02197nas a2200145   4500008004100000245014800041210006900189300001200258490000700270520164900277100002201926700002501948700002601973856005201999       1998                        eng d00aReproductive modes, ploidy distribution, and supernumerary chromosome frequencies of the flatworm Polycelis nigra (Platyhelminthes: Tricladida)0 aReproductive modes ploidy distribution and supernumerary chromos  a277-2850 v833 a<p>The hermaphroditic flatworm, Polycelis nigra, is characterized by two reproductive biotypes which differ with respect to ploidy; sexual individuals are diploid (n = 8, 2&times; = 16) and pseudogamous parthenogenetic individuals are polyploid (typically 3&times;). We have collected and karyotyped individuals from 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 sympatry, and identified purely diploid, mixed diploid-polyploid, and purely polyploid populations. Karyotype data show that in addition to the normal autosome complement, B chromosomes of differing morphology as well as stable aneuploid 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 submetacen- tric to metacentric, mitotically stable B chromosome, as well as a stable extra-A chromosome. Here, individuals having 1&ndash;3 B chromosomes were more abundant (61%) than those having no B&rsquo;s, implying that B chromosome infection has little detrimental effect when occurring in low numbers. Furthermore, 66% of individuals from this population possessed extra-A chromosomes, although it is unclear whether these elements are aneuploid autosomes or B chromosomes of different morphology. The ubiquity of these chromosomes, within asexuals in particular, is suggestive of a correlation between the origination of the elements and the evolution of polyploidy, or may reflect increased tolerance of parthenogenetic genomes to aneuploidy.</p>
1 aBeukeboom, Leo, W1 aSharbel, Timothy, F.1 aMichiels, Nicolaas, K  uhttps://continenticola.myspecies.info/node/131901937nas a2200169   4500008004100000245007700041210006900118300001400187490000700201520138400208100002201592700001701614700002401631700002001655700002601675856006601701       1996                        eng d00aPaternal inheritance of B chromosomes in a parthenogenetic hermaphrodite0 aPaternal inheritance of B chromosomes in a parthenogenetic herma  a646–6540 v773 a<p>B chromosomes are dispensable elements extra to the standard (A) chromosome complement. They have been described from many sexually reproducing species where they often exploit meiosis to accumulate from one generation to the next. Polycelis nigra is a simultaneously hermaphroditic flatworm that can reproduce either sexually or asexually through pseudogamous parthenogenesis: although individuals mate, the sperm is only used for egg activation but does not contribute genetically to the progeny. B chromosome frequencies were screened in populations from two North Italian lakes and found to vary from 27 to 90 per cent among pseudogamous parthenogenetic individuals. Because these frequencies were unexpectedly high for an asexual population, we looked at the transmission of B chromosomes in field-collected animals and controlled crosses. Animals that did not carry B chromosomes produced offspring with B chromosomes at frequencies of 6&ndash;34 per cent. Results from the crosses strongly suggest that B chromosomes are not only inherited maternally, as expected, but also paternally. This is a new mode of inheritance for a B chromosome. By preventing expulsion from the egg, this B chromosome seems to have acquired biparental transmission in an otherwise asexually reproducing host. We discuss the evolutionary consequences of this observation in more detail.</p>
1 aBeukeboom, Leo, W1 aSeif, Miriam1 aMettenmeyer, Thomas1 aPlowman, Amy, B1 aMichiels, Nicolaas, K  uhttp://www.nature.com/hdy/journal/v77/n6/abs/hdy1996192a.html