A karyometric analysis on diploid strains of three American species of Dugesia (subgenus Girardia), namely D. anceps, D. sanchezi and D. tigrina, has been carried out. In all these species the chromosome complement consists of 8 pairs of homologous chromosomes, forming a series gradually decreasing in length, the longest being approximately twice the shortest. In D. sanchezi the chromosomes are metacentric with very similar values of the centromeric index. In D. anceps and D. tigrina most of the chromosomes are metacentric; submetacentric chromosomes (two and one, respectively) are present among the smallest elements of the complement. The karyotypes of the three species, however, appear very similar and the slight differences observed are to be referred to minor chromosome rearrangements, such as pericentricinversions, occurring during the· evolution of these planarians.
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