Sperm numbers vary between inter- and intra-population matings of the grasshopper Chorthippus parallelus

Abstract

Comparing the reproductive output of intra- and inter-population matings is the most common way to assess whether post-mating reproductive isolation is caused by genetic incompatibilities. Such genetic incompatibility can however, only assume that the quantity of the post-mating signals involved does not differ between intra- and inter-population matings. This assumption may not be true because sexual selection predicts reduced mating effort towards low-quality mates and in many circumstances, allopatric partners are low-quality mates. Post-mating efforts may, therefore, be reduced in inter- compared to intra-population matings. Here, I test this crucial assumption by studying variation in one post-mating trait, sperm number, in crosses of two parapatric grasshopper populations. In both populations, males transferred fewer sperm to allopatric than sympatric females. If such plasticity with respect to population is common in other post-mating traits, differences between inter- and intra-population crosses may be more frequently caused by differences in sperm number rather than gamete incompatibility. Additionally, I found that sperm numbers declined less rapidly in the female storage organ of allopatric than sympatric females but its rate differed markedly between populations. This is discussed with respect to female adaptations to male traits.

Ejaculate size varies with remating interval in the grasshopper Chorthippus parallelus erythropus (Caelifera: Acrididae)

“…Ejaculate sizes were similar in mean and variation (as indicated by the large standard deviations - Table 1) to those measured in J males under similar or the same laboratory conditions , Reinhardt, 2001, 2006 Table 1). In order to account for the inter-male variability in the first ejaculate, ejaculate sizes in the second and third mating are presented graphically as the proportion of the first ejaculate.…”

“…Freshly cut grass blades of Dactylis glomerata were provided as food every two to three days. For further rearing details see Reinhardt & Köhler (1999), Reinhardt et al (1999), Reinhardt (2001Reinhardt ( , 2006.…”

“…Sperm numbers were analyzed as previously described Reinhardt, 2001Reinhardt, , 2006. Briefly, the spermatheca and its duct were removed from the female within 1 to 60 min after mating.…”

“…I also measure ejaculate size when there is a replenishment interval between matings. For illustrative reasons I also report the few data that became available on its sister taxon, the grasshopper Chorthippus parallelus parallelus because the two subspecies form a hybrid zone in southern Europe (Hewitt, 1988;Butlin, 1998;Hewitt, 2001, reviewed Butlin, 1998 and the two subspecies have diverged in several reproductive traits including mate searching, female sperm handling and testis size (Neems & Butlin, 1993;Tregenza et al, 2000aTregenza et al, ,b, 2002Reinhardt, 2006). This comparison and the differences in testis size resulted in the speculation that the two subspecies may differ in sperm allocation.…”

<b><i>Wolbachia</i></b> Induced Cytogenetical Effects as Evidenced in <b><i>Chorthippus parallelus</i></b> (Orthoptera)

“…It is difficult to evaluate whether this proportion of abnormal spermatids affects fertility or fecundity of affected individuals in terms of fitness. In our opinion, this reduction probably affects the number of viable sperm produced by infected males but not the number of expected descendants for a given population affected by this handicap: in insects, sperm is stored in the female's spermatheca, and therefore, we do not expect that this reduction in sperm number or quality in a given male will affect female's availability of sperm [Reinhardt, 2006]. However, it suggests selection against hybrid infected individuals which is indicative of a 'tension hybrid zone' [Barton and Hewitt, 1985].…”