Showing papers in "Heredity in 1993"

Sex determination in the Hymenoptera: a review of models and evidence

Abstract: The haploid males and diploid females of Hymenoptera have all chromosomes in the same proportions. This rules out most familiar sex-determining mechanisms, which rely on dosage differences at sex determination loci. Two types of model — genie balance and complementary sex determination (CSD) — have been invoked for Hymenoptera. Experimental studies provide no good evidence for genie balance models, which are contradicted by the detection of diploid males in 33 disparate species. Furthermore, recent advances have shown that sex determination in the best-studied diploid animals does not depend on genie balance, removing the original justification for hymenopteran genie balance models. Instead, several Hymenoptera have single-locus CSD. In this system, sex locus heterozyotes are female while homozygotes and hemizygotes are male. Single-locus CSD does not apply to several inbreeding species and this probably reflects selection against the regular production of diploid males, which are sterile. A multilocus CSD model, in which heterozygosity at any one of several sex loci leads to female development has also been proposed. To date, multilocus CSD has not been demonstrated but several biases against its detection must be considered. CSD can apply to thelytokous races as long as the cytogenetic mechanism permits retention of sex locus heterozygosity. However, some thelytokous races clearly do not have CSD. The distribution of species with and without CSD suggests that this form of sex determination may be ancestral in the Hymenoptera. However, phylogenetic analyses are hindered by the lack of data from several superfamilies and the fact that the internal phylogeny of the Hymenoptera remains controversial.

(CT) n and (GT) n microsatellites: a new class of genetic markers for Salmo trutta L. (brown trout)

Abstract: Thirteen (GT)n and four (CT)n microsatellite loci (n = 10 or more and n = 20 or more, respectively) have been isolated from a partial genomic library of brown trout and sequenced. On average, a (GT)n repeat sequence occurs approximately every 23 kb and a (CT)n repeat sequence every 76 kb in brown trout genome. Primers for DNA amplifications using the polymerase chain reaction (PCR) were synthesized for three single locus microsatellites. Mendelian inheritance of the observed polymorphisms was confirmed in full-sib families. Four brown trout populations (10 unrelated individuals per population) were screened for polymorphism with these three microsatellite loci. The total number of alleles detected in the four populations is five at one locus, six at the other two microsatellite loci and is three, on average, per population. Heterozygosities range from 0.18 to 0.74. The largest differences in allelic frequencies occurred between the Mediterranean and the Atlantic populations: this result is congruent with previous allozymic data. The gene-centromere distances of the three microsatellite markers were determined on gynogenetic lines: post-reduction rates range from 0.17 to 0.60. For all the three microsatellite loci, the primers designed from brown trout sequences can be used in another closely related species of salmonid, the rainbow trout (Oncorhynchus mykiss). This last aspect supports the view that microsatellite markers may have wide application in genetic studies in salmonid species and fishes in general.

Genetic variation in Bromus tectorum (Poaceae): comparison between native and introduced populations

Abstract: The amount and distribution of genetic variation in 51 native (Eurasian and northern African) populations of Bromus tectorum were assessed at 25 loci using starch gel electrophoresis and were compared with our previous results for introduced (North American) populations of this predominantly cleistogamous grass. More alleles and variable loci were detected across populations in the native range than in North American populations. Within populations, however, the level of polymorphism is higher in the introduced range than in the native range. Deviation from Hardy-Weinberg expectation among native populations is almost as severe as for introduced populations. Eurasian populations exhibit greater genetic differentiation than those in North America; populations from Southwest Asia are the most genetically differentiated. Comparison of single-locus genotypes suggests possible source populations in both Europe and Southwest Asia for the populations now so prominent in western North America. Genetic differences between introduced and native populations of B. tectorum stem from two opposing factors: the reduction in genetic variability across populations produced by founder effects combined with an increase in the within-population component of genetic variation from multiple introductions.

Polyandry and allele frequency differences between the sexes in the ant Formica aquilonia

Abstract: Genetic mother-offspring analyses based on six enzyme gene loci show that about 60 per cent of the females of the mound-building red wood ant Formica aquilonia mate with several males. The number of matings inferred from the offspring genotypes range from one to six, the arithmetic mean being 1.94. The mates do not contribute equally in the inseminations; in the case of two matings, one male is estimated to inseminate on average 77 per cent of the offspring. The average related-ness among the offspring of a single female is 0.6, corresponding to the effective number of matings of 1.43. Three of the six loci show remarkable allele frequency differences between the sexes. At each of these loci the males virtually lack the alleles present in females with frequencies 0.18 at Me, 0.25 at Pgk and 0.27 at Gpi. Segregation analyses indicate normal Mendelian inheritance at these loci and the difference between the sexes seem likely to result from selection.

Finite island model for organelle and nuclear genes in plants

Abstract: Recurrence equations for genetic diversities and differentiation were developed for hermaphrodite plant species in an island model of population structure. This was made possible by the definitions of diversities at all hierarchical levels from gamete to total population and by the definition of migration rates specific to plants for both nuclear and cytoplasmic genomes. Mating system was also incorporated. Numerical computations were used to compare equilibrium values of differentiation obtained with our equations with those predicted by classical formulas. We show that the differences (sometimes high) result from the interpretations of the definition of gene diversity in a population of finite size. We interpret it as the probability that two genes sampled with replacement are different alleles (instead of without replacement). The effects of several parameters (ploidy level, mode of inheritance, outcrossing rate, population size) on genetic subdivision were evaluated. Contrary to the situation in animals, plant migration is intrinsically asymmetrical because a gene transmitted to the next generation through the male gamete may migrate in the pollen grain and in the seed, whereas a gene transmitted through the female gamete can migrate only in the seed. As a consequence, mode of inheritance (in the case of cytoplasmic genes) and outcrossing rate have strong impacts on subdivision, especially when pollen migration is larger than seed migration (a likely situation in many plant species). Parameters estimated in a survey of oak populations (Quercus robur L.) were used to examine whether our understanding of a real situation could be improved by the model. In particular, the rate of return to equilibrium was studied after a perturbation, i.e. a temporary decrease of population sizes (a bottle-neck).

Why do maternally inherited microorganisms kill males

Abstract: Maternally-inherited male killing microorganisms are known in a number of insect species. We here discuss the evolutionary reasons for such behaviour through examining the ongoing dynamics of these elements. In cases where death occurs late in the life cycle, horizontal transmission associated with male death occurs but in others no such direct advantage to killing the individual appears to exist. The evidence that early male killing is analogous to primary sex ratio distortion through increasing the fitness with respect to producing females from increases in resources to surviving (female) offspring and through the prevention of inbreeding is discussed. The early life history of many of these species suggests an early resource advantage may accrue from male killing. Inbreeding avoidance appears to be an important factor in at least two cases. The potential for horizontal transmission to maintain these elements is also discussed. It is concluded that male killing appears to be an adaptive strategy for the microorganism in most cases.

Genetic variation and phylogenetic relationships among worldwide collections of the Russian wheat aphid, Diuraphis noxia (Mordvilko), inferred from allozyme and RAPD-PCR markers

Abstract: Genetic analyses were conducted on Diuraphis noxia (Mordvilko) populations collected from wheat, barley and other grasses from various countries throughout the world. These collections had been found to contain clones that differed in virulence from various cultivars, cuticular hydrocarbon profiles and life cycle characters. Discrete genetic markers analysed in this study included allozymes and arbitrary regions of the genome amplified by the polymerase chain reaction (RAPD-PCR). In all, 23 enzymes were evaluated; 17 of these enzymes representing 20 isozyme loci, were judged suitable for allozyme analysis. Polymorphisms were detected at three (15 per cent) loci: beta-esterase (beta-EST), phosphoglucose isomerase (PGI), and 6-phosphogluconate dehydrogenase (6-PGDH). The average expected heterozygosity amongst these loci was 4.9 per cent in the worldwide collection. Allozyme variation was absent within most populations, particularly within those countries where the species was recently introduced. Much greater genetic variation was detected when populations were analysed with RAPD-PCR. Populations were analysed with 69 polymorphic bands amplified by seven primers. All populations could be distinguished with this method. Cluster analyses indicated strong similarities between U.S.A. populations and collections from South Africa, Mexico, France and Turkey. The most variation was detected among populations from the Middle East and southern Russia.

The effects of forest fragmentation on genetic variation in Acer saccharum Marsh. (sugar maple) populations

Abstract: To investigate the effects of forest fragmentation on genetic variation in Acer saccharum Marsh. (Aceraceae) populations, aflozyme variation in eight populations in fragmented forest patches was compared to variation in eight population samples from extensive continuous forest. Contrary to theoretical predictions, increased genetic drift and inbreeding have not led to reduced within-individual or within-population genetic variation in patch populations. Polymorphism and individual heterozygosity were slightly higher in patch than continuous forest populations. However, overall, founder effects may have resulted in the loss of six alleles in the fragmented forest system. Maintenance of genetic variation in patch populations may be because of the limited number of generations since fragmentation occurred, or because populations in patches have not been reduced below the size of normal breeding associations for A. saccharum. Comparison of the distribution of alleles among populations, between patch and continuous forest, suggests that the higher genetic variation in patch populations may be the result of increased interpopulation gene flow.

Mapping and characterization of quantitative trait loci affecting resistance against second-generation European corn borer in maize with the aid of RFLPs

Abstract: Mapping and characterization of quantitative trait loci affecting resistance against second-generation European corn borer in maize with the aid of RFLPs

Identification of RAPD markers linked to a Rhynchosporium secalis resistance locus in barley using near-isogenic lines and bulked segregant analysis.

Abstract: Three hundred random sequence 10-mer primers were used to screen a pair of near-isogenic lines of barley and their donor parent for markers linked to genes conferring resistance to Rhynchosporium secalis. One primer was identified which reproducibly generated a product, SC10-65-H400, from the donor parent and the Rhynchosporium-resistant near-isogenic line but not from the recurrent parent. Segregation analysis on a barley doubled haploid population and examination of a further three near-isogenic lines, their donor and recurrent parents confirmed that this marker was linked to the Rhynchosporium resistance locus (Rh) on chromosome 3L. The presence or absence of SC10-65-H400 was subsequently used along with the resistance phenotype to identify two groups of individuals in the doubled haploid population which possessed alternative alleles at both loci and defined a genetic interval between these two markers. Based on that information two bulked DNA samples were constructed by combining equal amounts of DNA from five individuals from each group. The two bulks and doubled haploid parental lines were screened with 700 10-mer primers. Seven products were identified which were present in the ‘resistant’ bulk and parent and were absent in the susceptible samples. Segregation analysis established their association with Rh. In addition co-segregation of the linked markers with a set of chromosome arm specific RFLPs confirmed the location of the Rh locus on the long arm of barley chromosome 3.

Rapid detection of genetic variability in chrysanthemum (Dendranthema grandiflora Tzvelev) using random primers

Abstract: Genetic variation in chrysanthemum (Dendranthema grandiflora) was studied using a recently developed technique generating Random Amplified Polymorphic DNAs (RAPDs). It appeared that variation between cultivars was high and that the cultivars used could be distinguished from each other by using only two different primers. A family of cultivars, derived from one original cultivar by vegetative propagation, had identical fragment patterns. Because of the high level of polymorphism and clonal stability RAPD fragments are useful for cultivar identification. Genetic variability among related Dendranthema species was too high to study genetic distances either among cultivars within chrysanthemum or among species related to chrysanthemum.

Partial self-fertilization and inbreeding depression in two populations of Mimulus guttatus

Abstract: Partial self-fertilization and inbreeding depression in two populations of Mimulus guttatus

Genomic map of a diploid hybrid species

Abstract: Several fertile diploid plant species have been shown to be derived from hybridization between species with strong chromosomal sterility barriers, yet little is known about the genomic processes that accompany this mode of speciation. As a first step toward understanding these processes we have generated a detailed genetic linkage map for Helianthus anomalus, a diploid species derived via hybridization between H. annum and H. petiolaris. This was generated using 161 RAPD loci and one isozyme locus. The genetic markers were distributed into 18 linkage groups and cover 2338 cM. Analysis of the parental origin of each locus/allele revealed that 44 were originally derived from H. annuus, 37 were derived from H. petiolaris, 54 could have been derived from either parent, and 25 were unique to H. anomalus. In addition, the parental linkage groups were not transmitted or retained intact in the hybrid species. Rather, molecular markers originating from both parental species were interspersed within individual H. anomalus linkage groups. Although there are several limitations to analysing hybrid genomic composition using this approach, these results do raise the possibility that diploid hybrid species may be able to retain different portions and/or proportions of their parental species' genomes. This characteristic may provide diploid hybrid species with greater flexibility than allopolyploid species in terms of optimizing their genomes for a new ecological niche and may permit the production of several different diploid hybrid species from the same two parents, as appears to have happened in Helianthus.

Correlation between the individual heterozygosity of parents and their offspring.

Abstract: Specific formulations are derived for the correlation between the heterozygosity of a randomly mating parent and its offspring for a diallelic locus, and for the correlation when multiple loci are considered. The expected correlation is maximal, approaching r = 0.50, when allelic frequencies are highly asymmetric, and it is zero when the allelic frequencies are equal to 0.50. Parent-offspring correlations, up to a maximum of 0.50 for diallelic loci, indicate that levels of heterozygosity can respond to selection. Multilocus allozyme data from limber pine, Pinus flexilis, and from horses of standardbred and thoroughbred breeds are used to demonstrate correlations between a parent and its offspring. The Spearman rank correlation between the heterozygosity of a limber pine and the mean heterozygosity of her offspring is r = 0.45. Correlations in the horses range from r = 0.16 to 0.32.

Macrogeographic patterns of breeding system diversity in the Daphnia pulex group. I. Breeding systems of Canadian populations

Abstract: Efforts to deduce the evolutionary significance of sexual reproduction from the taxonomic and ecological distributions of asexuals rely on the presumption that reversions of asexual taxa to sexual reproduction are rare. There is growing evidence, however, that reversions may be likely as long as asexuals are diploid, and some sexual populations persist. However, few systems have been studied in sufficient detail to provide insight into the nature of interactions between the sexual and asexual components of a single taxon. The cladoceran crustaceans Daphnia pulex and D. pulicaria both show variation in breeding system with some populations reproducing by cyclic and others by obligate parthenogenesis. The present study used allozymic analysis to characterize the breeding systems of both species across Canada. Although F1 hybrids were prevalent between the species and invariably reproduced by obligate parthenogenesis, each parent species showed regional variation in its breeding system. D. pulex reproduces by obligate parthenogenesis in eastern Canada but switches to cyclic parthenogenesis in the west with an abrupt transition coincident with the forest/prairie ecotone. D. pulicaria shows a divergent pattern, with populations on the priaries reproducing by obligate parthenogenesis, while those in eastern Canada and the far west are cyclic parthenogens. The results make clear that breeding systems in both taxa tend to be uniform over large areas. The discordance in breeding system employed by each species at specific sites suggests that the patterns of breeding system variation are not linked to environmental factors, but are instead a consequence of the interplay between historical factors, such as dispersion patterns from glacial refugia and the sites of origin of mutations promoting the transition to asexuality.

Cytogenetics of Rutaceae. V. High chromosomal variability in Citrus species revealed by CMA/DAPI staining

Abstract: The CMA/DAPI chromosome banding pattern of six Citrus species was analysed. All of them showed a large amount of heterochromatin and heterozygosity. Most of the heterochromatin was CMA+/DAPI− and was located mainly in the terminal region of the long arms. Each individual was heterozygous for at least one chromosome pair and each species presented a different banding pattern. The greatest amount of information on karyotype variation can be obtained from the four largest pairs. Pairs I and III showed the highest amount of heterochromatin and heterozygosity whereas pair II was highly stable and almost completely lacked CMA+ heterochromatin. On the other hand, some features common to all species suggest a strong chromosome homoeology conservation. The CMA staining revealed some chromosome markers for each Citrus species that may be very useful in the identification of zygotic versus nucellar embryos, as well as for further evolutionary studies.

Heritability and 'evolvability' of Fitness Components in Callosobruchus maculatus

Abstract: Heritabilities of traits closely related to fitness (e.g. life-history traits) tend to be lower than those of morphological traits in natural populations. It is unclear, however, whether this pattern reflects relatively low additive-genetic variation, relatively high residual variation (i.e. non-additive and environmental effects) or both. A standard, half-sib breeding design was used to estimate narrow-sense heritabilities and coefficients of additive-genetic variation (CVA) for a life-history trait (total fecundity), a morphological trait (wing length) and a behavioural trait (allocation of eggs among available resources) in two outbred populations of the seed beetle Callosobruchus maculatus. Heritabilities of fecundity and wing length were high in both populations whereas the heritability of egg-laying behaviour was low but non-zero. In contrast, estimates of ‘evolvability’, which depend on CVA, were highest for fecundity, intermediate for egg-laying behaviour and lowest for wing length. These results resemble those from a recent survey of Drosophila studies and suggest that the typically low heritabilities of behavioural or life-history traits are better explained by high residual variation than by low additive-genetic variation. Phenotypic and genetic correlations were generally not significant except for a positive relationship between wing length and fecundity. No evidence was obtained of a trade-off or underlying functional relationship between the number of eggs laid and the pattern of egg dispersion among available resources.

Mitochondrial DNA variation and Haldane's rule in the Papilio glaucus and P. troilus species groups

Abstract: Variation in mitochondrial DNA (mtDNA) was surveyed, using restriction endonucleases, in all species of the Papilio glaucus and P. troilus groups (Lepidoptera: Papilionidae). Phylogenetic and distance relationships of mtDNA generally confirmed traditional species limits in the two species groups and compared favourably with a prior survey of their allozymes. The most notable exceptions were P. rutulus and P. eurymedon, which share their mtDNA, yet maintain numerous morphological and ecological distinctions in sympatry. In organisms with heterogametic females, like the Lepidoptera, the deficiencies of hybrid females that are expected on the basis of Haldane's rule should lead to congruence of mtDNA variants with species boundaries. In the P. glaucus group, the extent of mtDNA differences between species pairs generally coincided with the extent of the Haldane effect, as well as with the presence of major species differences on sex chromosomes (X effect).

Evolutionary cytogenetics of the Drosophila buzzatii species complex.

Abstract: The salivary gland chromosomes of 10 species in the Drosophila mulleri subgroup (repleta group) have been re-analysed. These include the eight members of the South American buzzatii and martensis clusters, previously ascribed to the mulleri complex, and the two Caribbean species D. stalkeri and D. richardsoni, previously comprising the stalkeri complex. The chief results can be summarized as follows. Inversion 3a is not present in the martensis cluster. Hence, there is no cytological link between this cluster, or the buzzatii cluster, and the rest of the mulleri complex. Accordingly, a new species complex, the buzzatii complex, is established with the two South American clusters. D. stalkeri and D. richardsoni share at least two inversions with all the species in the buzzatii and martensis clusters, and produce hybrids in interspecific crosses with many of them. This indicates a close phylogenetic relationship. Therefore, D. stalkeri and D. richardsoni are incorporated as a cluster within the newly erected buzzatii complex. A phylogenetic tree illustrating the chromosomal evolution of the buzzatii complex is presented and all the previous cytological information concerning its members is reviewed.

A general model for the genetic control of copper tolerance in Silene vulgaris : evidence from crosses between plants from different tolerant populations

Abstract: Copper tolerance in Silene vulgaris seems to be controlled by two major genes. One segregates only in crosses to non-tolerants but never in crosses between tolerants originating from different isolated populations. The second segregates only in crosses to plants from the most tolerant population. The level of tolerance in tolerant plants seems to be controlled by two additional genes, which are hypostatic to the first major gene. They segregate in crosses to non-tolerants but not in crosses between equally homozygous tolerant plants from different populations. It is argued that all the genes are involved in the control of an exclusion mechanism operating at the plasmalemma.

The effect of zinc contamination from electricity pylons. Genetic constraints on selection for zinc tolerance

Abstract: Zinc tolerance, based on root growth in zinc-containing solutions, was examined in samples of 30 Agrostis capillaris L. individuals collected from populations beneath five electricity pylons in North Wales. The populations were found to vary in zinc tolerance. Three of them contained individuals whose tolerance ranged from low to high, one population consisted of individuals with low to moderate tolerance, whilst the fifth population (ZK-180) contained no tolerant individuals. Differences in zinc tolerance were also found between seedlings raised from five seed populations collected from plants growing in the uncontaminated pasture areas adjacent to four of those pylons, and one further pylon in the same area. The population adjacent to pylon ZK-180 consistently showed little sign of tolerant individuals. Samples of seeds collected from the populations in the uncontaminated areas surrounding each pylon, and likely to have been the origin of the pylon populations were grown at a concentration of 15 μg Zn cm−3, and root lengths of 105 randomly chosen seedlings from each were measured. The means and variance of tolerances of four of the five did not differ, but all were significantly higher than that adjacent to pylon ZK-180. Two cycles of selection and breeding for increased zinc tolerance were carried out using seed samples collected from four of the five populations of adults collected beneath pylons examined previously, and including population ZK-180. Marked increases in tolerance were effected in three of the four populations; no corresponding increase in tolerance was achieved for population ZK-180. It is concluded from these findings that the lack of zinc tolerance beneath pylon ZK-180 is due to the absence of appropriate variation in zinc tolerance within that population, and in potential colonizing material adjacent to it, a situation which contrasts with that for the other three pylons.

The effect of serial founder-flush cycles on quantitative genetic variation in the housefly

Abstract: The effect of serial founder-flush cycles on quantitative genetic variation in the housefly

Comparison of inbreeding and outbreeding in hermaphroditic Arianta arbustorum (L.) (land snail)

Abstract: Inbreeding depression was studied in the outcrossing hermaphroditic land snail Arianta arbustorum, by comparing the reproductive performance of full-sib pairs (inbreeding) and pairs of unrelated individuals (outbreeding). Fecundity (number of eggs laid) was not found to differ significantly between inbreeding and outbreeding pairs under laboratory conditions, but fertility (the proportion of eggs that hatched) and the number of hatchlings produced by inbreeding pairs were significantly lower than those of outbreeding pairs. Estimates of reduction in the fertility of eggs (0.37) and in the number of hatchlings produced (0.56) suggested significant inbreeding depression in full-sib crosses. Inbred offspring reared in the garden had a higher mortality than outbred offspring reared in the same environment, with an inbreeding depression of 0.30 in the first year after hatching; but no significant difference was found when offspring from both groups were kept in the laboratory. The results support the hypothesis that cross-fertilization in simultaneous hermaphrodites is maintained by strong inbreeding depression.

Genetics of life history in Daphnia magna . I. Heritabilities at two food levels

Abstract: Broad- and narrow-sense heritabilities for several life-history traits were estimated from 23 mother-daughter pairs of Daphnia magna at two food levels. Sexually produced daughter clones were obtained from a field collection of ephippial females (mothers) and the subsequent hatching of their ephippial eggs (daughters). Mother and daughter clones were maintained by parthenogenetic reproduction. Heritabilities of adult body-length of eight successive instars were the highest estimates of all in good food, but the lowest of all in poor food. For clutch size and body-length of offspring from the first six clutches, narrow- and broad-sense heritabilities were about equal and lower in poor food than in good food. The amount of genetic variation present would allow a response to selection on clutch size and offspring length in both environments but adult length only in good food. For age at maturity we found no additive genetic variance. We found no difference in broad-sense heritabilities between mother clones, representing the last generation after a period of asexual reproduction, and their sexually produced offspring. This suggests that genetic variance does not increase after one sexual generation or that it was not reduced before. Differences in heritabilities between environments are discussed with reference to the enlarged phenotypic variances that result from variation in juvenile instar number. Targeted growth could explain the pronounced differences in the heritabilities of adult length between environments.

Inheritance of self-compatibility in almond

Abstract: The incidence of self-compatibility in 742 almond trees, resulting from 25 inter-cultivar crossings in which at least one of the parents is self-compatible (Genco and Tuono), was determined by means of microscopic observation of the pollen tube growth in the pistil. The data obtained are consistent with the assumption that self-incompatibility in the genus Prunus is controlled by a single, multi-allelic gene whose expression in the pollen is gametophytic. The 100 per cent occurrence of self-compatible specimens in the cross where the Ferragnes cultivar is the female parent is due to the existence of an allele shared with the self-compatible cultivars used. The high number of progeny examined and the method used to ascertain the self-compatibility make the results very reliable, as opposed to other less precise methodologies or tests with few seedlings.

Heritability of floral traits for the perennial wild flower Penstemon centranthifolius (Scrophulariaceae): clones and crosses

Abstract: Floral and reproductive traits frequently influence pollinator behaviour and plant reproductive success and are therefore thought to be under selection. However, for wild plant populations there is little information on the amount of genetic variation for such traits. We estimated broad- and narrow-sense heritability of floral and reproductive traits for two populations of the perennial wildflower Penstemon centranthifolius, using a paternal half-sib crossing design in one case, and clonal propagation of ramets from different genets in the other. In both designs, most traits were significantly heritable, including nectar production, corolla length and width, inflorescence length and total flower production. Flowering date was significantly heritable for the crosses, but not for the clones, while nectar sugar concentration was not significantly heritable in either design. Genetic correlations were in some cases strong, and in some cases differed greatly from the corresponding phenotypic correlations. These results indicate that P. centranthifolius harbours a large reservoir of genetic variance which could support an evolutionary response to phenotypic selection.

Constraints on the evolution of attractive traits: genetic (co)variance of zebra finch bill colour

Abstract: We estimated the heritability and genetic correlation between male and female bill colour in a laboratory population of zebra finches (Taeniopygia guttata) in order to examine the potential genetic constraints on the evolution of a sexually dimorphic trait. The heritability estimates of bill colour from regressions of offspring on single parents ranged from h2 = 0.34 to 0.73 and all but one of these estimates were significantly greater than zero. The restricted maximum likelihood heritability estimates for full- and half-siblings were significant for females (h2 = 0.48) but not significant for males (h2 = 0.45). The maximum likelihood estimates indicate that there is little dominance genetic variance for bill colour. The large genetic correlation between male and female bill colour (rg = 0.91) combined with opposing selection on male and female bill colour indicates that the evolution to sex-specific optima may proceed very slowly.

Nuclear DNA content in F1 hybrids of maize

Abstract: The nuclear DNA content was determined in two separate experiments for 19 maize inbred lines and 26 maize F1 hybrids Ten inbred lines were initially screened in the first experiment An 11 per cent difference in nuclear DNA amount was observed between the two lines with the lowest amounts of DNA and two lines with the largest genome sizes All possible hybrid combinations were made among these four lines In all cases, the genome sizes of the F1 hybrids were not significantly different from their expected parental means In several cases, however, F1 plants of specific crosses were not uniform in genome size In these crosses, genome sizes observed ranged from the genome size of the low parental genome to the largest parental genome These results indicate an instability in F1 genome sizes in certain maize crosses In order to corroborate this hypothesis, a second experiment was performed To remove any biases with respect to genome size, 14 F1 maize hybrids were selected solely on the basis of their heterotic response with no regard for their genome sizes or the genome sizes of their parental inbred lines The nuclear DNA content of the nine parental lines and 14 hybrids was determined In most of the crosses, the nuclear DNA content of the F1 hybrids was not significantly different from their respective parental means However, in five parental combinations, the mean nuclear DNA content of the F, hybrids was significantly higher than their respective parental means The combined results of this study support the hypothesis of instability in nuclear DNA content in F1 hybrids of maize This instability appears not to be universal in all maize hybrids but is restricted to specific parental combinations

Variation and inheritance of ribosomal DNA in Pinus sylvestris L. (Scots pine)

Abstract: Variability in the ribosomal DNA (rDNA) gene family was surveyed in Scots pine (Pinus sylvestris L.) populations from southern and northern Finland. A total of 97 trees were studied with three restriction enzymes, revealing 13 variable rDNA phenotypes. Each rDNA phenotype represents the combined genotype of the eight rDNA loci (NORs, nucleolus organizer regions) that are present in the Scots pine genome. The studied Scots pine populations were equally variable and there was no evidence of geographic differentiation. Of the total rDNA diversity of the species, 86 per cent was found within populations (including within-individual variability) and 14 per cent was found between populations. Within individual trees, one to four rDNA repeat types were distinguished. No variation was found in the coding regions but variable restriction sites were identified in the IGS and transcribed spacer regions. The inheritance pattern of an rDNA variant carrying a 0.4 kb deletion in the transcribed spacer region was studied. The deletion-carrying rDNA variants were distributed non-randomly across the NOR loci and showed regular Mendelian segregation in the progeny. The observed distribution pattern of rDNA variability suggests that the rate of intrachromosomal homogenization is greater than that of interchromosomal homogenization in Scots pine.

Diapause in Drosophila melanogaster females: a genetic analysis

Abstract: Female Drosophila melanogaster exhibit ovarian diapause at low temperatures and short day lengths. We found that D. melanogaster isofemale lines from Windsor (Ontario, Canada) had a significantly higher percentage of females in diapause than did those from Cartersville (Georgia, U.S.A.). To investigate the heredity of this trait, we performed a 16-reciprocal cross analysis using two extreme isofemale lines called W and C. We found that diapause in D. melanogaster is inherited as a simple autosomal recessive trait with the C response (less flies in diapause) completely dominant to the W one. Maternal and cytoplasmic factors did not affect differences in diapause in these lines. The result of our genetic analysis of diapause in D. melanogaster opens may avenues for the genetic dissection of this ecologically relevant trait.