Reproductive modes, ploidy distribution, and supernumerary chromosome frequencies of the flatworm Polycelis nigra (Platyhelminthes : Tricladida)

The hermaphroditic flatworm, Polycelis nigra, is characterized by two repro ductive biotypes which differ with respect to ploidy; sexual individuals ar e diploid (n = 8, 2x = 16) and pseudogamous parthenogenetic individuals are polyploid (typically 3x). We have collected and karyotyped individuals fro m 15 sampling sites (13 in mid to northern Italy, one in Great Britain and one in The Netherlands). We found that biotypes can exist alone or in sympa try, and identified purely diploid, mixed diploid-polyploid, and purely pol yploid populations. Karyotype data show that in addition to the normal auto some complement, B chromosomes of differing morphology as well as stable an euploid chromosomes (extra-A) were found almost exclusively in polyploids ( 11 of 12 sites). We extensively sampled Lago di Toblino (northern Italy), a pure polyploid population characterized by a submetacentric to metacentric , mitotically stable B chromosome, as well as a stable extra-A chromosome. Here, individuals having 1-3 B chromosomes were more abundant (61%) than th ose having no B's, implying that B chromosome infection has little detrimen tal effect when occurring in low numbers. Furthermore, 66% of individuals f rom this population possessed extra-A chromosomes, although it is unclear w hether these elements are aneuploid autosomes or B chromosomes of different morphology. The ubiquity of these chromosomes, within asexuals in particul ar, is suggestive of a correlation between the origination of the elements and the evolution of polyploidy, or may reflect increased tolerance of part henogenetic genomes to aneuploidy.