Heterosis in Cepaea

A test has been made of the association of heterozygosity with shell breadth in the polymorphic snail Cepaea nemoralis. The material was collected by C. B. Goodhart from a series of paired sites at which individuals reached different adult breadths. Dominant phenotypes, in which a large fraction was heterozygous, had a greater breadth and lower variance than recessive phenotypes regardless of whether the measurement was of shell ground colour, banding or the double recessive vs. the rest. Most of the difference was contributed by samples from the habitat where animals reached the largest size. The result is consistent with existence of heterotic sections of chromosome that include the colour and banding loci, and may help to explain the persistence of the polymorphism.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 49–53.     

Generation mean analyses for flowering and maturity in Indian mustard (Brassica junces (L.) Czern & Coss)

Summary Components of generation means were partitioned for days to flower initiation and maturity in three crosses of Indian mustard (Brassica juncea (L.) Czern and Coss) cultivars. A linked digenics model was adequate for flowering in cross II and maturity in Cross I. All three types of digenic interactions among the linked pairs of genes, additive X additive (i), additive X dominance (j) and dominance X dominance (1), contributed significantly in the inheritance of flowering in cross II and maturity in cross I. A complete association among the genes of greater effects in higher mean parent was detected for flowering in cross II and maturity in Cross I. Duplicate epistasis was evident for flowering in Crosses I and II and maturity in Crosses I and III.Inadequacy of all the fitted models for days to flowering in Cross III and maturity in Cross II indicated the presence of higher order interactions.Part of PhD (plant breeding) Thesis, submitted by senior author to GBPUAT, Pantnagar (Nainital) U.P., India (unpublished). Research paper No. 4262     

Differential inactivation and methylation of a transgene in plants by two suppressor loci containing homologous sequences

In a previous study on doubly transformed tobacco plants, we observed the unexpected inactivation in trans of T-DNA-I (encoding Kan^rNOS) following the introduction into the same genome of an unlinked copy of T-DNA-II (encoding Hyg^rOCS). This inactivation, which probably resulted from interactions between homologous regions on each T-DNA, was correlated with methylation in the nos _pro, which controlled the expression of both the nptII and nos genes. In this paper, we show that the inactivation and methylation of the nos _pro nptII gene in the presence of a suppressor T-DNA-II locus can be either complete (epistasis) or partial (cellular mosaicism). In plants showing partial suppression, the strength of the Kan^r phenotype, which apparently reflected the proportion of cells expressing the nptII gene, was inversely correlated with the degree of methylation of the nos _pro. The extent of nos _pro methylation decreased progressively in successive generations as suppressor T-DNA-II loci were crossed out. The strength of the Kan^r phenotype was improved and nos _pro methylation was less extensive in first generation Kan^r progeny obtained from outcrossing with untransformed tobacco than from self-fertilization.     

EPISTASIS AS A SOURCE OF INCREASED ADDITIVE GENETIC VARIANCE AT POPULATION BOTTLENECKS

The role of epistasis in evolution and speciation has remained controversial. We use a new parameterization of physiological epistasis to examine the effects of epistasis on levels of additive genetic variance during a population bottleneck. We found that all forms of epistasis increase average additive genetic variance in finite populations derived from initial populations with intermediate allele frequencies. Average additive variance continues to increase over many generations, especially at larger population sizes (N = 32 to 64). Additive-by-additive epistasis is the most potent source of additive genetic variance in this situation, whereas dominance-by-dominance epistasis contributes smaller amounts of additive genetic variance. With additive-by-dominance epistasis, additive genetic variance decreases at a relatively high rate immediately after a population bottleneck, rebounding to higher levels after several generations. Empirical examples of epistasis for murine adult body weight based on measured genotypes are provided illustrating the varying effects of epistasis on additive genetic variance during population bottlenecks.     

CLINES FOR HYBRID DYSFUNCTION IN A GRASSHOPPER HYBRID ZONE

Two subspecies of the grasshopper Chorthippus parallelus meet in the Pyrenees forming a hybrid zone several kilometers wide. Crosses between the two pure taxa result in sterile male offspring and normal females (i.e., Haldane's rule applies). However, no such dysfunction has been detected in hybrid males collected through the center of the hybrid zone. By assessing the level of dysfunction in the offspring of reciprocal crosses, it was possible to map clines for the genes responsible for dysfunction through the zone. This analysis shows that there is no abrupt transition between incompatible genomes in the field. Crosses were also made between females collected from a transect spanning the hybrid zone and pure males of both subspecies. This reveals noncoincident clines for dysfunction near the center of the hybrid zone such that the dysfunction expressed in the offspring of these crosses is less than expected from simple models. More complex models involving interaction among genes must be invoked. Also, the possibility exists that since the postglacial contact of these two grasshopper taxa, hybrid dysfunction has become ameliorated by the evolution of modifiers. This hybrid zone is thought to be a tension zone, maintained by a balance between selection against hybrid genotypes and dispersal into the zone center. The lessening of hybrid disadvantage over time through the breakdown of epistatic interactions by recombination or through modification could account for the general lack of dysfunction in field collected hybrids today.     

A case of congenital syphilis during the colonial period in Mexico City

Congenital syphilis has been diagnosed very seldom in ancient populations. The case that we examined comes from San Jeronimo's Church (17th and 18th centuries AD; Mexico City). Coffin 43 contained an incomplete skeleton of an approximately 2-year-old infant. The pathological lesions of this skeleton include bilateral osteochondritis, diaphyseal osteomyelitis, and osteitis and/or periostitis on the long bones. The radiographic appearance depicts symmetrical osteomyelitic foci, particularly at the proximal extremity of both tibiae (Wimberger's sign). The skull exhibits hydroceph-aly and periosteal changes on the vault, and the unerupted upper incisors evince dental hypoplasia and other pathological alterations reminiscent of Hutchinson's incisors. All these features strongly suggest a case of early Congenital syphilis. © 1995 Wiley-Liss, Inc.     

The genetics of phenotypic plasticity. IV. Chromosomal localization

The contributions of each chromosome to the traits thorax size and plasticity of thorax size as affected by temperature in Drosophila melanogaster were measured. A composite stock was created from lines previously subjected to selection on thorax size or plasticity of thorax size. A chromosome extraction was performed against a uniform background lacking genetic variation, provided by a stock of marked balancer flies. With regard to amount of plasticity, chromosome I and the balancer stock showed no plasticity, the composite stock showed the greatest plasticity, and chromosomes II and III were intermediate. Chromosome I showed significant genetic variation for thorax size at both 19° C and 25° C, but not for plasticity, while chromosome II showed significant genetic variation for plasticity, but not for thorax size. Chromosome III showed significant genetic variation for both thorax size and plasticity. We tested the predictions of three models of the genetic basis of phenotypic plasticity: overdominance, pleiotropy, and epistasis. The results support the epistasis model, in agreement with earlier work. The amount of developmental noise was correlated with phenotypic plasticity at 25° C, in agreement with earlier work. A negative correlation was found at 19° C for chromosome II, contrary to earlier work.     

The genetics of colour-patterns in the grasshopper Chorthippus brunneus

Several alleles were found to determine the colour of the dorsal pronotum in Chorthippus brunneus; there was evidence for at least two loci (C and V). Brown (C^B)was the universal recessive and green (C^C) was dominant to all other colours. The white allele (C^W)was codominant with green(C^G)and purple (C^P). Wing-patterns were determined by a separate, probably linked locus (W). A dominant plain wing-pattern (W^P) was associated with colours other than brown. Striped(W^S)and mottled(W^mo) were codominant and a plain recessive allele (W^P) was also found. All three alleles were associated with the brown phenotype. A purple-sided allele (S^Pu) was sometimes obmd with C^pu.. S^Pu was dominant to brown sides (S^B), A series of markings on the dorsal and lateral pronotum (linea intermedia, fascia postocularis, linea media, carina media and zona lateralis) were investigated and found to be controlled at separate loci which may be linked to W. These characters were expressed by dominant alleles. Epistatic effects by modifier loci were shown to have an important effect on the determination of wing phenotype. Allele Wo⁺, for example, suppressed the stripe-wing pattern, linea media, carina media and zona lateralis. It was concluded that colour patterns appear to be under genetic control and that dominant alleles were rare in the wild. Changes in shades of colours were shown to be age-dependent and minor.     

The genetics of evolutionary radiations

With the realization that much of the biological diversity on Earth has been generated by discrete evolutionary radiations, there has been a rapid increase in research into the biotic (key innovations) and abiotic (key environments) circumstances in which such radiations took place. Here we focus on the potential importance of population genetic structure and trait genetic architecture in explaining radiations. We propose a verbal model describing the stages of an evolutionary radiation: first invading a suitable adaptive zone and expanding both spatially and ecologically through this zone; secondly, diverging genetically into numerous distinct populations; and, finally, speciating. There are numerous examples of the first stage; the difficulty, however, is explaining how genetic diversification can take place from the establishment of a, presumably, genetically depauperate population in a new adaptive zone. We explore the potential roles of epigenetics and transposable elements (TEs), of neutral process such as genetic drift in combination with trait genetic architecture, of gene flow limitation through isolation by distance (IBD), isolation by ecology and isolation by colonization, the possible role of intra‐specific competition, and that of admixture and hybridization in increasing the genetic diversity of the founding populations. We show that many of the predictions of this model are corroborated. Most radiations occur in complex adaptive zones, which facilitate the establishment of many small populations exposed to genetic drift and divergent selection. We also show that many radiations (especially those resulting from long‐distance dispersal) were established by polyploid lineages, and that many radiating lineages have small genome sizes. However, there are several other predictions which are not (yet) possible to test: that epigenetics has played a role in radiations, that radiations occur more frequently in clades with small gene flow distances, or that the ancestors of radiations had large fundamental niches. At least some of these may be testable in the future as more genome and epigenome data become available. The implication of this model is that many radiations may be hard polytomies because the genetic divergence leading to speciation happens within a very short time, and that the divergence history may be further obscured by hybridization. Furthermore, it suggests that only lineages with the appropriate genetic architecture will be able to radiate, and that such a radiation will happen in a meta‐population environment. Understanding the genetic architecture of a lineage may be an essential part of accounting for why some lineages radiate, and some do not.