MULTIPLE SUPERNUMERARY CHROMOSOMES IN THE PSEUDOGAMOUS PARTHENOGENETIC FLATWORM POLYCELIS-NIGRA - LINEAGE MARKERS OR REMNANTS OF GENETIC LEAKAGE

Polycelis nigra is a free-living simultaneous hermaphroditic flatworm that has amphimictic and pseudogamous parthenogenetic biotypes. Sexual individuals are always diploid (2n = 16) and pseudogamous parthenogen s are polyploid (usually triploid). Two types of supernumerary chromos omes are found in parthenogens, those resembling autosomes (''A-like'' ) and typical B chromosomes, both of which reach frequencies in popula tions of close to 100%. Experiments measuring the transmission rates o f the B chromosomes indicated that they are potentially inherited via the male line, escaping expulsion by pseudogamous parthenogenesis. Thi s study used the C-banding technique to demonstrate (i) that there is a single morphologically distinct B chromosome (B1) and (ii) that then are two ''A-like'' chromosomes that can be considered B chromosomes ( B2 and B3) and which are not simple polysomics of one of the eight aut osomes. As there is no genetic exchange between pseudogamous parthenog enetic lineages, two different individuals carrying a similar B morph must either have received it through common ancestry (a lineage marker ) or have acquired it horizontally from another parthenogenetic lineag e (leakage). C-banding further revealed intra-individual heteromorphy for band regions on chromosomes 5 and 8. This supports the karyotypic observation that oogenesis is preceded by premeiotic chromosome doubli ng followed by pairing of replicate homologues.