Showing papers in "Heredity in 1979"
TL;DR: It is shown that a hybrid zone involving a single locus can move in response to a selective imbalance between the two homozygotes, and also to variation in population density and dispersal rate.
Abstract: This paper investigates the dynamic behaviour of hybrid zones which are maintained by a balance between dispersal and selection against hybrids. In the first section it is shown that a hybrid zone involving a single locus can move in response to a selective imbalance between the two homozygotes, and also to variation in population density and dispersal rate. It can be trapped by natural barriers, and so an allele which is selected against when rare cannot advance, even if it is advantageous when common. The continuous model used in deriving these results is shown to be a good approximation to the stepping-stone model, provided that the cline contains several demes. The effect of stochastic forces on multi-locus hybrid zones is then considered. An expression giving the shift in position after an arbitrary perturbation in gamete frequency is derived. Using this formula, it is found that sampling drift is negligible unless the zone includes few organisms and involves few loci. Random variations in population structure are the dominant force, and could allow considerable movement in an even environment. However, natural barriers can still trap hybrid zones, and so it is likely that they will remain roughly where they first formed.
479 citations
TL;DR: It is shown that a linked cline induces a step in the frequency of a neutral allele, which is proportional to the gradient in neutral allele frequency and to the inverse of the recombination rate.
Abstract: The effect of a cline as a barrier to gene flow at closely linked, weakly selected loci is investigated using a continuous diffusion model. It is shown that a linked cline induces a step in the frequency of a neutral allele, which is proportional to the gradient in neutral allele frequency and to the inverse of the recombination rate. A barrier to dispersal or a local region of low abundance has a similar effect (Nagylaki, 1976). The cline will block the flow of neutral alleles over a region of chromosome roughly 2√s/t map units long. However, a slightly advantageous allele will be little affected, and must be very tightly linked to be delayed for long.
329 citations
TL;DR: It was found that a strong positive correlation between the crossabilities of wheat varieties with H. bulbosum and with rye exists, reinforcing the conclusion that the crossability of wheat with both species is governed, at least in part, by the same genetic system.
Abstract: Two experiments to investigate the crossabilities of varieties of bread wheat, Triticum aestivum, with Hordeum bulbosum are described. In the first a range of varieties having low and high crossability with rye, Secale cereale, were used. It was found that a strong positive correlation between the crossabilities of wheat varieties with H. bulbosum and with rye exists. In the second experiment the genetics of crossability was investigated using chromosome substitution lines of the non-crossable variety, Hope, into the cross-able variety Chinese Spring. Two chromosomes, namely 5A and 5B of Hope, were found to reduce markedly the crossability of Chinese Spring. These chromosomes were those identified by Riley and Chapman (1967) as being responsible for the crossability of Chinese Spring with rye, reinforcing the conclusion that the crossability of wheat with both species is governed, at least in part, by the same genetic system. However, other chromosomes were also shown to affect the crossability of Chinese Spring, although to a much lesser extent. The use of this interspecific cross to produce wheat dihaploid lines for breeding purposes is discussed.
156 citations
TL;DR: There is always some critical number of active S alleles that ensures the elimination of mutant alleles with no activity in pollen, irrespective of how many active alleles are present, provided the inbreeding depression is small enough.
Abstract: We have examined a model proposed by East (1929) for the evolution of gametophytic self-incompatibility allele systems, starting from populations with a self-fertility allele. The original self-fertility allele is not eliminated from the population when three active S alleles have been incorporated, but self-fertility alleles can coexist with S alleles. In order to examine this co-existence more fully, we studied the invasion of self-incompatible populations containing various numbers of S alleles (all assumed to be equally frequent) by three possible types of self-compatibility factors that could arise by mutation at the S locus. We find that there is always some critical number of active S alleles that ensures the elimination of mutant alleles with no activity in pollen; this number depends on die inbreeding depression. Alleles abolishing the stigma activity can spread, irrespective of how many active alleles are present, provided the inbreeding depression is small enough. Some of the biological implications of these results are discussed.
135 citations
133 citations
TL;DR: Two models of gynodioecy controlled by cytoplasmic factors and nuclear genes are studied and neither permits stable coexistence of the two types of factor in a population.
Abstract: Two models of gynodioecy controlled by cytoplasmic factors and nuclear genes are studied. In one model, there is a male-sterility determining cytoplasm which produces its effect only in the absence of a restorer allele at a nuclear locus. In the other model, either the cytoplasmic factor or the nuclear gene can produce male-sterility independently of the other. Neither model permits stable coexistence of the two types of factor in a population.
124 citations
TL;DR: The breeding behaviour of fallow deer is described and shown to make a father-daughter incest likely and an inclusive fitness argument is developed to account for incest in polygynous species.
Abstract: It is assumed by many zoologists that most animals avoid inbreeding because of the associated deleterious effects. The breeding behaviour of fallow deer is described and shown to make a father-daughter incest likely. By treating incest as a form of altruistic behaviour on the part of females, an inclusive fitness argument is developed to account for incest in polygynous species. It is shown that a substantial (1/3) reduction in individual fitness is required if incest is to be selected against.
91 citations
TL;DR: The maximum likelihood method for estimating linkage disequilibrium from genotypic data for randomly mating populations is studied and it is recommended that the cubic be solved completely.
Abstract: The maximum likelihood method for estimating linkage disequilibrium from genotypic data for randomly mating populations is studied. Instead of iterative methods for finding a root of the cubic equation for one of the gametic frequencies (Hill, 1974), it is recommended that the cubic be solved completely. For data with some missing genotypic classes, it is further recommended that explicit solutions for the cubic be used.
83 citations
TL;DR: There is therefore counter-selection on fathers to produce diploid sons, and if the right genetic variation arises this countering selection will revert male haploidy to diploids, which applies to large, random-mating populations but not necessarily to inbred populations.
Abstract: In arthropods and rotifers a variety of genetic systems share the common property that males transmit only their mother's genome while females transmit genomes of both parents. Many of these have long been recognised because of the haploidy of males, but there are also some species in which males are diploid and yet transmit only the maternal genome. It is shown that the evolution of these maternal-genome-transmitting males (“haploid” males) is governed by the simple selective principle that maternal alleles occur at twice the frequency in gametes of haploid sons as in gametes of diploid sons. Under a simple constraint on the fitness of haploid males, selection favours mothers which produce haploid sons merely because they transmit maternal alleles at a greater rate than do diploid sons. The advantage to mothers in producing haploid sons is at the expense of the paternal genome, which is not transmitted by haploid sons. There is therefore counter-selection on fathers to produce diploid sons, and if the right genetic variation arises this countering selection will revert male haploidy to diploidy. These arguments apply to large, random-mating populations but not necessarily to inbred populations.
74 citations
TL;DR: Comparison of legumins from different Pisum types showed variability, both in the net surface charge of the whole molecule and in the nature of subunit heterogeneity on two-dimensional gels, which stressed the usefulness of two- dimensions in the genetic analysis of pea seed storage protein structure.
Abstract: Genetic variability in the structure of the α-subunits of legumin from Pisum —a two-dimensional gel electrophoresis study
71 citations
TL;DR: From these results it is recommended that seed for reforestation should not be moved more than 140 m elevation, 1·6 degrees latitude, or 2·7 degrees longitude in northern Idaho and eastern Washington.
Abstract: Growth, phenology and frost tolerance of seedlings from 50 populations of Douglas-fir (Pseudotsuga menziesii var glauca) were compared in 12 environments Statistical analyses of six variables (bud burst, bud set, 3-year height, spring and fall frost injuries, and deviation from regression of 3-year height on 2-year height) showed that populations not only differed in mean performance, but also reacted differently to the environmental gradient Most of the population-environment interaction was attributable to heterogeneous regressions of population means on environmental means For all variables except growth rate, the variance of heterogeneous regression coefficients was explained by convergence of regression lines to a common point on the environmental gradient Consequently, mean values for populations were significantly correlated with regression coefficients Thus, main effects of populations in those single environments that induced the greatest mean differences reflected the interaction Multiple regression analyses associated adaptive differentiation of populations with geographic and ecologic characteristics of the seed source Differentiation was controlled primarily by elevation and secondarily by latitude Whereas longitude was a minor factor, habitat types accounted for no differentiation beyond that associated with elevation, a factor closely correlated with habitat types From these results it is recommended that seed for reforestation should not be moved more than 140 m elevation, 1·6 degrees latitude, or 2·7 degrees longitude in northern Idaho and eastern Washington
TL;DR: The supplementary DNA fraction which distinguishes outbreeding from inbreeding species has, at most, surprisingly little effect upon the growth and development of the Lolium hybrids.
Abstract: Inbreeding species of Lolium have about 40 per cent more nuclear DNA than the outbreeding species. Even so inbreeders may be readily hybridised with outbreeders to produce viable F1's which, in many cases, are fertile, producing F2 progenies on intercrossing or backcross progenies when crossed with the parents. Recombination between and within chromosomes at meiosis in the F1 results in the quantitative segregation of DNA to the nuclei of gametes and of F2 and backcross progenies. An assay of the effects of the DNA variation among these progenies showed no influence of DNA amount upon the expression of 19 phenotypic characters, ranging from early seedling growth to flowering. There was, however, a very slight reduction from expectation in the mean DNA amount of backcross and F2 progenies. Apart from this slight effect on viability, either of gametes or of zygotes with high nuclear DNA content, our assay shows that the supplementary DNA fraction which distinguishes outbreeding from inbreeding species has, at most, surprisingly little effect upon the growth and development of the Lolium hybrids.
TL;DR: In the hypothesis of speciation where premating isolating mechanisms are supposed to evolve as a response to selection against hybrids, there is some doubt as to whether genetic variation for assortative mating would exist, and whether it would always respond to selection.
Abstract: A single locus mass-action model of assortative mating, with comments on the process of speciation
TL;DR: 6-phosphogluconate dehydrogenase (6PGD) is sex-linked in Heliconius butterflies, and this confirms that sex- linked genes in lepidoptera, as in birds, are not dosage-compensated.
Abstract: 6-phosphogluconate dehydrogenase (6PGD) is sex-linked in Heliconius butterflies. Within each of two species tested, the specific activity of 6PGD in males (the homogametic sex) is approximately twice that in females. This confirms that sex-linked genes in lepidoptera, as in birds, are not dosage-compensated. This absence of dosage compensation may be the basis for the frequent female-limitation of mimicry, and explains the peculiarity that the loci involved are never sex-linked, whereas male-limited sexual characters can be both sex-linked and autosomal.
TL;DR: Mating between morphs is frequency dependent so that it is the rare morph in the population, regardless of colour, which mates more frequently and appears to be a further method by which the polymorphism in this species is maintained.
Abstract: The nature of the factors maintaining the colour polymorphism in Adalia bipunctata has been the subject of much discussion and in some populations a tendency for the melanic morphs to mate more frequently than the non-melanics has been reported. The morph frequencies among mating pairs in wild populations of A. bipunctata in England have been scored and analysed in order to discover whether mating between morphs is a random process. Data from the literature are also analysed in the same way and the methods of analysis adopted by earlier workers are discussed. It is concluded that mating between morphs is frequency dependent so that it is the rare morph in the population, regardless of colour, which mates more frequently. This appears to be a further method by which the polymorphism in this species is maintained.
TL;DR: There is no statistically significant relationship between vernalisation response and resistance to freezing to temperatures of −6° and −12°C, so that in spite of the fact that chromosomes of of homoeologous group 5 are implicated in the control of both vernAlisation requirement and cold resistance there is no evidence that the characters are determined by the same genetic factors.
Abstract: Vernalisation response in wheat is controlled by genes located on chromosomes 5A and 5D. The variety Chinese Spring has a winter or late allele (vrn1) on chromosome 5A and a spring or early allele (Vrn3) on 5D. Increasing the dosage of the former has no significant effect on vernalisation response, but when the dosage of the latter is increased the response is reduced and flowering takes place earlier in unvernalised plants. A background of alien cytoplasm from Aegilops ovata prolongs the vegetative phase of development and also increases the vernalisation response, although to a lesser extent than chromosomal changes. The same chromosomes are involved in the control of cold resistance. Alleles on 5A and 5D of Chinese Spring condition poor tolerance to low temperatures, while those on 5A of Cheyenne and 5D of Hope confer a higher degree of resistance. Cytoplasmic as well as chromosomal factors play an important part in determining the level of resistance of difference genotypes. Within the group of genotypes studied, there is no statistically significant relationship between vernalisation response and resistance to freezing to temperatures of −6° and −12°C, so that in spite of the fact that chromosomes of of homoeologous group 5 are implicated in the control of both vernalisation requirement and cold resistance there is no evidence that the characters are determined by the same genetic factors.
TL;DR: The variation in the proportion of the DNA coding for ribosomal RNA in three flax genotrophs, and the inheritance of this proportion in crosses between them were determined, and DNA variation was observed and lines could be selected from within a genotrophic with differing proportions of rDNA.
Abstract: The variation in the proportion of the DNA coding for ribosomal RNA in three flax genotrophs, and the inheritance of this proportion in crosses between them were determined. DNA variation was observed and lines could be selected from within a genotroph with differing proportions of rDNA. The degree of variation increased when the genotrophs were crossed, the extent of the increase being dependent on the particular genotrophs involved in the cross. The variation extended into the F2 and F3 generations. The mean values of F2 and F3 families were not highly correlated with the value of the parental plant from which they were derived. In one cross the F2 and F3 means were close to the average value of the two parents involved in the original cross. In the other cross, the F1, F2 and F3 means were all less than the average value of the two original parents.
Journal Article•
TL;DR: It is concluded that cytochrome f is synthesised in intact isolated pea chloroplasts and was resistant to ribonuclease.
Abstract: Chloroplasts, isolated from the leaves of 7-day-old pea seedlings, were incubated in the light with [35S]methionine or [3H]leucine. After extraction from the washed chloroplast membranes using a mixture of ethyl acetate, ethanol and ammonia, cytochrome f was precipitated with a monospecific antiserum and resolved by gradient polyacrylamide gel electrophoresis in sodium dodecylsulphate. The cytochrome f band was identified by its intrinsic fluorescence in ultraviolet light and was shown to be radioactive by autoradiography or fluorography of dried polyacrylamide gel. One-dimensional peptide mapping of the products of papain hydrolysis confirmed that the radioactivity was an integral part of cytochrome f. The incorporation of [35S]methionine into cytochrome f was inhibited by D(-)threo-chloramphenicol but not by cycloheximide and did not occur in the dark. The synthesis was resistant to ribonuclease. It is concluded that cytochrome f is synthesised in intact isolated pea chloroplasts.
TL;DR: Modifications of chemical composition of surface waxes are induced by the recessive alleles of the gl-4,gl-8, gl-15 and gl-18 loci, which may affect a very early stage of the biosynthetic pathway leading to long chain compounds or, alternatively interfere with the supply of wax precursors.
Abstract: Modifications of chemical composition of surface waxes are induced by the recessive alleles of the gl-4, gl-8, gl-15 and gl-18 loci. Mutant gl-4 blocks the elongation of long chain wax molecules at the C30—C32 step, gl-8 and gl-18 mimic the action of gl-1 and gl-7, and may affect a very early stage of the biosynthetic pathway leading to long chain compounds or, alternatively interfere with the supply of wax precursors, gl-15 shows both an unusual spectrum of n-alkanes, in that the percentage of even chain compounds is higher than normal, and an abnormal synthesis of esterified primary alcohols of C16 and C18 chain lengths.
TL;DR: It appears, however, that the loss of duplicate gene expression occurs shortly following duplication, and once some structural or regulatory genetic divergence occurs, the duplicated loci become relatively stable.
Abstract: Theoretical studies of the rate of fixation of null alleles at duplicate loci were carried out with both deterministic numeric analysis and computer simulations assuming an intermediate selective optimum of the number of active gene copies. A relatively rapid loss of duplicate gene expression occurs with this model under a wide variety of conditions, including incomplete recessiveness of the null allele and linkage. It differs from previous models of gene loss, which are based on mutation pressure, because the rate of gene loss is accelerated with increasing population size. Identification of the mode of selection responsible for the observed loss of gene duplication in polyploid fish is not possible at present. It appears, however, that the loss of duplicate gene expression occurs shortly following duplication. Once some structural or regulatory genetic divergence occurs, the duplicated loci become relatively stable.
TL;DR: The simulations illustrated show that a plausible model for mate selection can generate data on the similarity of twins and their spouses which are remarkably consistent with a transitive model for the effects of mate selection.
Abstract: The simulations illustrated show that a plausible model for mate selection can generate data on the similarity of twins and their spouses which are remarkably consistent with a transitive model for the effects of mate selection. This is, biological considerations impose constraints upon the relative values of correlations which are not foreseen, for example, by the some advocates of conventional path models although they might be predicted by common sense. In particular, the correlation between the spouses of twins is expected to be non-zero under a model of phenotypic assortment and turns out to be approximately equal to the product of the twin correlation and the square of the marital correlation. The relative magnitudes of the correlations derived from an empirical study of such relationships should enable models of phenotypic assortment to be tested more rigorously. Including both identical and non-identical twins in the sample studied should permit the inherited and cultural components of the mating system to be identified with more conviction. In the event of one sex playing a more significant role in mate selection for particular traits, such studies should reveal diagnostic patterns of familial correlations as long as male and female twins and their spouses are analysed separately. If the analysis is restricted to phenotypic correlations of the parents, the qualitative findings do not appear to be greatly affected by selection due to assortative mating although a reduction in variance is to be expected if a large proportion of individuals is unable to mate. In such cases twins will also be significantly concordant for mating. The consequences of such varied regimes of assortation for the population structure and the relationship between traits in subsequent generations remain the object of future inquiry.
TL;DR: It was found that apomixis has no effect on the maintenance of polymorphism in species which otherwise out-cross, but in Species which partially or completely self, the possibility of the selective maintenance of balanced polymorphism was enhanced.
Abstract: Populations undergoing selfing, outcrossing and apomixis were studied. Genotypic transition equations were established which allow the properties of equilibrium populations to be discussed. Particular attention was paid to the continued maintenance of heterozygosity. It was found that apomixis has no effect on the maintenance of polymorphism in species which otherwise out-cross, but in species which partially or completely self, the possibility of the selective maintenance of balanced polymorphism was enhanced.
TL;DR: The patterns of chromosomal gametic disequilibria differ on the two sides of the zone, providing evidence of asymmetrical hybrid breakdown and supporting the hypothesis of one-way introgression of autosomes from the ”Torresian” into the “Moreton” race.
Abstract: The “Moreton” and “Torresian” races of Caledia captiva are parapatrically distributed in south-east Queensland. Although no obvious morphological differences exist between these taxa, there is a very high level of karyotypic divergence. An analysis of the structure of the contact zone has revealed that the transition from one race to the other occurs very abruptly, with approximately 65 per cent of the change in racial frequencies occurring in a 200 m interval. The total width of the zone of hybridisation is 1 k and within this area, mating occurs randomly within and between the races. Analyses of the karyotypic data have established that there is rapid elimination of “Moreton” autosomes on the “Torresian” side of the zone, but that “Torresian” autosomes are retained on the “Moreton” side. Further, the patterns of chromosomal gametic disequilibria differ on the two sides of the zone, providing evidence of asymmetrical hybrid breakdown and supporting the hypothesis of one-way introgression of autosomes from the “Torresian” into the “Moreton” race. A deterministic model examines the effect of hybrid breakdown in a contact zone.
TL;DR: Zein, the storage protein accumulated in maize endosperm, is composed of several polypeptides which can be separated by isoelectric-focusing (IEF) analysis, and three bands were shown to be controlled by factors localised on the analysed chromosome segments.
Abstract: Zein, the storage protein accumulated in maize endosperm, is composed of several polypeptides which can be separated by isoelectric-focusing (IEF) analysis. Much IEF zein variability is found when analysing different inbred lines—the number of bands varies from 8 to 15 and can occupy about 27 positions—and F1 endosperms show an additive parental pattern. Chromosomal location of factors controlling the phenotypic expression of eight IEF bands was studied by means of translocations between A and B chromosomes and trisomic stocks, which allowed a large part of the genome to be explored. Three bands were shown to be controlled by factors localised on the analysed chromosome segments: one close to the R locus on chromosome 10, one on the short arm of chromosome 4 and one on a segment composed of portions from the short arm of chromosome 9 and the long arm of chromosome 4. The band controlled by the factor on chromosome 10 occupies the same IEF position as one which was found to be controlled by a factor on chromosome 7 near the opaque-2 locus. However, on the basis of SDS electrophoresis, these two polypeptides were found to have different molecular weights. The results obtained are discussed in relation to the molecular origin of zein heterogeneity.
TL;DR: The evolution of X-chromosomal control of both the visual and olfactory components of the courtship communication system of C. eurytheme and C. philodice is discussed and compared to the mode of inheritance of other communication systems.
Abstract: The inheritance of male pheromone production in the sulphur butterflies Colias eurytheme and C. philodice was determined by gas chromatographic analyses of wing extracts from various genotypes derived from interspecific crosses. A wing ultraviolet-reflectance pattern (found only in C eurytheme males) which is controlled by an X-linked gene served as a marker for determining the relative influence of this chromosome v. the autosomes on pheromone production. The data indicate that production of the most important C. philodice wing compounds, three different n-hexyl esters, is controlled by one or more autosomal genes that are at least codominant. The X-chromosome carries most of the genes or the gene controlling production of the most important C. eurytheme compound, 13-methyl heptacosane. Expression of this compound in hybrids displays a codominant pattern. Because the wing ultraviolet-reflectance pattern of male C. eurytheme is an important courtship signal, it appears that all of the genes controlling the male courtship signals of this species are inherited as a co-adapted gene complex on the X-chromosome. The evolution of X-chromosomal control of both the visual (wing UV-reflectance pattern) and olfactory (13-methyl heptacosane) components of the courtship communication system of C. eurytheme is discussed and compared to the mode of inheritance of other communication systems.
TL;DR: The results are consistent with the hypothesis that speciation is accompanied by quantitative changes in the repeated sequence complements of genomes, and using a DNA probe from Aegilops speltoides that contains the most highly repeated sequences, diploid Aegilop species could be distinguished from diploids Triticum species.
Abstract: The families of repeated sequences in the genomes of a range of Triticum and Aegilops species have been compared. All the genomes are very similar. However, using a DNA probe from Aegilops speltoides that contains the most highly repeated sequences, diploid Aegilops species could be distinguished from diploid Triticum species. Different Aegilops species' DNAs also hybridise to differing extents with this probe. The results are consistent with the hypothesis that speciation is accompanied by quantitative changes in the repeated sequence complements of genomes. Most if not all of the families of repeated sequences in hexaploid wheat can be detected in Aegilops speltoides (related to the B genome) and in Aegilops squarrosa (related to the D genome). However, some families of repeated sequences of hexaploid wheat were not found in Triticum monococcum (related to the A genome). Some of the most highly repeated sequences of hexaploid wheat are preferentially concentrated in the B genome. These sequences are useful as probes for distinguishing the three diploid genomes of hexaploid wheat.
TL;DR: The utilisation of gynogenesis in genetic analysis and chromosome mapping in carp resulted in a new colour mutant, controlled by two genes, and only fish with double recessive alleles pprr showed the mutant coloration.
Abstract: The present paper reports the utilisation of gynogenesis in genetic analysis and chromosome mapping in carp. A new colour mutant was analysed with the help of gynogenetic haploid, diploid and biparental F1 and F2 populations. The mutant character was controlled by two genes, P and R, and only fish with double recessive alleles pprr showed the mutant coloration. The gene—kinetochore (G–K) distances of the genes were established. G–K distances were measured for two other genes—transferrin and scaliness.
TL;DR: The associations of alleles at different electrophoretic loci observed in barley and wild oats can be explained by hitch-hiking due to the mating system, and it appears difficult to differentiate experimentally between the hitch hiking and the previously proposed coadaptation hypotheses.
Abstract: The associations of alleles at different electrophoretic loci observed in barley and wild oats can be explained by hitch-hiking due to the mating system. In other words, selection at other loci in the genome may have resulted in gametic disequilibrium or a change in gametic disequilibrium at neutral loci in these highly self-fertilised plants. This explanation appears to be a simple alternative hypothesis to account for the change in gametic disequilibrium in barley and the presence of gametic disequilibrium in wild oats. However, it appears difficult to differentiate experimentally between the hitch hiking and the previously proposed coadaptation hypotheses.
TL;DR: The evidence suggests that the trait of cyanogenesis confers protection against herbivory by aphids and, possibly, other insects at the seedling stage, but feeding of the insects studied would not lead directly to seedling mortality of either phenotype.
Abstract: Insect herbivores colonizing cyanogenic and acyanogenic white clover plants were investigated. With the exception of aphids, the herbivores studied were infrequent visitors and failed to discriminate between the two phenotypes. Aphids, however, were two to three times more abundant on acyanogenic than cyanogenic plants. Evidence of visible insect feeding damage was identical for both phenotypes. The evidence suggests that the trait of cyanogenesis confers protection against herbivory by aphids and, possibly, other insects. Such protection would be most important at the seedling stage, but feeding of the insects studied would not lead directly to seedling mortality of either phenotype. Instead, mortality of weakened seedlings may result from other causes later in the season.