Analysis of the phenotypic effects of B chromosomes in a natural population of Metagagrella tenuipes (Arachnida : Opiliones)

Phenotypic effects of B chromosomes in a natural population of Metagagrella tenuipes (Arachnida: Opiliones) were studied. Mean number of Bs per indivi dual in the population studied was 6.0, and remained stable during two succ essive summers of 1997 and 1998. In contrast to the number of B chromosomes , ratios between individuals possessing odd and those possessing even numbe rs of Bs changed during both collection seasons: the proportion of harvestm en with an even number of Bs decreased from June-July to October-November. A possible reason for this may be a difference in susceptibility to parasit es between B-odd and B-even harvestmen. In the group of B-even individuals the percentage of infected harvestmen in the June-July samples was much hig her compared to the B-odd group. In addition, the infection rate in the B-e ven group decreased more sharply than among B-odd harvestmen. In the group of B-even harvestmen infection was associated with reduced body size, where as no such association was found among B-odd harvestmen. In the group of B- even individuals there was a U-shaped relationship between number of Bs and the probability of being infected by parasites, and an inverted-U-shaped r elationship between body size and number of Bs. No such associations were f ound in the group of B-odd harvestmen. Seasonal selection is suggested to b e a main factor contributing to the B-chromosome polymorphism in M. tenuipe s.