Phenotypic fitness effects of B chromosomes in the pseudogamous parthenogenetic planarian Polycelis nigra

B chromosomes are elements extra to the standard (A) chromosomes. Their frequencies in populations are determined by their transmission rates and effects on host fitness. Most B chromosomes are considered to be genomic parasites having transmission drive and being detrimental to their carriers. In some populations of the hermaphroditic planarian Polycelis nigra a small metacentric B chromosome is very abundant. These populations are pseudogamous parthenogenetic, meaning that sperm is needed to activate egg development, but sperm chromosomes are eliminated from the egg after fertilization. Previous transmission studies have shown that paternally inherited B chromosomes occasionally escape expulsion from the egg. Here, we compared a number of phenotypic fitness aspects of worms, with and without B chromosomes, taken from the field. B chromosomes did not significantly affect adult body size, cocoon fertility, cocoon size, hatching time and number of young per cocoon. In one of three samples cocoon production decreased significantly with increasing B number. In one of two populations, young from mothers with B chromosomes grew significantly slower than those of mothers without Bs. The dynamics of this B chromosome appear to conform to the parasitic model: Bs accumulate through occasional paternal inheritance despite being selected against through negative effects on cocoon production and juvenile growth.