Aim To analyse the zoogeography of the shallow-water holothuroids of the western Indian Ocean (WIO). Based on this analysis we ask to what extent differences in species' ability to disperse across potential barriers provide an explanation for holothuroid zoogeography.Location Shallow-waters (50 m isobaths) of the WIO, extending from Suez to Cape Town and from the coastline of East Africa upward to 65°E.Methods Data for the analysis were obtained from Samyn's (2003) monograph on the shallow-water sea cucumbers of the WIO. A species presence/absence matrix with a resolution of 1°latitude/longitude was constructed. These cells were assigned to eight coarser operational geographical units, which were delimited on the basis of published faunistic and geological borders. The analytical zoogeographical methods employed were cluster analysis on several b-diversity coefficients and parsimony analyses of endemicity. The influence of life-history strategies on the distribution pattern was analysed through examination of latitudinal and longitudinal gradients, and by plotting cumulative curves for species number against range size. ResultsThe shallow-water holothuroid fauna of the WIO can be split into several biogeographical units. To the north, we found evidence that the northern Red Sea holothuroid fauna differs from that of the southern Red Sea. The latter has closest affinity with south-east Arabia and the Persian Gulf, and thus the biogeographical barrier of Bab-el-Mandab nowadays seems to be of minor importance. The cold upwelling at the east coast of Somalia forms an effective barrier for holothuroids and especially those with lecitothrophic (short-lived) larvae. Even though the circumtropical biogeographical pattern is not well resolved, important taxonomic turnovers suggest that it is composed of several distinct subprovinces. Taxonomic turnover is at least partially dictated by the dispersion capacity of the different orders.Main conclusions This study concludes that the WIO is best split into at least three biogeographical realms: (1) the Red Sea and associated Arab Basin, (2) the asymmetrical circumtropical region stretching from the horn of Africa to southern Mozambique, and (3) southern Africa. Conspicuous differences in dispersal abilities of the three dominant orders are identified. The biogeography of the WIO is best explained by: (1) species' dispersion ability, (2) the prevalent current patterns, and (3) to a lesser, geographically limited extent, recent geological history. As a serendipitous discovery, we found that Rapoport's rule does not hold in the WIO.

A collaborative study of three laboratories compared the induction of aneuploidy by X-rays in human lymphocytes and fibroblasts. The induction of non-disjunction versus chromosome loss by X-rays was investigated using a variety of aneuploidy detection methods. Chromosome loss was determined by fluorescence in situ hybridization (FISH) with pan-centromeric probes in cytochalasin-B-blocked binucleated cells. Chromosome non-disjunction was estimated by FISH with chromosome-specific centromeric probes in binucleated interphase cells. Chromosomes were counted in parallel in lymphocyte metaphase cells; chromosome counts of the whole karyotype and counts of chromosomes 2 and 8 using chromosome paints. A major observation in spontaneous non-disjunction frequencies concerned the clear difference in frequencies observed between the two painted chromosomes in the same primary cells. When cells were irradiated elevated frequencies were observed for all the different cytogenetic endpoints. Although only a small number of the micronuclei were positive for the centromeric signal and presumably contained whole chromosomes, the absolute number %oC+ increased with dose. Higher rates of non-disjunction were found for irradiated cells; in fibroblasts a statistically significant increase was observed at a dose of 0.5 Gy. The detection of hyperdiploidy by means of chromosome counts and chromosome painting revealed an increase from doses of 1 Gy and higher. Comparison of the different methods detecting different endpoints indicates that non-disjunction may be an important mechanism leading to spontaneous and X-ray-induced aneuploidy. The relative radiosensitivity of aneuploidy induction was compared in two types of primary human cells - lymphocytes and fibroblasts. For chromosome loss both cell types showed similar results, whereas for non-disjunction fibroblasts seemed to be more sensitive. However, these differences may reflect a different sensitivity in the scoring methods used.

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