197 JOURNAL OF ORTHOPTERA RESEARCH 2010, 19(2) M. BAKKALI, M.I. MANRIQUE-POYATO, M.D. LÓPEZ-LEÓN, F. PERFECTTI, J. CABRERO AND J.P.M. CAMACHO Abstract We analyse here three components of reproductive success (egg fertility, egg clutch size and embryo clutch size) in several temporal samples from different Spanish and Moroccan populations of the grasshopper Eyprepocnemis plorans. The analysis of spatial and temporal variation suggests that egg clutch size, but neither embryo clutch size nor egg fertility, depends significantly on both year of sampling and population of origin. While the former effect could mainly be due to year-to-year variation in food availability (essential to egg production but not to hatching success), the spatial variation may also include population-dependent genetic factors. We also tested the effect of the presence of supernumerary (B) chromosomes carried by many individuals in most natural populations of this species. We found a slight but significant decrease in egg fertility associated with the presence of B chromosomes. We discuss possible causes of the observed variation for these three reproductive traits at both spatial and temporal levels, as well as the effect of B-chromosome presence as a parasitic element disturbing reproduction of carrier females. Key words B chromosomes, clutch size, fertility, grasshopper, Eyprepocnemis plorans, Orthoptera Introduction About 15% of eukaryote species carry supernumerary (B) chro- mosomes in addition to members of the standard chromosome complement (A chromosomes). Two models have been put forward to explain B-chromosome maintenance in natural populations: the parasitic (Östergren 1945) and the heterotic (Darlington 1958, White 1973) models. Both assume that B frequency may reach equilibrium as a consequence of the action of two opposite forces, i.e., drive with harmful effects of the B chromosome in the parasitic model, and beneficial effects at low number of Bs, but harmful effects at high B numbers, in the heterotic model. Most B chromosome systems where transmission and fitness effects have been sufficiently studied, fit the parasitic model (for a recent review, see Camacho 2005). A minority of Bs, however, show characteristics being compatible with the heterotic model, such as providing resistance to pathogens, e.g., Nectria haematococca (Miao et al. 1991) (see other examples in Camacho 2005) or being beneficial for survival, as shown for the nondriving B chromosome in the plant Allium schoenoprasum (Plowman & Bougourd 1994). A variant of the parasitic model not assuming equilibrium for B frequency, was built on the basis of population dynamics studies in the grasshopper Eyprepocnemis plorans. In this species, the first analyses of B-chromosome transmission showed an absence of drive for the three main B variants (López-León et al. 1992). Subsequent experiments, however, found that one of these variants (B 2 ) showed significant drive when B-carrying females were crossed to males from a B-lacking population, thus suggesting that B drive was sup- pressed in the B-carrying population (Herrera et al. 1996). This led Camacho et al. (1997) to propose that the same B chromosome may show drive or not, depending on the population evolutionary stage, since parasitic B chromosomes may lose drive due to the evolution of modifier genes in the A chromosomes — a scenario previously suggested by Shaw (1984) and demonstrated by Shaw and Hewitt (1985) and Nur and Brett (1985, 1987). Therefore, B chromosome frequency in natural populations is not necessarily at equilibrium, but may change as B chromosomes pass through several stages, i.e., parasitic, drive-suppression and neutral- ized stages (Camacho et al. 1997). At any stage, the B chromosome can mutate to a new variant and when this generates a variant being able to drive, the near-neutral cycle restarts, thus prolonging the life of the B-chromosome polymorphism. Repeated generation of new variants and recovery of drive allow the polymorphism to persist in the populations. B chromosomes in E. plorans actually show high mutation rates (López-León et al. 1993, Bakkali & Camacho 2004), which putatively facilitate the substitution of neutralized Bs by derived driving variants. It is worth mentioning that this kind of polymorphism regeneration was evidenced in E. plorans (Zurita et al. 1998). Furthermore, the near-neutral model has been considered to explain the long-term evolution of other B-chromosome systems (González-Sánchez et al. 2003, Jones et al. 2008) and other selfish genetic elements (Johnson 1997); Frank (2000) highlighted its importance as a paradigm for the evolution of polymorphisms of attack-defence between parasitic elements and their hosts. An important aspect of the research on parasitic B chromosomes is to ascertain whether they impose some load on host fitness. At the cytological level, B chromosomes of Spanish E. plorans popula- tions were found to increase chiasma frequency (Camacho et al. 1980, Camacho et al. 2002), and the number of active nucleolus organizer regions (NOR) (Cabrero et al. 1987, López-León et al. 1995), although the relationship between these traits and fitness is unknown. Analyses of several exophenotypic traits, however, failed to show any effect of B chromosomes (Camacho et al. 1980, Martín-Alganza et al. 1997). However, B chromosomes in E. plorans have been reported to decrease egg fertility (Muñoz et al. 1998, Zurita et al. 1998) and to increase the formation of abnormal spermatids (Suja & Rufas 1989, Teruel et al. 2009), although it is not clear that this latter effect significantly decreases male fertility. In the grasshopper Myrmeleotettix maculatus, sperm dysfunction in B-carrying males seems to be associated with a poor transmission of B chromosomes (Hewitt 1973a,b; Hewitt et al. 1987). In high parallelism with Spanish Bs, both parasitic (driving) and neutral- Effects of B chromosomes on egg fertility and clutch size in the grasshopper Eyprepocnemis plorans M. BAKKALI, M.I. MANRIQUE-POYATO, M.D. LÓPEZ-LEÓN, F. PERFECTTI, J. CABRERO AND J.P.M. CAMACHO Departamento de Genética, Facultad de Ciencias, Universidad de Granada, 18071, Granada, Spain. Email: jpmcamac@ugr.es Submitted June 2, 2010, accepted September 6, 2010 Journal of Orthoptera Research 2010,19(2): 197-203