PHENOTYPIC FITNESS EFFECTS OF B-CHROMOSOMES IN THE PSEUDOGAMOUS PARTHENOGENETIC PLANARIAN POLYCELIS-NIGRA

B chromosomes are elements extra to the standard (A) chromosomes. Thei r frequencies in populations are determined by their transmission rate s 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 Po lycelis nigra a small metacentric B chromosome is very abundant. These populations are pseudogamous parthenogenetic, meaning that sperm is n eeded to activate egg development, but sperm chromosomes are eliminate d from the egg after fertilization. Previous transmission studies have shown that paternally inherited B chromosomes occasionally escape exp ulsion 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 f ertility, cocoon size, hatching time and number of young per cocoon. I n one of three samples cocoon production decreased significantly with increasing B number. In one of two populations, young from mothers wit h B chromosomes grew significantly slower than those of mothers withou t Bs. The dynamics of this B chromosome appear to conform to the paras itic model: Bs accumulate through occasional paternal inheritance desp ite being selected against through negative effects on cocoon producti on and juvenile growth.