Biomedical subjects

P M Brakefield

Publications and source records attributed to P M Brakefield.

32 records · Page 2Linked to original sources

PubMed2001

Sequestration of host plant glucosinolates in the defensive hemolymph of the sawfly Athalia rosae.

Interactions between insects and glucosinolate-containing plant species have been investigated for a long time. Although the glucosinolate-myrosinase system is believed to act as a defense mechanism against generalist herbivores and fungi, several specialist insects use these secondary metabolites for host plant finding and acceptance and can handle them physiologically. However, sequestration of glucosinolates in specialist herbivores has been less well studied. Larvae of the tumip sawfly Athalia rosae feed on several glucosinolate-containing plant species. When larvae are disturbed by antagonists, they release one or more small droplets of hemolymph from their integument. This "reflex bleeding" is used as a defense mechanism. Specific glucosinolate analysis, by conversion to desulfoglucosinolates and analysis of these by high-performance liquid chromatography coupled to diode array UV spectroscopy and mass spectrometry, revealed that larvae incorporate and concentrate the plant's characteristic glucosinolates from their hosts. Extracts of larvae that were reared on Sinapis alba contained sinalbin, even when the larvae were first starved for 22 hr and, thus, had empty guts. Hemolymph was analyzed from larvae that were reared on either S. alba, Brassica nigra, or Barbarea stricta. Leaves were analyzed from the same plants the larvae had fed on. Sinalbin (from S. alba), sinigrin (B. nigra), or glucobarbarin and glucobrassicin (B. stricta) were present in leaves in concentrations less than 1 micromol/g fresh weight, while the same glucosinolates could be detected in the larvae's hemolymph in concentrations between 10 and 31 micromol/g fresh weight, except that glucobrassicin was present only as a trace. In larval feces, only trace amounts of glucosinolates (sinalbin and sinigrin) could be detected. The glucosinolates were likewise found in freshly emerged adults, showing that the sequestered phytochemicals were transferred through the pupal stage.

Adaptation, Physiological↗

PubMed2001

Lack of response to artificial selection on the slope of reaction norms for seasonal polyphenism in the butterfly Bicyclus anynana.

The tropical butterfly Bicyclus anynana shows adaptive phenotypic plasticity in response to wet-dry seasonality. The wet season form (WSF) has a conspicuous wing pattern with large eyespots, whereas the dry season form (DSF) lacks eyespots and therefore has a more cryptic appearance. Temperature is the main factor controlling this difference: rearing larvae at a low (<19 degrees C) temperature in the laboratory results in the DSF, whereas rearing at a high (>23 degrees C) temperature induces the WSF. We applied truncation selection in opposite directions in successive generations reared at two alternating temperatures (18.5 degrees C and 23.5 degrees C) to increase (for two High Plasticity (HP) lines), and decrease (for two Low Plasticity (LP) lines) wing pattern plasticity. Plasticity was assessed by partitioning full-sib families over four rearing temperatures (18.5 degrees C, 20.5 degrees C, 21.5 degrees C and 23.5 degrees C). Several wing pattern elements were measured for which the first principal component (PC1) provides a useful summary. The slopes of reaction norms for PC1 were significantly steeper in the HP lines than in the LP lines; however, the selection lines did not always differ significantly from the unselected stock. The results of crosses between the replicates of the selection lines gave no indication for effects of inbreeding. We argue that high, positive genetic correlations across temperatures retard a response to selection in opposite directions in different environments. This is discussed with respect to potential evolutionary constraints in natural populations in these butterflies.

Adaptation, Biological↗

PubMed2000

Evolutionary dynamics of declining melanism in the peppered moth in the Netherlands.

Populations of Biston betularia in the region of The Netherlands around Leiden and Rotterdam were resampled. A comparison of three sets of data for 1969-1973, 1988 and 1999 enabled a further examination of declines in melanism. Unlike parallel changes for the black carbonaria form of this species in urban regions of Britain, those inThe Netherlands involve substantial changes in frequencies of at least two of the intermediate insularia morphs as well as an increase in the non-melanic typica morph. The darkest of the three insularia morphs has shown a transitory pulse of increased frequency in The Netherlands. The dynamics are discussed in relation to the history of air pollution and to straightforward predictions about selection.

Adaptation, Physiological↗

PubMed2000

Butterfly wing pattern mutants: developmental heterochrony and co-ordinately regulated phenotypes.

Butterfly wings are colored late in development, when pigments are synthesized in specialized wing scale cells in a fixed developmental succession. In this succession, colored pigments are deposited first and the remaining areas are later melanized black or brown. Here we studied the developmental changes underlying two wing pattern mutants, firstly melanic mutants of the swallowtail Papilio glaucus, in which the yellow background is turned black, and secondly a Spotty mutant of the satyrid Bicyclus anynana, which carries two additional eyespots. Despite the very different pattern changes in these two mutants, they are both associated with changes in rates of scale development and correspondingly, the final color pattern. In the melanic swallowtail, background scales originally destined to become yellow (normally developing early and synthesizing papiliochrome) show delayed development, fail to make papiliochrome, and subsequently melanize at the same time as scales in the wild-type black pattern. In the B. anynana eyespot, scale maturation begins with the central white focus, then progresses to the surrounding gold ring and later finishes with melanization of the black center. Mutants showing additional eyespots display accelerated rates of scale development (corresponding to new eyespots) in wing cells not normally occupied by eyespots. Thus by either delaying or accelerating rates of scale development, the final color, or position, of a wing pattern element can be changed. We propose that this heterochrony of scale development is a basic mechanism of color pattern formation on which developmental mutants act to change lepidopteran color patterns.

Animals↗

PubMed2000

The genetic basis of eyespot size in the butterfly Bicyclus anynana: an analysis of line crosses.

The tropical butterfly Bicyclus anynana shows wide variation in the size of the eyespots on the ventral side of its wings. Differences in the environmental temperature during late larval and early pupal stages are a major source of this variation, but variation also exists within temperatures. Using lines selected at a single temperature for large and small eyespots, and a number of crosses derived from these lines, we studied the genetic basis of eyespot size in B. anynana. We applied Lande's modification of the Castle-Wright (C-W) estimator to estimate the minimum number of genes contributing to the difference between the two lines. Estimates indicated that at least five genes are involved. As the C-W estimator is based on a number of simplifying assumptions, we tested to what extent one of these assumptions (additive gene action) is actually met. Joint-scaling tests indicated that the assumption of additivity is not satisfied and that dominance and probably epistasis play a role. Because reciprocal crosses were available we looked for evidence for sex-linkage and cytoplasmic effects. No evidence for cytoplasmic effects was found, but the data were consistent with the presence of one or more loci on the X-chromosome. The results are discussed in the context of the current model of eyespot formation.

Animals↗

PubMed2000

DNA in situ hybridization on polytene chromosomes of Simulium sanctipauli at loci relevant to insecticide resistance.

A DNA technique for in situ hybridization developed by Kumar & Collins (1994) for use on polytene chromosomes of adult Anopheles mosquitoes (Diptera: Culicidae) was modified for use with Simulium larval salivary gland chromosomes (Diptera: Simuliidae). Cloned fragments of several Simulium genes (coding for aspartate amino transferase, cytochrome P450 and DNA polymerase) were successfully mapped physically by assigning specific band locations in Simulim sanctipauli V. & D. This represents the first attempt at locating genes beyond the resolution of linkage to inversions in any blackfly species.

Animals↗

PubMed2000

Inbreeding depression and genetic load in laboratory metapopulations of the butterfly Bicyclus anynana.

We investigated the effects of inbreeding on various fitness components and their genetic load in laboratory metapopulations of the butterfly Bicyclus anynana. Six metapopulations each consisted of four subpopulations with breeding population sizes of N = 6 or N = 12 and migration rate of m = 0 or m = 0.33. Metapopulations were maintained for seven generations during which coancestries and pedigrees were established. Individual inbreeding coefficients at the F7 were calculated and ranged between 0.01 and 0.51. Even though considerable purging had occurred during inbreeding, the genetic load remained higher than that of many outbreeding species: approximately two lethal equivalents were detected for egg sterility, one for zygote survival, one for juvenile survival, and one for longevity. Severe inbreeding depression occurred after seven generations of inbreeding, which jeopardized the metapopulation survival. This finding suggests that the purging of genetic load by intentional inbreeding cannot be recommended for the genetic conservation of species with a high number of lethal equivalents.

Animals↗

PubMed2000

Evidence of multiple mating and hybridization in Simulium damnosum s.l. (Diptera: Simuliidae) in nature.

Beakers with aeration were used to rear individual (single) egg batches of Simulium damnosum (Theobald) s.l. to larvae and adults. Chromosomal analysis of the progeny of individual females indicated that multiple mating occurred within the S. damnosum complex and that hybridization occurred between S. damnosum s.s. and S. sirbanum (Vajime & Dunbar) in nature. These results indicate that possibly more extensive hybridization may be taking place in some areas of the Onchocerciasis Control Program than previously reported. Dramatic changes in the rates of hybridization among members of the S. damnosum complex possibly may lead to the formation of new cytotypes.

Animals↗

PubMed1999

Inbreeding of bottlenecked butterfly populations. Estimation using the likelihood of changes in marker allele frequencies.

Polymorphic enzyme and minisatellite loci were used to estimate the degree of inbreeding in experimentally bottlenecked populations of the butterfly, Bicyclus anynana (Satyridae), three generations after founding events of 2, 6, 20, or 300 individuals, each bottleneck size being replicated at least four times. Heterozygosity fell more than expected, though not significantly so, but this traditional measure of the degree of inbreeding did not make full use of the information from genetic markers. It proved more informative to estimate directly the probability distribution of a measure of inbreeding, sigma2, the variance in the number of descendants left per gene. In all bottlenecked lines, sigma2 was significantly larger than in control lines (300 founders). We demonstrate that this excess inbreeding was brought about both by an increase in the variance of reproductive success of individuals, but also by another process. We argue that in bottlenecked lines linkage disequilibrium generated by the small number of haplotypes passing through the bottleneck resulted in hitchhiking of particular marker alleles with those haplotypes favored by selection. In control lines, linkage disequilibrium was minimal. Our result, indicating more inbreeding than expected from demographic parameters, contrasts with the findings of previous (Drosophila) experiments in which the decline in observed heterozygosity was slower than expected and attributed to associative overdominance. The different outcomes may both be explained as a consequence of linkage disequilibrium under different regimes of inbreeding. The likelihood-based method to estimate inbreeding should be of wide applicability. It was, for example, able to resolve small differences in sigma2 among replicate lines within bottleneck-size treatments, which could be related to the observed variation in reproductive viability.

Alleles↗

PubMed1997

The genetics and development of an eyespot pattern in the butterfly Bicyclus anynana: response to selection for eyespot shape.

The normally circular eyespots on the wing of the butterfly Bicyclus anynana were selected to become elliptical in two divergent lines, with antero-posterior elongation of the eyespots in one line and proximodistal elongation in the other. Selection was continued for nine generations, and symmetrical realized heritabilities of approximately 15% were achieved initially. The elliptical eyespot shapes characteristic of each line were still produced when the signaling center of the eyespot (the focus) was surgically rotated by 90 or 180 degrees or when an eyespot was induced ectopically by localized damage. We conclude that selection changed general properties of the epidermis that responds to signals emanating from the eyespot focus but did not affect the mechanism of focal signaling.

Animals↗

PubMed1996

Development, plasticity and evolution of butterfly eyespot patterns.

The developmental and genetic bases for the formation, plasticity and diversity of eyespot patterns in butterflies are examined. Eyespot pattern mutants, regulatory gene expression, and transplants of the eyespot developmental organizer demonstrate that eyespot position, number, size and colour are determined progressively in a developmental pathway largely uncoupled from those regulating other wing-pattern elements and body structures. Species comparisons and selection experiments suggest that the evolution of eyespot patterns can occur rapidly through modulation of different stages of this pathway, and requires only single, or very few, changes in regulatory genes.

Adaptation, Biological↗

PubMed1995

Eyespot development on butterfly wings: the epidermal response to damage.

Eyespot colour patterns decorate the wings of many butterfly species. The eyespot is specified in the early pupal epidermis by signals from a central "focus," and it has been suggested that the focus is the source of a diffusible morphogen gradient. We show that ectopic eyespots can be induced in nonfocal positions throughout the distal, but not the proximal, wing epidermis of Bicyclus anynana by mild epidermal damage inflicted at 12-18 hr (into a 6- to 7-day pupal period). Damage may lower, locally and transiently, the threshold for response to morphogen. Here, we have tested two predictions of the gradient model. As predicted, mild damage close to the focus (parafocal) locally extends the eyespot, making it markedly asymmetrical, even after early operations (1 or 6 hr), when remote epidermis is non-responsive. Early parafocal operations also have an unexpected result, however, reducing the extent of the eyespot in other directions, perhaps through a long-range "wound effect" on the signaling activity of the focal cells. The model also correctly predicts that increasing the severity of a nonfocal operation will prolong the transient damage and hence give ectopic eyespots after early (1- or 6-hr) operations. We do not, however, find the expected size increase in ectopics induced by the later (12 or 18 hr) severe damage. Similarly, we demonstrate the predicted effect of early (1 or 6 hr) mild damage in increasing the response to a subsequent (18 hr) operation at the same position. The gradient model is therefore supported by most aspects of eyespot induction in response to epidermal damage.

Animals↗

PubMed1995

Eyespot development on butterfly wings: the focal signal.

The eyespot colour pattern on butterfly wings is specified in the early pupal epidermis by signals from a central "focus." In Bicyclus anynana we show that a small square of focal epidermis, grafted to a range of distal wing sites, induces eyespot formation in surrounding host tissue. Signaling is limited to the focus, and even an adjacent (parafocal) graft does not maintain its normal fate (of contributing to the eyespot) and does not influence its surroundings. Along the wing, there is an abrupt change in the epidermis, as a focus grafted to a proximal site provokes no host response. The results of several grafting experiments demonstrate that their different response properties are autonomous to small areas of the distal and proximal epidermis and that the nonresponding proximal tissue can nonetheless transmit the focal sign. The Bicyclus dorsal forewing has a small anterior and a large posterior eyespot, and we show that this results mainly from a difference in focal signals, not in the epidermal response. A grafted posterior focus induces a large eyespot, whereas an anterior focus induces a small eyespot. Furthermore, the anterior and posterior eyespots differ in proportions, and this difference also depends on the identity of the focus, not on the responding epidermis. Eyespots are specified over many cell diameters from the focus by a mechanism which could consist of one long-range signal, such as a morphogen gradient or of a cascade of short-range interactions initiated by the focus. Focal control of the difference in size and proportion between the anterior and posterior eyespot is more readily compatible with a gradient mechanism. Neither model, however, readily explains why the pattern induced by a grafted focus is smaller, but its peripheral gold annulus is broader than in the corresponding control eyespot. Also, there is no direct evidence for long-range gradients, in the butterfly wing or any other insect epithelium.

Animals↗

PubMedDate not supplied

Correlations between scale structure and pigmentation in butterfly wings.

We examined the correlation between color and structure of wing scales in the nymphalid butterflies Bicyclus anynana and Heliconius melpomene. All scales in B. anynana are rather similar in comparison to the clear structural differences of differently pigmented scales in H. melpomene. Where scale structural differences in H. melpomene are qualitative, they seem to be quantitative in B. anynana. There is a "gradient" in the density of some structural elements, the cross ribs, in the scales of B. anynana: black, gold, and brown scales show progressively lower cross rib density within an individual. There is, however, high individual variation in the absolute cross rib densities (i.e., scales with a particular color and cross rib density in one individual may have a different color but similar density in another individual). By ectopically inducing color pattern during early pupal development, we examined whether a scale's color and its microstructure could be uncoupled. The effect of these manipulations appears to be different in B. anynana and H. melpomene. In Bicyclus, "black" scales induced by wing damage at an ectopic location normally containing brown scales acquire both an intermediate structure and color between that of brown and normal black scales. In Heliconius, however, intermediate colors or scale structure were never observed, and scales with an altered color (due to damage) always have the same structure as normal scales with that color. The results are discussed on the basis of gene expression patterns, variability in rates of scale development and pigment, and scale sclerotization pathways.

Analysis of Variance↗