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T Tregenza

Publications and source records attributed to T Tregenza.

11 recordsLinked to original sources
PubMed2001

Dynamic mimicry in an Indo-Malayan octopus.

During research dives in Indonesia (Sulawesi and Bali), we filmed a distinctive long-armed octopus, which is new to science. Diving over 24 h periods revealed that the 'mimic octopus' emerges during daylight hours to forage on sand substrates in full view of pelagic fish predators. We observed nine individuals of this species displaying a repertoire of postures and body patterns, several of which are clearly impersonations of venomous animals co-occurring in this habitat. This 'dynamic mimicry' avoids the genetic constraints that may limit the diversity of genetically polymorphic mimics but has the same effect of decreasing the frequency with which predators encounter particular mimics. Additionally, our observations suggest that the octopus makes decisions about the most appropriate form of mimicry to use, allowing it to enhance further the benefits of mimicking toxic models by employing mimicry according to the nature of perceived threats.

Animals↗
PubMed2001

Sexual selection and speciation.

The power of sexual selection to drive changes in mate recognition traits gives it the potential to be a potent force in speciation. Much of the evidence to support this possibility comes from comparative studies that examine differences in the number of species between clades that apparently differ in the intensity of sexual selection. We argue that more detailed studies are needed, examining extinction rates and other sources of variation in species richness. Typically, investigations of extant natural populations have been too indirect to convincingly conclude speciation by sexual selection. Recent empirical work, however, is beginning to take a more direct approach and rule out confounding variables.

Journal Article↗
PubMed2000

Genetic compatibility, mate choice and patterns of parentage: invited review.

There is growing interest in the possibility that genetic compatibility may drive mate choice, including gamete choice, particularly from the perspective of understanding why females frequently mate with more than one male. Mate choice for compatibility differs from other forms of choice for genetic benefits (such as 'good genes') because individuals are expected to differ in their mate preferences, changing the evolutionary dynamics of sexual selection. Recent experiments designed to investigate genetic benefits of polyandry suggest that mate choice on the basis of genetic compatibility may be widespread. However, in most systems the mechanisms responsible for variation in compatibility are unknown. We review potential sources of variation in genetic compatibility and whether there is any evidence for mate choice driven by these factors. Selfish genetic elements appear to have the potential to drive mate compatibility mate choice, though as yet there is only one convincing example. There is abundant evidence for assortative mating between populations in hybrid zones, but very few examples where this is clearly a result of selection against mating with genetically less compatible individuals. There are also numerous cases of inbreeding avoidance, but little evidence that mate choice or differential fertilization success driven by genetic compatibility occurs between unrelated individuals. The exceptions to this are a handful of situations where both the alleles causing incompatibility and the alleles involved in mate choice are located in a chromosome region where recombination is suppressed. As yet there are only a few potential sources of genetic compatibility which have clearly been shown to drive mate choice. This may reflect limitations in the potential for the evolution of mate choice for genetic compatibility within populations, although the most promising sources of such incompatibilities have received relatively little research.

Animals↗
PubMed2000

Patterns of trait divergence between populations of the meadow grasshopper, Chorthippus parallelus.

To understand the process of speciation, we need to identify the evolutionary phenomena associated with divergence between populations of the same species. A powerful approach is to compare patterns of trait differences between populations differing in their evolutionary histories. A recent study of genetic divergence between populations of the meadow grasshopper Chorthippus parallelus, from different locations around Europe has allowed us to use this species to investigate which aspects of evolutionary history are associated with divergence in morphology and mating signals. During the last glaciation C. parallelus was confined to a number of refugia in southern Europe and has subsequently recolonized the northern part of the continent. This process of isolation followed by range expansion has created populations differing markedly in their evolutionary pasts--some have been isolated from one another for thousands of years, others have undergone repeated founder events, and others now live in sympatry with a closely related species. Using laboratory-reared grasshoppers from 12 different populations with a range of evolutionary histories, we quantify differences in morphology, chemical signals, and male calling-song. The observed pattern of divergence between these populations is then compared with the pattern predicted by hypotheses about what drives divergence. This comparison reveals that long periods in allopatry and processes associated with repeated founder events are both strongly associated with divergence.

Animals↗
PubMed2000

The origins of premating reproductive isolation: testing hypotheses in the grasshopper Chorthippus parallelus.

There are many proposed routes for the origin of premating reproductive isolation, but few systematic studies aimed at testing their relative importance. Accumulated information about the biogeographical history of the European meadow grasshopper, Chorthippus parallelus, has allowed us to make a planned series of comparisons among populations aimed at distinguishing the contributions of some of these hypotheses. We have compared the effects on assortative mating of long-term isolation in glacial refugia, founder events during postglacial colonization, and sympatry with a closely related species. A likelihood-based analysis allowed us to separate effects of variation in male and female mating propensity among populations from variation in mate choice leading to assortative mating. All three effects contributed significantly to the overall variation in mating pattern in a set of 21 pairwise comparisons among seven populations. Male cuticular composition, but not other candidate signals, was significantly associated with the level of assortative mating. Of the hypotheses for the origin of reproductive isolation, only the predictions of the founder hypothesis explained a significant amount of the variation in assortative mating. This does not rule out the possiblity that there may be some other explanation. Having established the pattern of divergence, it is possible to generate hypotheses that explain our results at least as well as the founder hypothesis. However, because many such post hoc hypotheses are possible, they cannot be tested with this dataset. On this basis, our results favor the hypothesis that some aspect of the colonization process tends to accelerate divergence in mating signals leading to premating reproductive isolation. This could be accomplished through any one of several mechanisms. Colonization involves many bottlenecks as new populations are established at the edge of the range by long-distance migrants. Genetic effects may be important, but these bottlenecks may also alter the conditions under which mates are found and chosen, as suggested by Kaneshiro. At the same time, the colonizing populations may encounter novel environmental challenges.

Animals↗
PubMed1998

Levels of genetic polymorphism: marker loci versus quantitative traits.

Species are the units used to measure ecological diversity and alleles are the units of genetic diversity. Genetic variation within and among species has been documented most extensively using allozyme electrophoresis. This reveals wide differences in genetic variability within, and genetic distances among, species, demonstrating that species are not equivalent units of diversity. The extent to which the pattern observed for allozymes can be used to infer patterns of genetic variation in quantitative traits depends on the forces generating and maintaining variability. Allozyme variation is probably not strictly neutral but, nevertheless, heterozygosity is expected to be influenced by population size and genetic distance will be affected by time since divergence. The same is true for quantitative traits influenced by many genes and under weak stabilizing selection. However, the limited data available suggest that allozyme variability is a poor predictor of genetic variation in quantitative traits within populations. It is a better predictor of general phenotypic divergence and of postzygotic isolation between populations or species, but is only weakly correlated with prezygotic isolation. Studies of grasshopper and planthopper mating signal variation and assortative mating illustrate how these characters evolve independently of general genetic and morphological variation. The role of such traits in prezygotic isolation, and hence speciation, means that they will contribute significantly to the diversity of levels of genetic variation within and among species.

Alleles↗
PubMed1997

Definitive evidence for cuticular pheromones in a cricket

The Orthoptera include many species established as important model systems in the study of animal behaviour, particularly in relation to communication and mating systems. Although most interest has focused on auditory communication, increasing circumstantial evidence suggests that there may be a widespread additional communication channel in the form of cuticular contact pheromones. Using the field cricket, Gryllus bimaculatuswe conducted a behavioural assay which demonstrated that males can distinguish the sex of conspecifics using such a channel. Male response to females (courtship song) was completely abolished by using an organic solvent to remove cuticular hydrocarbons and associated compounds from a stimulus female. It was subsequently restored by painting the washed female with the dissolved extract. This technique controls for the possibility, inherent in previous tests, that the lack of response to washed body parts might be due to the washing process itself. The composition of the cuticles of males and females was analysed using gas chromatography. This revealed that the two sexes differ markedly in the quantities of the majority of the compounds found in the cuticular extract that had previously been shown to be used in mate recognition. This suggests that mate recognition is likely to be due to the relative concentrations of several cuticular compounds, rather than a single 'sex pheromone'. It supports previous assertions of the existence of contact pheromones in the Orthoptera, suggesting that they may be widespread in this group.Copyright 1997 The Association for the Study of Animal Behaviour1997The Association for the Study of Animal Behaviour

Journal Article↗