Mimetic gain in batesian and Müllerian mimicry

Summary Starting from field investigations and experiments on mimetic butterfly populations a model for two mimetic species is developed. The model comprises various features such as the growth rates and carrying capacities of the two species, their unpalatability to predators, the recruitment and the training of the predators and, most important, the similarity of the two mimetic species. The model ranges from pure Batesian to pure Müllerian mimicry over a spectrum of possible cases. The mimetic gain is introduced as the relative increase in equilibrium density in a mimetic situation as compared to a situation where mimicry is not present. The dependence of this quantity on parameters as growth rate, carrying capacity, unpalatability, and similarity is investigated using numerical methods.     

Spatial mosaic and interfacial dynamics in a Müllerian mimicry system

Uncovering why spatial mosaics of mimetic morphs are maintained in a Müllerian mimicry system has been a challenging issue in evolutionary biology. In this article, we analyze the reaction diffusion system that describes two-species Müllerian mimicry in one- and two-dimensional habitats. Due to positive frequency-dependent selection, a local population first approaches the state where one of the comimicking patterns predominates, which is followed by slow movement of boundaries where different patterns meet. We then analyze the interfacial dynamics of the boundaries to find whether a stable cline is maintained and to obtain the wave speed if the cline is unstable. The results are: (1) In a spatially uniform habitat the morph with greater base fitness spreads both in one and two species system. (2) The strength of cross-species interaction determines whether the mimetic morph clines of model and mimic species coalesce into the same geographical region or pass through each other. The joint wave speed of clines decreases by increasing the number of comimicking species in the mimicry ring. (3) In spatial heterogeneous habitats, stable clines can be maintained due to the balance between the base fitness gradient and the biased gene flow by negative curvature of boundary. This allows the persistence of a spatial mosaic even if one of the morphs is in every place advantageous over the other. A balanced cline is also maintained if there is a gradient in the population density. (4) A new advantageous morph occurring at a local region is doomed to go to extinction in a finite time if the "radius" of initial distribution is below a threshold. Possible applications to the heliconiine butterfly mimicry ring, heterozygous disadvantage systems of chromosomal rearrangement and hybrid zone, the third phase of Wright's Shifting Balance theory, and cytoplasmic incompatibility are discussed.     

Müllerian mimicry in aposematic spiny plants

Müllerian mimicry is common in aposematic animals but till recently, like other aspects of plant aposematism was almost unknown. Many thorny, spiny and prickly plants are considered aposematic because their sharp defensive structures are colorful and conspicuous. Many of these spiny plant species (e.g., cacti and Agave in North American deserts; Aloe, Euphorbia and acacias with white thorns in Africa; spiny plants in Ohio; and spiny members of the Asteraceae in the Mediterranean basin) have overlapping territories, and also similar patterns of conspicuous coloration, and suffer from the evolutionary pressure of grazing by the same large herbivores. I propose that many of these species form Müllerian mimicry rings.Key words: aposematic coloration, defense, evolution, herbivory, müllerian mimicry, spines, thornsAposematic (warning) coloration is a biological phenomenon in which poisonous, dangerous or otherwise unpalatable organisms visually advertise these qualities to other animals. The evolution of aposematic coloration is based on the ability of target enemies to associate the visual signal with the risk, damage or non-profitable handling, and later to avoid such organisms as prey. Typical colors of aposematic animals are yellow, orange, red, purple, black, white or brown and combinations of these.^1–5 Many thorny, spiny and prickly plant species were proposed to be aposematic because their sharp defensive structures are usually colorful (yellow, orange, red, brown, black, white) and/or associated with similar conspicuous coloration.^5–22 Animal spines also have similar conspicuous coloration and were proposed to be aposematic.^1,5,17,23Several authors have proposed that mimicry of various types helps in plant defense, e.g.,^9,24–34 More specifically, Müllerian mimicry was already proposed to exist in several defensive plant signaling systems. The first was for several spiny species with white-variegated leaves.^8,10 The second was for some tree species with red or yellow poisonous autumn leaves.³⁵ The third cases are of a mixture of Müllerian and Batesian mimicry, of thorn auto-mimicry found in many Agave species.⁸Here I propose that many species of visually aposematic spiny plants of the following taxa: (1) Cactaceae, (2) the genus Agave, (3) the genus Aloe, (4) African thorny members of the genus Euphorbia, (5) African acacias with white thorns, (6) spiny vascular plants of southeastern Ohio, (7) spiny Near Eastern plants with white variegation on their leaves, (8) Near Eastern members of the Asteraceae with yellow spines, form Müllerian mimicry rings of spiny plants.To consider the existence of Müllerian mimicry rings in aposematic organisms, two factors are needed: (1) a similar signal, and (2) an overlapping distribution in respect to the territory of predators in animals, or herbivores in plants. I will show below that for the plant taxa proposed here to form Müllerian mimicry rings, both criteria operate.The accumulating data about the common association of plant defenses by spines with visual conspicuousness, along with the fact that many such species overlap in their habitat, raises the possibility of the broad phenomenon of existence of Müllerian mimicry rings in plants. Even from the limited number of publications proposing visual aposematism in spiny plants, the operation of vegetal Müllerian mimicry rings seems to be obvious. The phenomenon can now be traced to both the Old World (Asia, Africa and Europe) and the New World (North America). The best-studied cases include Cactaceae and the genera Agave, Aloe and Euphorbia,⁶ African acacias with white thorns,^12,15 Near Eastern spiny plants with white variegation on their leaves,^7,11 aposematic spiny vascular plants of southeastern Ohio,¹⁶ and many spiny Mediterranean species of the Asteraceae with yellow spines.²²In the four spiny taxa (Cactaceae and the genera Agave, Aloe and Euphorbia) that were the first to be proposed as visually aposematic⁶ there is a very strong morphological similarity. In cacti, there are two types of conspicuousness of spines that are typical of many plant species: (1) colorful spines, and (2) white spots, or white or colorful stripes, associated with spines on the stems. These two types of aposematic coloration also dominate the spine system of Agave, Aloe and Euphorbia. The fact that many species of three of these four spiny taxa (Agave, Aloe and Euphorbia) are also poisonous^36–38 further indicates their potential to form Müllerian mimicry rings.I propose that each of these groups for itself and some of these groups (e.g., Cactaceae and the genus Agave in North America; Aloe, Euphorbia and acacias in east and south Africa) that have overlapping distribution and share at least some of the herbivores, form Müllerian mimicry rings.The first Müllerian mimicry ring is of cacti and Agave that have an overlapping distribution over large areas in North America.^37,39 The large herbivores in North America disappeared not so long ago in evolutionary time scales and seem to have shaped the spiny defense of these plant taxa.⁴⁰The second Müllerian mimicry ring is of the spiny and thorny members of the African genera Aloe, Euphorbia and certain acacias with very conspicuous white thorns, which partly overlap in distribution and share various large mammalian herbivores.^12,15,36,41The third Müllerian mimicry ring is the outcome of the common presence of aposematic coloration in spiny vascular plants of southeastern Ohio,¹⁶ with color patterns in thorns and spines similar to those of Cactaceae and the genera Agave, Aloe and Euphorbia described in Lev-Yadun.⁶The next case of potential operation of Müllerian mimicry ring of spiny plants with overlapping territories that suffer from the same large herbivores, but on a much smaller geographical scale, has recently been proposed for several spiny species with white-variegated leaves,⁷ and later for more than 20 spiny species in the flora of Israel that have white markings associated with their spines.¹¹The last case of a probable Müllerian mimicry ring was described by Ronel et al.²² who while studying the spine system of Near Eastern spiny members of the Asteraceae, found 29 spiny species with yellow spines, and additional such species are expected to occur. Since some of these species and others with yellow spines also grow in southern Europe, it is clear that the same phenomenon is also common there.I conclude that Müllerian mimicry rings seem to be very common in plants, and that it is probable that many other spiny plants that form Müllerian mimicry rings are waiting to be studied. Such defensive rings are probably also formed by poisonous plants that share similar colors or odors.     

Conservative coevolution of Müllerian mimicry in a group of rift lake catfish

Biological mimicry has long been viewed as a powerful example of natural selection's ability to drive phenotypic evolution, although continuing debates surround the mechanisms leading to its development and the nature of these mimetic relationships. Müllerian mimicry, in which unpalatable species derive a mutual selective benefit through evolved phenotypic similarity, has alternatively been proposed to evolve through either a two-step process initiated by a large mutational change, or through continuous gradual evolution toward a common aposematic phenotype. I exposed a model predatory fish species to two species of endemic Lake Tanganyikan Synodontis to provide evidence for aposematism and the presence of Müllerian mimicry in these species. Predators quickly became conditioned to avoid the venomous catfish and did not discriminate between the two species when they were switched, supporting a hypothesis of functional Müllerian mimicry in this group of similarly colored fish. Ancestral state reconstructions and statistical comparisons of color pattern divergence in Tanganyikan Synodontis indicate that Müllerian mimicry in these catfish has developed through diversification of an aposematic common ancestor with subsequent conservative mutualistic coevolution among its daughter lineages, rather than advergent evolution of a mimic toward a nonrelated model, as assumed by widely accepted models of Müllerian mimicry evolution.     

Batesian, quasi-Batesian or Müllerian mimicry? Theory and data in mimicry Research

In this paper I argue that the nature of mimetic relationships remains contentious because there are insufficient data to enable full evaluation of theoretical models. There is, however, a growing appreciation of the need to draw together empirical studies to provide foundations for theoretical work. I review some recent data that considers the responses of predators to changing numbers of defended prey items and the nature of mimicry along a palatability spectrum. A simple model of predator behaviour is constructed which combines assumptions from Pavlovian learning studies with traditional ‘number dependent’ learning models. This model has two important properties. First it shows that Pavlovian assumptions can be represented in a simple model which generates interesting predictions. Second it indicates some areas that still need detailed empirical study – most importantly perhaps is the way that predators respond to prey with different levels of edibility. This revised version was published online in July 2006 with corrections to the Cover Date.     

Convergent evolution in the genetic basis of Müllerian mimicry in heliconius butterflies

The neotropical butterflies Heliconius melpomene and H. erato are Müllerian mimics that display the same warningly colored wing patterns in local populations, yet pattern diversity between geographic regions. Linkage mapping has previously shown convergent red wing phenotypes in these species are controlled by loci on homologous chromosomes. Here, AFLP bulk segregant analysis using H. melpomene crosses identified genetic markers tightly linked to two red wing-patterning loci. These markers were used to screen a H. melpomene BAC library and a tile path was assembled spanning one locus completely and part of the second. Concurrently, a similar strategy was used to identify a BAC clone tightly linked to the locus controlling the mimetic red wing phenotypes in H. erato. A methionine rich storage protein (MRSP) gene was identified within this BAC clone, and comparative genetic mapping shows red wing color loci are in homologous regions of the genome of H. erato and H. melpomene. Subtle differences in these convergent phenotypes imply they evolved independently using somewhat different developmental routes, but are nonetheless regulated by the same switch locus. Genetic mapping of MRSP in a third related species, the “tiger” patterned H. numata, has no association with wing patterning and shows no evidence for genomic translocation of wing-patterning loci.     

Investigating Müllerian mimicry: predator learning and variation in prey defences

Inexperienced predators are assumed to select for similarity of warning signals in aposematic species (Müllerian mimicry) when learning to avoid them. Recent theoretical work predicts that if co-mimic species have unequal defences, predators attack them according to their average unpalatability and mimicry may not be beneficial for the better defended co-mimic. In this study, we tested in a laboratory environment whether a uniform warning signal is superior to a variable one in promoting predator learning, and simultaneously whether co-mimics are preyed upon according to their average unpalatability. There was an interaction of signal variation and unpalatability but inexperienced birds did not select for signal similarity in artificial prey; when the prey was moderately defended a variable signal was even learnt faster than a uniform one. Due to slow avoidance learning, moderately defended prey had higher mortality than highly defended prey (although this was not straightforward), but mixing high and moderate unpalatability did not increase predation compared with high unpalatability. This does not support the view that predators are sensitive to varying unpalatability. The results suggest that inexperienced predators may neither strongly select for accurate Müllerian mimicry nor affect the benefits of mimicry when the co-mimics are unequally defended.     

Müllerian mimicry among bees and wasps: a review of current knowledge and future avenues of research

Many bees and stinging wasps, or aculeates, exhibit striking colour patterns or conspicuous coloration, such as black and yellow stripes. Such coloration is often interpreted as an aposematic signal advertising aculeate defences: the venomous sting. Aposematism can lead to Müllerian mimicry, the convergence of signals among different species unpalatable to predators. Müllerian mimicry has been extensively studied, notably on Neotropical butterflies and poison frogs. However, although a very high number of aculeate species harbour putative aposematic signals, aculeates are under-represented in mimicry studies. Here, we review the literature on mimicry rings that include bee and stinging wasp species. We report over a hundred described mimicry rings, involving a thousand species that belong to 19 aculeate families. These mimicry rings are found all throughout the world. Most importantly, we identify remaining knowledge gaps and unanswered questions related to the study of Müllerian mimicry in aculeates. Some of these questions are specific to aculeate models, such as the impact of sociality and of sexual dimorphism in defence levels on mimicry dynamics. Our review shows that aculeates may be one of the most diverse groups of organisms engaging in Müllerian mimicry and that the diversity of aculeate Müllerian mimetic interactions is currently under-explored. Thus, aculeates represent a new and major model system to study the evolution of Müllerian mimicry. Finally, aculeates are important pollinators and the global decline of pollinating insects raises considerable concern. In this context, a better understanding of the impact of Müllerian mimicry on aculeate communities may help design strategies for pollinator conservation, thereby providing future directions for evolutionary research.     

Localization of Müllerian mimicry genes on a dense linkage map of Heliconius erato

We report a dense genetic linkage map of Heliconius erato, a neotropical butterfly that has undergone a remarkable adaptive radiation in warningly colored mimetic wing patterns. Our study exploited natural variation segregating in a cross between H. erato etylus and H. himera to localize wing color pattern loci on a dense linkage map containing amplified fragment length polymorphisms (AFLP), microsatellites, and single-copy nuclear loci. We unambiguously identified all 20 autosomal linkage groups and the sex chromosome (Z). The map spanned a total of 1430 Haldane cM and linkage groups varied in size from 26.3 to 97.8 cM. The average distance between markers was 5.1 cM. Within this framework, we localized two major color pattern loci to narrow regions of the genome. The first gene, D, responsible for red/orange elements, had a most likely placement in a 6.7-cM region flanked by two AFLP markers on the end of a large 87.5-cM linkage group. The second locus, Sd, affects the melanic pattern on the forewing and was found within a 6.3-cM interval between flanking AFLP loci. This study complements recent linkage analysis of H. erato's comimic, H. melpomene, and forms the basis for marker-assisted physical mapping and for studies into the comparative genetic architecture of wing-pattern mimicry in Heliconius.     

The wave speed of intergradation zone in two-species lattice Müllerian mimicry model

A spatially explicit model is studied to analyse the movement of coupled clines in two-species Müllerian mimicry system as exemplified by the comimicking helicoiine butterflies in Central-South America Heliconius erato and Heliconius melpomene. In this system, a pair of comimicking wing patterns of two species (mimicry ring) is found in a geographical region but another pair of wing patterns is found in a different geographical region. The distribution of mimicry rings thus forms a spatial mosaic in a large geographical scale, and the mechanism responsible for their stable maintenance has been a long-standing question in evolutionary biology. We here examine the speed of the movement of boundaries that divide the regions inhabited by different mimetic morphs in each comimicking species, by assuming coupled two-state stochastic cellular automatons where the flipping rate of the site occupied by a mimetic morph depends on the local density of the same morph and of the comimicking morph in the other species. The speed of cline movement shows a complex dependence on the coupling parameter between mimetic species--greater coupling of comimicking morphs between species slows down the cline movement only when the reduction in predation rate exhibits diminishing return to the increase of local mimetic morph density. The analytical predictions are confirmed by the results of Monte Carlo simulations. The speed of advance is quite different from that predicted from the conventional reaction-diffusion model, indicating that demographic stochasticity plays a critical role in determining the speed of cline movement. We also examine if the spatial heterogeneity in migration rate can stably maintain clines.     

Spatial mosaic formation through frequency-dependent selection in Müllerian mimicry complexes

Although contemporary models of Müllerian mimicry have considered the movement of interfacial boundaries between two distinct mimetic forms, and even the possibility of polymorphisms in two patch systems, no model has considered how multiple forms of Müllerian mimics might evolve and be maintained over large geographical areas. A spatially explicit individual-based model for the evolution of Müllerian mimicry is presented, in which two unpalatable species are distributed over discrete cells within a regular lattice. Populations in each cell are capable of genetic drift and experience localized dispersal as well as frequency-dependent selection by predators. When each unpalatable prey species was introduced into a random cell and allowed to spread, then mimicry evolved throughout the system in the form of a spatial mosaic of phenotypes, separated by narrow "hybrid zones". The primary mechanism generating phenotypic diversity was the occasional establishment of new mutant forms in unoccupied cells and their subsequent maintenance (and spread) through frequency-dependent selection. The mean number of discrete clusters of the same morph that formed in the lattice was higher the higher the intensity of predation, and higher the lower the dispersal rate of unpalatable prey. Under certain conditions the hybrid zones moved, in a direction dependent on the curvature of their interfacial boundaries. However, the mimetic mosaics were highly stable when the intensity of predation was high and the rate of prey dispersal was low. Overall, this model highlights how a stable mosaic of different mimetic forms can evolve from a range of starting conditions through a combination of chance effects and localized frequency-dependent selection.     

Müllerian mimicry: an examination of Fisher's theory of gradual evolutionary change

In 1927, Fisher suggested that Müllerian mimicry evolution could be gradual and driven by predator generalization. A competing possibility is the so-called two-step hypothesis, entailing that Müllerian mimicry evolves through major mutational leaps of a less-protected species towards a better-protected, which sets the stage for coevolutionary fine-tuning of mimicry. At present, this hypothesis seems to be more widely accepted than Fisher's suggestion. We conducted individual-based simulations of communities with predators and two prey types to assess the possibility of Fisher's process leading to a common prey appearance. We found that Fisher's process worked for initially relatively similar appearances. Moreover, by introducing a predator spectrum consisting of several predator types with different ranges of generalization, we found that gradual evolution towards mimicry occurred also for large initial differences in prey appearance. We suggest that Fisher's process together with a predator spectrum is a realistic alternative to the two-step hypothesis and, furthermore, it has fewer problems with purifying selection. We also examined the factors influencing gradual evolution towards mimicry and found that not only the relative benefits from mimicry but also the mutational schemes of the prey types matter.     

The evolution of a Müllerian mimic in a spatially distributed community

Strong positive density-dependence should lead to a loss of diversity, but warning-colour and Müllerian mimicry systems show extraordinary levels of diversity. Here, we propose an analytical model to explore the dynamics of two forms of a Müllerian mimic in a heterogeneous environment with two alternative model species. Two connected populations of a dimorphic, chemically defended mimic are allowed to evolve and disperse. The proportions of the respective model species vary spatially. We use a nonlinear approximation of Müller's number-dependent equations to model a situation where the mortality for either form of the mimic decreases hyberbolically when its local density increases. A first non-spatial analysis confirms that the positive density-dependence makes coexistence of mimetic forms unstable in a single isolated patch, but shows that mimicry of the rarer model can be stable once established. The two-patch analysis shows that when models have different abundance in different places, local mimetic diversity in the mimic is maintained only if spatial heterogeneity is strong, or, more interestingly, if the mimic is not too strongly distasteful. Therefore, mildly toxic species can become polymorphic in a wider range of ecological settings. Spatial dynamics thus reveal a region of Müllerian polymorphism separating classical Batesian polymorphism and Müllerian monomorphism along the mimic's palatability spectrum. Such polymorphism-palatability relationship in a spatial environment provides a parsimonious hypothesis accounting for the observed Müllerian polymorphism that does not require quasi-Batesian dynamics. While the stability of coexistence depends on all factors, only the migration rate and strength of selection appear to affect the level of diversity at the polymorphic equilibrium. Local adaptation is predicted in most polymorphic cases. These results are in very good accordance with recent empirical findings on the polymorphic butterflies Heliconius numata and H. cydno.     

Molecular phylogenetic evidence for a mimetic radiation in Peruvian poison frogs supports a Müllerian mimicry hypothesis.

Examples of Müllerian mimicry, in which resemblance between unpalatable species confers mutual benefit, are rare in vertebrates. Strong comparative evidence for mimicry is found when the colour and pattern of a single species closely resemble several different model species simultaneously in different geographical regions. Todemonstrate this, it is necessary to provide compelling evidence that the putative mimics do, in fact, form a monophyletic group. We present molecular phylogenetic evidence that the poison frog Dendrobates imitator mimics three different poison frogs in different geographical regions in Peru. DNA sequences from four different mitochondrial gene regions in putative members of a single species are analysed using parsimony, maximum-likelihood and neighbour-joining methods. The resulting hypotheses of phylogenetic relationships demonstrate that the different populations of D.imitator form a monophyletic group. To our knowledge, these results provide the first evidence for a Müllerian mimetic radiation in amphibians in which a single species mimics different sympatric species in different geographical regions.     

Advergence in Müllerian mimicry: the case of the poison dart frogs of Northern Peru revisited

Whether the evolution of similar aposematic signals in different unpalatable species (i.e. Müllerian mimicry) is because of phenotypic convergence or advergence continues to puzzle scientists. The poison dart frog Ranitomeya imitator provides a rare example in support of the hypothesis of advergence: this species was believed to mimic numerous distinct model species because of high phenotypic variability and low genetic divergence among populations. In this study, we test the evidence in support of advergence using a population genetic framework in two localities where R. imitator is sympatric with different model species, Ranitomeya ventrimaculata and Ranitomeya variabilis. Genetic analyses revealed incomplete sorting of mitochondrial haplotypes between the two model species. These two species are also less genetically differentiated than R. imitator populations on the basis of both mitochondrial and nuclear DNA comparisons. The genetic similarity between the model species suggests that they have either diverged more recently than R. imitator populations or that they are still connected by gene flow and were misidentified as different species. An analysis of phenotypic variability indicates that the model species are as variable as R. imitator. These results do not support the hypothesis of advergence by R. imitator. Although we cannot rule out phenotypic advergence in the evolution of Müllerian mimicry, this study reopens the discussion regarding the direction of the evolution of mimicry in the R. imitator system.     

Testing Müllerian mimicry: an experiment with wild birds

Experiments with wild birds feeding on pastry 'prey' were performed to test competing theories of Müllerian mimicry Conventional theories predict that all resemblances between defended prey will be mutually advantageous and, hence, Müllerian. In contrast, unconventional theories predict that, if there are inequalities in defences between mimetic species, the less well-defended prey may dilute the protection of the better defended species in a quasi-Batesian manner. This unconventional prediction follows from an assumption that birds learn about the edibilities of prey using rules of Pavlovian learning. We report on two experiments, each lasting 40 days, which showed that a moderately defended prey can dilute the protection of a better defended mimic in a quasi-Batesian fashion, but can add protection to a mimic which has the same moderate levels of defence. These results match predictions of unconventional theories of mimicry and go some way to resolving the long-running arguments over the nature of Müllerian mimicry.     

Contacts between Wolffian and Müllerian cells at the tip of the outgrowing Müllerian duct in rat embryos

The developing Müllerian duct was studied at the light microscopic as well as the electron microscopic level in rat embryos, especially in the section of the terminal bud and its tip, where Wolffian and Müllerian duct are enclosed by a common basal membrane. In this zone desmosomes can be found among Wolffian cells and also among Müllerian cells. In addition, we found cell contacts between Müllerian and Wolffian cells, namely short electron-dense segments on adjacent surfaces or disc-shaped thickenings within opposite plasma membranes, as well as fusions of the plasmalemmata over short distances. Until now, these cell contacts have not been described in rat embryos.     

Variants of the anti-Müllerian hormone gene in a compound heterozygote with the persistent Müllerian duct syndrome and his family

Summary The human genome contains a large number of interspersed simple repeat sequences that are variable in length and can therefore serve as highly informative, polymorphic markers. Typing procedures include conventional multilocus and single locus probing, and polymerase chain reaction aided analysis. We have identified simple sequences in a cosmid clone stemming from the human Y chromosome and consisting of (gata)_n repeats. We have compared these with two equivalent simple repeat loci from chromosome 12. After amplifying the tandemly repeated motifs, we detected between four and eight different alleles at each of the three loci. Codominant inheritance of the alleles was established in family studies and the informativity of the simple repeat loci was determined by typing unrelated individuals. The polymorphisms are suitable for application in linkage studies, practical forensic case work, deficiency cases in paternity determination, and for studying ethnological questions. The mutational mechanisms that bring about changes in simple repeats located both on the autosomes and on the sex chromosomes, are discussed.Professor Dr. Otto Prokop (Humboldt-Universität Berlin) on the occasion of his 70th birthday     

Müllerian inhibiting substance production and testicular migration and descent in the pouch young of a marsupial

The ontogeny of Müllerian inhibiting substance (MIS) production by the developing testis of an Australian marsupial, the tammar wallaby (Macropus eugenii), was determined during pouch life using an organ-culture bioassay of mouse fetal urogenital ridge. This information was related to the morphological events during testicular migration and descent. MIS biological activity was found in testes (but not ovaries or liver) of pouch young from 2 to 85 days of age. MIS production had commenced by day 2, which is within a day of the first gross morphological signs of testicular differentiation. Müllerian duct regression occurred between 10 and 30 days, which partly coincided with testicular migration to the inguinal region and enlargement of the gubernacular bulb (15 to 30 days). These observations are consistent with the hypothesis that MIS may be involved in testicular transabdominal migration. The epididymis commenced development and growth only after the testis had descended through the inguinal ring. This provides no support for the suggestion that the epididymis is involved in testicular descent into the scrotum. The basic sequence of events in post-testicular sexual differentiation in the wallaby is sufficiently similar to that seen in eutherian mammals to make it an excellent experimental model for future studies of testicular differentiation, migration and descent.     

Tasting the difference: do multiple defence chemicals interact in Müllerian mimicry?

Müllerian mimicry, where two unpalatable species share a warning pattern, is classically believed to be a form of mutualism, where the species involved share the cost of predator education. The evolutionary dynamics of Müllerian mimicry have recently become a controversial subject, after mathematical models have shown that if minor alterations are made to assumptions about the way in which predators learn and forget about unpalatable prey, this textbook case of mutualism may not be mutualistic at all. An underlying assumption of these models is that Müllerian mimics possess the same defence chemical. However, some Müllerian mimics are known to possess different defence chemicals. Using domestic chicks as predators and coloured crumbs flavoured with either the same or different unpalatable chemicals as prey, we provide evidence that two defence chemicals can interact to enhance predator learning and memory. This indicates that Müllerian mimics that possess different defence chemicals are better protected than those that share a single defence chemical. These data provide insight into how multiple defence chemicals are perceived by birds,and how they influence the way birds learn and remember warningly coloured prey. They highlight the importance of considering how different toxins in mimicry rings can interact in the evolution and maintenance of Müllerian mimicry and could help to explain the remarkable variation in chemical defences found within and between species.