Supernumerary chromosomes (B-chromosomes) have been found in numerous specimens of the planarian Dugesia benazzii collected in some localities of Corsica (Bravone, Conca, Golo, Patrimonio and Oso). These chromosomes are of small size, about one halif of the smallest chromosome of the standard karyotype; they are metacentric or slightly heterobrachial, devoid of satellites and secondary constrictions, euchromatic and are present in both somatic and germ cells. Various individuals of the same population as well as various cells of the same organism may contain a variable number of B-chromosomes (from O to 4). The B-chromosomes in metaphase I oocytes appear always as univalents and may sometimes acquire a ring-shaped configuration. lndividuals exhibiting B-chromosomes in their cells belong to populations which reproduce mainly by fission and possess highly polisomic chromosomal sets. However, supernumerary chromosomes are absent in other races of planarians which present karyological and reproductive characteristics simlar to those of the populations described above. The same is true for eudiploid individuals, even if they belong to a population in which some specimens exhibit B-chromosomes. As far as my observations go, the specimens possessing supernumerary chromosomes are present in areas of Corsica which correspond to the distribution of the diploid amphimictic biotype; such specimens, however, have not been found in areas in which polyploid biotypes are present. The possible significance of B-chromosomes, in relation to the intraspecific variability of these populations of planarians, is discussed.
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