Karyology and karyometric analysis of an Afrotropical freshwater planarian (Platyhelminthes, Tricladida)

The karyology and karyotype analysis of the Afrotropical species Dugesia aethiopica is reported. A double chromosome complement is displayed in a single individual, triploid (3n - 27 + 1-2 B-chromosomes) and diploid, (2n = 18 + 0-1 B-chromosome) with a basic number of n = 9. The occurrence of a double chromosome number, involving both somatic and germ cell lines, fits the condition of a diffuse mosaicism or mixoploidy. The coexistence of heteromorphisms between homologous chromosomes, polysomic and monosomic conditions, and the presence of B chromosomes could be considered a condition of aneuploidy. The constant re-occurrence of mixoploidy in D. aethiopica represents an adaptive mechanism whose persistence could be favoured by the asexual reproductive mode. The new finding confirms the existence of two different chromosomal lineages in African planarians with n = 8 and n = 9.