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M G Ritchie

Publications and source records attributed to M G Ritchie.

17 recordsLinked to original sources
PubMed2006

An unusual phylogeography in the bushcricket Ephippiger ephippiger from Southern France.

Pleistocene glaciations have played a major role in species divergence. The bushcricket Ephippiger ephippiger shows unusual patterns of intraspecific variation in multiple traits across Southern Europe. This is centred in Southern France, and evidence implies that it results from secondary contact after differentiation in Pleistocene refugia. However, the possible time scales involved, locations of the refugia and patterns of expansion remain obscure. This study sequenced the COII (507 BP) and cyt b (428 BP) mitochondrial genes to examine the intraspecific phylogeography of Western European samples of E. ephippiger. A minimum evolution tree revealed little resolution between described subspecies of E. ephippiger. Strikingly, populations from the Pyrenees and Mediterranean coastal region contained a complex genetic structure corresponding to major river valleys, independent of the traditional taxonomy. Samples of the subspecies E. e. vitium formed a distinct clade, perhaps supporting their taxonomic status. However, other forms (cruciger and cunii) were not genetically distinct, which is surprising given differences in their morphology and behaviour. The extent of the genetic divergence between Pyreneen valleys is unexpectedly deep, with average Tamura-Nei distances of around 14% (net distances of 11%) separating the main clades of coding COII sequences. Cyt b showed a similar pattern, but was confounded by some non-coding probable pseudogenes. If a conventional insect molecular clock is applied, these cryptic clades must pre-date the Pleistocene, and hypotheses for their history are discussed. However, mtDNA divergence in Ephippiger is not evolving in a clock-like manner, because a likelihood ratio test rejects clock assumptions for the COII sequences.

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PubMed2005

Experimental manipulation of sexual selection and the evolution of courtship song in Drosophila pseudoobscura.

Courtship song serves as a sexual signal and may contribute to reproductive isolation between closely related species. Using lines of Drosophila pseudoobscura experimentally selected under different sexual selection regimes, we tested whether increased promiscuity and enforced monogamy led to evolutionary changes in courtship song elements. In D. pseudoobscura, males produce both a low and high rate repetition song. We found that both song types diverged after selection and that the direction of changes was consistent with ordered hypotheses of predicted directions of change under the different mating system structures. In particular, latency to the initiation of song and duration of the interpulse interval (IPI) decreased in highly promiscuous lines and increased in monogamous lines. These results suggest that courtship song may rapidly evolve under different mating system structures, representing either functional evolution of more stimulatory song under conditions of strong sexual selection, or might result from increased courtship vigor, or represent correlated evolution. Some speciation theory predicts that increased sexual selection should result in increased variance of traits between allopatric populations, facilitating reproductive isolation. We also found that courtship song elements were not equally variable between replicate lines.

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PubMed2005

Are solitary and gregarious Mormon crickets (Anabrus simplex, Orthoptera, Tettigoniidae) genetically distinct?

Phase polyphenisms are usually thought to reflect plastic responses of species, independent of genetic differences; however, phase differences could correlate with genetic differentiation for various reasons. Mormon crickets appear to occur in two phases that differ in morphology and behaviour. Solitary individuals are cryptic and sedentary whereas gregarious individuals form bands, migrate, and are aposematically coloured. These traits have been thought to be phenotypically plastic and induced by environmental conditions. However, there has been no previous investigation of the extent of genetic differences between solitary and gregarious populations of this widespread North American species. We sequenced two mitochondrial genes, COII and COIII, in samples of Mormon crickets from gregarious populations west of the continental divide and solitary mountain populations primarily east of the divide. Sequencing revealed two genetically distinct clades that broadly correspond with the solitary eastern populations and the mainly gregarious western populations. We used coalescent modelling to test the hypothesis that the species consists of two deep genetic clades, as opposed to a series of equally distinct populations. Results allowed us to reject the null hypothesis that a radiation independent of phase produced these clades, and molecular clock estimates indicate the time of divergence to be approximately 2 million years ago. This work establishes that the solitary populations found in the mountains on the eastern slope are part of a clade that is genetically distinct from the western populations, which are primarily gregarious, and the implications of this apparent correlation between phase and genetic differentiation are discussed.

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PubMed2005

Patterns of speciation in endemic Mexican Goodeid fish: sexual conflict or early radiation?

Currently there is much interest in the potential for sexual selection or conflict to drive speciation. Theory proposes that speciation will be accelerated where sexual conflict is strong, particularly if females are ahead because mate choice will accentuate divergence by limiting gene flow. The Goodeinae are a monophyletic group of endemic Mexican fishes with an origin at least as old as the Miocene. Sexual selection is important in the Goodeinae and there is substantial interspecific variability in body morphology, which influences mate choice, allowing inference of the importance of female mate choice. We therefore used this group to test the relationship between sexual dimorphism and speciation rate. We quantified interspecific variation in sexual dimorphism amongst 25 species using a multivariate measure of total morphological differentiation between the sexes that accurately reflects sexual dimorphism driven by female mate choice and also used a mtDNA-based phylogeny to examine speciation rates. Comparative analyses failed to support a significant association between sexual dimorphism and speciation rate. In addition, variation in the time course of speciation throughout the whole clade was also examined using a similar tree containing 34 extant species. A constant rates model for the growth of this clade was rejected, but analyses instead indicated a decline in the rate of speciation over time. These results support the hypothesis of an early expansion of the group, perhaps due to an early radiation influenced by the key innovation of live bearing, or the prevalence of Miocene volcanism. In general, support for the role of sexual selection in generating patterns of speciation is proving equivocal and we argue that vicariance biogeography and adaptive radiations remain the most likely determinants of major patterns of diversification of continental organisms.

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PubMed2002

Quantitative trait loci affecting a courtship signal in Drosophila melanogaster.

Courtship plays a major role in the sexual isolation of species, yet the genetics underlying courtship behaviour are poorly understood. Here we analyse quantitative trait loci (QTL) for a major component of courtship song in recombinant inbred lines derived from two laboratory strains of Drosophila melanogaster. The total variance among lines exceeds that between parental strains, and is broadly similar to that seen among geographic strains of the Cosmopolitan form of this species. Previous studies of the quantitative genetics of fly song have implied a polygenic additive inheritance with numerous genes spread throughout the genome. We find evidence for only three significant QTLs explaining 54% of the genetic variance in total. Thus there is evidence for a few large effect genes contributing to the genetic variance among lines. Interestingly, almost all of the candidate song genes previously described for D. melanogaster do not coincide with our QTLs.

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PubMed2001

Mitochondrial DNA variation and GIS analysis confirm a secondary origin of geographical variation in the bushcricket Ephippiger ephippiger (Orthoptera: Tettigonioidea), and resurrect two subspecies.

Geographic variation within species can originate through selection and drift in situ (primary variation) or from vicariant episodes (secondary variation). Most patterns of subspecific variation within European flora and fauna are thought to have secondary origins, reflecting isolation in refugia during Quaternary ice ages. The bushcricket Ephippiger ephippiger has an unusual pattern of geographical variability in morphology, behaviour and allozymes in southern France, which has been interpreted as reflecting recent primary origins rather than historical isolation. Re-analysis of this variation using Geographical Information Systems (GIS) suggests a possible zone of hybridization within a complex pattern of geographical variation. Here we produce a genetic distance matrix from restriction fragment length polymorphism (RFLP) bandsharing of an approximately 4.5 kb fragment of mitochondrial DNA (mtDNA), and compare this with predictions resulting from the GIS analysis. The mtDNA variation supports a postglacial origin of geographical variation. Partial Mantel test comparisons of genetic distances with matrices of geographical distance, relevant environmental characteristics and possible refugia show refugia to be the best predictors of genetic distance. There is no evidence to support isolation by distance. However, environmental contrasts do explain significant variation in genetic distance after allowing for the effect of refugial origin. Also, a neighbour-joining tree has a major division separating eastern and western forms. We conclude that the major source of variation within the species is historical isolation in glacial refugia, but that dispersal, hybridization and selection associated with environmental features has influenced patterns of mtDNA introgression. At least two valid subspecies can be defined.

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PubMed2001

Characterization of female preference functions for Drosophila montana courtship song and a test of the temperature coupling hypothesis.

Female mate preferences are a major cause of diversity and elaboration in male sexual traits. Here we characterize the shape of female preference functions for pulse length and carrier frequency of the courtship song of Drosophila montana by fitting both parametric and nonparametric functions to the incidence of female receptive gestures to synthetic song. Preference functions for both traits are strongly directional. That for pulse length is linear and favors short pulses, whereas that for carrier frequency is stabilizing in shape, but would exert directional preferences favoring males with high carrier frequency. The preference for carrier frequency has probably evolved under sexual selection, but reasons for the preference for short pulses are less apparent. We also examine the effect of ambient temperature on the carrier frequency of male song and on the preference function for carrier frequency. For many similar acoustic communication systems, temperature coupling, a compensatory effect of temperature on preference functions, is thought to maintain coordination between preferences and signals. However, although the carrier frequency of D. montana song is highly dependent on environmental temperature, there is no temperature coupling of the female preference function. We suggest that temperature coupling may often arise due to a common effect of temperature on song and preference, rather than be an advantageous characteristic whose function is to maintain coordination in temperature-affected communication systems.

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PubMed2000

The inheritance of female preference functions in a mate recognition system.

Mate recognition systems (MRSs) play a major role in sexual selection and speciation, yet few studies have analysed both male and female components in detail. Here, female preference functions have been characterized for the tettigoniid bushcricket Ephippiger ephippiger, and the inheritance of male song and female preference functions followed in crosses between subspecies. Songs are disproportionately determined by sex-linked genes. However, there is no evidence for a role of maternally derived sex-linked genes in female preference or of maternal effects. At the genetic level, there is a mismatch between peak preferences and male song, consistent with an evolutionary history of persistent directional preferences. Such a pattern of inheritance could contribute to the process of speciation via the evolution of new MRSs.

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PubMed1996

The shape of female mating preferences.

The "shape" of a female mating preference is the relationship between a male trait and the probability of acceptance as a mating partner. The shape of preferences is important in many models of sexual selection, mate recognition, communication, and speciation, yet it has rarely been measured precisely. Here I examine preference shape for male calling song in a bushcricket (katydid). Preferences change dramatically between races of a species, from strongly directional to broadly stabilizing (but with a net directional effect). Preference shape generally matches the distribution of the male trait. This is compatible with a coevolutionary model of signal-preference evolution, although it does not rule out an alternative model, sensory exploitation. Preference shapes are shown to be genetic in origin.

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PubMed1996

Polygenic control of a mating signal in Drosophila.

Recent findings show that traits which influence speciation in the melanogaster subgroup of Drosophila can be determined by relatively few loci and/ or be sex-linked. Using backcrosses between D. simulans and D. mauritiana, we show that an acoustic mating signal which plays an important role in courtship (the interpulse interval of the 'lovesong') has a polygenic determination. There is no evidence of single genes of large effect, a disproportionate contribution of the sex chromosome or epistasis. The probability of finding single genes with a large effect on sexual isolation may vary with the mode of signalling involved.

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PubMed1994

Genetic variability of the interpulse interval of courtship song among some European populations of Drosophila melanogaster.

The interpulse interval of the courtship song of Drosophila melanogaster is a character which may play a significant role in mating success and reproductive isolation. Here we examine the variability of interpulse interval among replicated laboratory strains of D. melanogaster. There is no significant variation among populations of different geographical origin. This suggests that interpulse interval is subject to strong selection, as the populations are known to differ for other characters. One population, however, was sufficiently different to allow a genetic analysis. Reciprocal F1s and backcrosses implied that the variance was predominantly additive and autosomal. Possible sources of selection on interpulse interval are discussed.

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PubMed1994

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.

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PubMed1993

Female song preference and the period gene in Drosophila.

Mutations at the period (per) locus in Drosophila melanogaster alter rhythmic components of the male courtship song. We have examined the mating speed of females homozygous for mutant per 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 system.

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PubMed1992

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 hybrid Ephippiger 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.

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