Michael G. Ritchie

Artificial selection for a courtship signal in Drosophila melanogaster

The mean inter-pulse interval of the song of maleDrosophila melanogasteris a courtship signal which shows relatively little variability in natural populations and is important to male mating success. Artificial selection was carried out on this trait for six generations, producing a line of flies that had a mean inter-pulse interval outside the normal range for the species. The stock used was derived from isofemale lines taken from a natural population in which other studies had suggested heritability for this trait was low. The response to selection was asymmetric, with a greater response for longer inter-pulse interval. Analysis of other changes between the lines suggests that the genes selected for act fairly specifically on inter-pulse interval, reducing both the mean inter-pulse interval and the number of pulses produced. Analysis of F2 and back-cross generations suggested a polygenic additive architecture without a significant non-additive component or disproportionate role of sex-linked genes on mean inter-pulse interval. Contrary to expectations, variance was greater in the F1 than in F2 or back-cross generations, and the realized heritability was much greater than the evolvability (coeYcient of additive genetic variation). The results suggest that behavioural traits with low variability still retain the potential for rapid evolution, perhaps because environmental sources of variation counteract fixation. ? 1996 The Association for the Study of Animal Behaviour

Assortative mating across a hybrid zone in Chorthippus parallelus (Orthoptera: Acrididae)

Two subspecies of the grasshopper Chorthippus parallelus meet and form a hybrid zone in the Pyrenees. Both strong hybrid dysfunction and a behavioural difference occur, which would seem to make the zone a suitable candidate for speciation by reinforcement. One of the classic ways of looking for this is to test for increased levels of assortative mating between populations from close to the region of contact. Here we show that, with virgin insects, such assortment decreases as one approaches the centre of the zone in C. parallelus. The pattern is different upon remating, with non‐virgins showing a pattern more like that predicted by reinforcement. Overall there is little evidence for reinforcement. We argue that other tests of the model may be more appropriate for stable hybrid zones.

Variation in female mate preference across a grasshopper hybrid zone

A method for studying variation in female preference among populations free from the confounding effects of variation in male characters is described. This method is applied to sixteen populations in a transect across a hybrid zone in the grasshopper, Chorthippus parallelus. Significant variation in preference is revealed among populations. The pattern of this variation is analysed in the context of the reinforcement model of speciation. While the data do indicate an increased homogamic preference on one side of the zone relative to more distant populations, this pattern cannot be distinguished statistically from an abrupt transition in preference over a distance of less than 1 km. We argue that either of these patterns implies that selection operates on female preferences in the hybrid zone.

FEMALE SONG PREFERENCE AND THE PERIOD GENE IN DROSOPHILA

Mutations at theperiod (per) locus inDrosophila melanogaster alter rhythmic components of the male courtship song. We have examined the mating speed of females homozygous for mutantper alleles when presented with artificial mutant songs. Mutant females retain a preference for wild-type over mutant songs, thus male song and female preference are probably under separate genetic control. In contrast,per-mutant females from an established laboratory stock which had been maintained for nearly two decades appear to have an enhanced response to the corresponding mutant song in that they no longer discriminate against mutant song. These results are discussed in terms of the “genetic coupling” and “coevolution” theories of complementarity between male and female components of communication systems.

Causation, fitness effects and morphology of macropterism in Chorthippus parallelus (Orthoptera: Acrididae)

ABSTRACT. 1. Chorthippus parallelus (Zett.) displays an apparent‘flight polyphenism’despite the fact that macropterous individuals cannot fly.

Are differences in song responsible for assortative mating between subspecies of the grasshopper Chorthippus parallelus (Orthoptera: Acrididae)?

Abstract Chorthippus parallelus parallelus and C. p. erythropus mate assortatively in the laboratory. Previously it had been assumed that differences in the songs of the males would be responsible for this assortment. However, in the present study, C. p. erythropus females continued to mate assortatively, and readily, with males that had been made dumb by the removal of their wings. Furthermore, this assortment broke down, and matings occurred less readily, following the removal of the antennae of both sexes. With C. p. parallelus the converse applied, i.e. assortment continued after the antennae were removed and mating occurred less readily in the absence of song. This implies that a substantial degree of differentiation (a change in the main channel of communication) has occurred between the mate recognition systems of these subspecies without leading to speciation.

Variation in male song and female preference within a population of Ephippiger ephippiger (Orthoptera: Tettigoniidae)

Species-specific patterns of insect calling song are commonly assumed to show low variability. In the bushcricket Ephippiger ephippiger, male song shows large geographical variation in syllable number, and females strongly prefer song typical of their own population. Here it is shown that there is also large variation in syllable number within a population of E. ephippiger. Two factors associated with this variability were identified: eclosion sequence (which was positively correlated with mean male syllable number) and male-male interactions (with chorusing males producing more variable song than those singing in isolation). Similar variability was not evident in female preference as females did not show significant heterogeneity when offered a choice of recorded songs representing the main syllable types from the population, nor did overall preference vary amongst the main song types. A variety of factors, including genetic variability, developmental plasticity, male interactions and weak female preferences, could be responsible for this phenotypic variability.

Reproductive isolation and the period gene of Drosophila

The identification of genes of large effect on ecologically important traits is an important aim of molecular ecology. The period gene of Drosophila is a candidate for a gene with a large influence on premating isolation between Drosophila species, as it determines species specific aspects of courtship behaviour. Strains of D. melanogaster are available which have been genetically transformed with the period gene of either D. melanogaster or D. simulans. Here we show that D. melanogaster females do not discriminate between two such strains. This suggests that period may only make a small contribution to total premating isolation between these species. We discuss the use of genetically transformed strains in assessing the influence of single genes on complex traits.

Behavioral coupling in tettigoniid hybrids (Orthoptera)

Studies of the mating behavior of male and female F1 hybrids between closely related taxa can provide information concerning the genetic control of characters that play a major role in speciation. Orthoptera have been used previously for such studies. Hybrid crickets show behaviors which are broadly intermediate to the parentals but hybrid grasshoppers may retain parental behavior patterns. This study examines the behavior of hybridEphippiger ephippiger bushcrickets, the third major orthopteran group. The differences in male song and female preference are probably both mainly additive and male song differences not sex linked. Thus, given a choice, hybrid females would prefer to mate with hybrid males, an example of “behavioral coupling.” The evolutionary inferences which can be drawn from studies of F1 hybrids between closely related taxa are discussed.

Broad‐scale mapping of a hybrid zone between subspecies of Chorthippus parallelus (Orthoptera: Acrididae)

Many species are subdivided into geographical races or subspecies whose ranges do not overlap but may meet. Where ranges do meet, hybrid zones may form, and these have now been described in a large number of species (Barton & Hewitt, 1985,1989; Hewitt, 1988). The majority of these hybrid zones are believed to be maintained by a balance between dispersal and selection against hybrids and to have formed as a result of secondary contact between populations that had accumulated genetic differences in allopatry (Barton & Hewitt, 1985). The evidence for a secondary origin, rather than divergence in sifu in response to environmental selection pressures, is mainly indirect and includes the coincidence of clines in many characters, the absence of ecological correlates of cline position on a local scale, and the positions of zones on a broad scale (Endler, 1977; Barton & Hewitt, 1985, 1989; Hewitt, 1988). The present course of a hybrid zone can often be interpreted in terms of range expansion from glacial refugia into new areas of suitable habitat and this interpretation can give useful insight into the history of the interacting taxa (for examples see Hewitt, 1989). Two subspecies of the grasshopper Chorthippus parallelus meet and hybridize in the Pyrenees (Butlin & Hewitt, 1985; Hewitt, 1990). C.p.parallelus occurs over most of Europe and east into Asia but is replaced in the Iberian Peninsula by C.p.erythropus. The two taxa differ in many characters (reviewed by Hewitt, 1990) and in the laboratory produce sterile F l male offspring but fertile female hybrids (Hewitt et al., 1987). Clines for several characters have been mapped in detail in two localities Col de la Quillane, Pyrenees-Orientales and Col du Pourtalet,