Abstract
The structure of the holocentric chromosomes of the rosy apple aphid, Dysaphis plantaginea (2n = 12), and pear-grass aphid, Melanaphis pyraria (2n = 8), was studied using C-banding, NOR, Giemsa and fluorochrome staining, and fluorescent in situ hybridization (FISH). Contrary to the equilocal distribution of heterochromatin typical of monocentric chromosomes, in both species C-banding evidenced a tendency of highly repetitive DNAs to be restricted to the X chromosomes. Silver staining and FISH, using a 28S rDNA probe, located rDNA genes on one telomere of each X chromosome, the only brightly fluorescent C-positive sites revealed by CMA3 staining, whereas all other heterochromatic C-bands were DAPI positive. Both species showed a noticeable amount of rDNA heteromorphism. Mitotic recombination is proposed as a possible mechanism responsible for the variation in size of rDNA.
Highlights
Most eukaryotes possess monocentric chromosomes characterised by a single localized centromere, which, under a microscope, appears as a primary constriction in the chromosome
The present paper reports the results of a cytogenetic study of two pests of pome orchards: the rosy apple aphid, Dysaphis plantaginea (Passerini), which is an important pest in western European apple orchards and troublesome in organic apple production (Cross et al, 2006), and the brown pear aphid, Melanaphis pyraria, which in addition to causing direct damage transmits barley yellow dwarf luteovirus (El Yamani & Hill, 1991)
C-banding treatment in Melanaphis pyraria revealed one main heterochromatic band located in a telomeric position on the X chromosomes
Summary
Most eukaryotes possess monocentric chromosomes characterised by a single localized centromere, which, under a microscope, appears as a primary constriction in the chromosome. Species with holocentric chromosomes are recorded in both the plant and animal kingdoms, including insects and nematodes, which is thought to be a result of convergent evolution (Dernburg, 2001). Notwithstanding their uniqueness, holocentric chromosomes have not been intensively studied, except in the nematode worm Caenorhabditis elegans (for a review see Maddox et al, 2004), Mitotic chromosomes can be observed in aphid embryonic tissues. For this reason, aphids represent a suitable insect model for determining the differences and similarities in the structure and activity of holocentric and monocentric chromosomes. These resources will provide novel opportunities for investigating many peculiar features of aphid biology, such as aphid-plant interactions and insecticide resistance in aphid species of agricultural importance
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