Asymmetrical homogamy and unequal sex ratio from reciprocal mating-order crosses between Chorthippus parallelus subspecies

Abstract

To study the evolution of this Pyrenean hybrid zone following postglacial secondary contact, a crossing programme was performed involving sequential mating of single females by males of both subspecies in both orders. Progeny were scored as embryos using Acridine orange and C-banding to identify their male parent. This revealed a number of significant departures from straightforward inheritance. There was an excess of pure over hybrid progeny. This homogamy is produced at fertilization but may have been determined before through sperm preference or competition. There was also sperm precedence with the second mating producing most of the progeny. This may be due to sperm ageing or differential storage and utilization by the female. The fundamental sex ratio in the progeny was biased towards males. Of particular interest is the finding that homogamy is markedly greater when C.p. parallelus is the mother, suggesting some form of differential biochemical recognition. This asymmetry should have important consequences for the dynamics of the hybrid zone. All these results are discussed in the light of the divergence of the two subspecies and the mixing of their genomes in the hybrid zone.

Females of the grasshopper Chorthippus parallelus (Zett.) do not remate for fresh sperm

“…Fresh sperm is more motile, and more motile sperm has a higher velocity (Aitherton et al 1979) and shows a greater fertilization capacity (Lillie 1919;Roche et al 1968;Lanmann 1968;Miller et al 1969;Lodge et al 1971;Smith & Lodge 1987;Levitan et al 1991;Andre¨& Lindegarth 1995). Bella et al (1992) found slight evidence for sperm ageing in the competitive ability in C. parallelus by comparing the proportion of o¡spring fathered by the second male to mate (P 2 -values) from homogametic and heterogametic matings, but no correlation of sperm age with fertilization success was detected. Although sperm ageing processes and consequences within males and females might be quite di¡erent (Lanmann 1968), there seems to be a similar e¡ect of a decrease in sperm quality aged in the testes of human males.…”

“…Females of this and related species mate at a maximum of one time per oviposition (Renner 1952;Haskell 1958;Hartmann & Loher 1996; K. Reinhardt and G. KÎhler, unpublished data; but see Bella et al 1992) as they are not receptive between copulation and oviposition (Renner 1952;Hartmann & Loher 1996). They can reject males for more than one oviposition (Haskell 1958;Kriegbaum & Von Helversen 1992;K.…”

“…A sharp decrease in fertilization capacity, an obvious character of sperm quality, has been demonstrated in many external (Lillie 1919;Levitan et al 1991;Andre¨& Lindegarth 1995) and internal fertilizers (Lanmann 1968;Lodge et al 1971;Smith & Lodge 1987). Another consequence of sperm ageing could be a di¡er-ential survival according to the sex chromosomes as suggested by Hewitt et al (1989) and Bella et al (1992). Thus, sperm ageing in the female can lead to a biased o¡spring sex ratio (Sapp & Martin-DeLeon 1992;Corkery & Martin-DeLeon 1996).…”

“…We carried out experiments that allowed us to separate the e¡ects of sperm age and laying order by comparing a treatment of females with free access to males with one where females were mated only once. Since last-male sperm precedence is commonly found in grasshoppers (Simmons & Siva-Jothy 1998) and also in our study species (Bella et al 1992;K. Reinhardt, unpublished data) it is reasonable to assume that multiply mated females always have a certain level of fresh sperm and that the most recently received sperm is used for fertilization.…”

Negatively assorted gamete fertilization for supernumerary heterochromatin in two grasshopper species

“…Few exceptions to random gamete fertilization, however, have been reported up to now, especially in animals where sperm success in fertilization seems to depend little or not at all on the genotype (Muller & Settles, 1927;Zimmering et al, 1970). Homogamy caused by preferential fertilization, however, has been reported in a few cases of hybridization between closely related animals (Hewitt et a!., 1987(Hewitt et a!., , 1989Bella et a!., 1992;Gregory & Howard, 1994;Wade et al, 1994). In plants, however, the pollen-tube growth velocity could play a significant role in competition between pollen grains to fertilize the macrospore.…”