining of mitotic metaphases of the aphid A. pisum reveals the presence of argentophilic bridges connecting the two X chromosomes. The presence of nucleolar material connecting sex chromosomes seems to be quite a common phenomenon in organisms belonging to very different phyla, and suggests a role of nucleolar proteins in chromosome association and disjunction. In somatic cells of A. pisum, bridges connecting X chromosomes are detectable not only after silver staining but also after CMA3 staining. This finding suggests that GC rich DNA is involved in this type of association. Molecular analysis of rDNA intergenic spacers shows several 247 bp repeats containing short sequences having a high level of homology with the chi sequence of Escherichia coli and with the consensus core region of human hypervariable minisatellites. Moreover, each 247 bp repeat presents a perfect copy of a promoter sequence for polymerase I. These aphid repeats show structural homologies with a 240 bp repeat, which is considered to be responsible for sex chromosome pairing in Drosophila, not only in view of their common presence within rDNA spacers but also for their length and structure. The presence of chi sequences in the IGS of A. pisum, by promoting unequal crossing-over between rDNA genes, could thus give rise to the nucleolar organizing region (NOR) heteromorphism described in different aphid species. Although X pairing at NORs is fundamental in aphid male determination, the presence of heteromorphism of rDNA genes does not inhibit male determination in the A. pisum clone utilized for our experiments.

The Supernumerary Chromosome of Man

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