Paternal inheritance of B chromosomes in a parthenogenetic hermaphrodite

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. Polycells 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 6-34 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.