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Abstract
The species of Odonata are cytogenetically characterised by possessing holokinetic chromosomes, a post-reductional meiosis, an XX/X0 (female/male) sex chromosome mechanism, m-chromosomes, and only one chiasma per bivalent. Chromosome studies were performed on males of Erythrodiplax nigricans and Coryphaeschna perrensi from Argentina. Erythrodiplax nigricans has n=12+XO and lacks m-chromosomes, while C. perrensi has 2n=27, n=13+XO, m-chromosomes and a large autosomal pair associated with the nucleolus. The meiotic behaviour of both species follows the general pattern of the order: the X chromosome is positively heteropycnotic during early prophase I; bivalents regularly show only one chiasma; all chromosomes migrate synchronously and almost parallel to the equatorial plane at anaphase I; at metaphase II the X chromosome is present in all the cells as a consequence of the post-reductional division, lies outside the metaphasic plate, and migrates asynchronously with the autosomes at anaphase II. In C. perrensi, the largest bivalent exhibits two chiasmata in a large proportion of cells, which is a very rare feature among dragonflies. Heterochromatin characterisation with DAPI-CMA banding reveals that C. perrensi does not show fluorescent banding, except for a CMA bright band at one telomeric region of the largest bivalent, associated with the NOR region; in E. nigricans, autosomes have small AT-rich telomeric blocks, except for the smallest pair, which exhibits conspicuous bands in both telomeric regions, one being GC-rich and the other AT-rich. Taking into account that the m-chromosomes have been found in other E. nigricans populations, their absence in the studied population may be due to the presence of such heterochromatic blocks.