Fitness effect analysis of a heterochromatic supernumerary segment in the grasshopper Eyprepocnemis plorans

Several components of fitness were analysed in relation to the presence of a supernumerary chromosome segment (SCS) in two natural populations of the grasshopper Eyprepocnemis plorans, including clutch size, egg fertility, eg g and embryo productivity and survivability from embryo to adult, and SCS t ransmission through males. The results have shown the absence of a signific ant relationship between SCS presence and these fitness components, with th e single exception of egg fertility which decreases significantly in SCS fe males with mating shortage. This fertility decrease is thus expected to be relevant for the population dynamics of the SCS only in low-density populat ions, those in which it is difficult for females to find a male to copulate with before each egg-batch is ready to be laid. The analysis of the SCS tr ansmission through males showed no significant differences between expected and observed SCS frequencies. The SCS polymorphism seems to be at a status close to neutrality in respect to fitness, but its slight disadvantage in transmission through females carrying B chromosomes predicts that the polym orphism should tend to disappear, unless SCS recurrent amplification, or an other undiscovered force, counteracts this tendency.