Gene duplication,allelic diversity,selection processes and adaptive value of MHC class II <Emphasis Type="Italic">DRB</Emphasis> genes of the bank vole, <Emphasis Type="Italic">Clethrionomys glareolus</Emphasis>

The generation and maintenance of allelic polymorphism in genes of the major histocompatibility complex (MHC) is a central issue in evolutionary genetics. Recently, the focus has changed from ex situ to in situ populations to understand the mechanisms that determine adaptive MHC polymorphism under natural selection. Birth-and-death evolution and gene conversion events are considered to generate sequence diversity in MHC genes, which subsequently is maintained by balancing selection through parasites. The ongoing arms race between the host and parasites leads to an adaptive selection pressure upon the MHC, evident in high rates of non-synonymous vs synonymous substitution rates. We characterised the MHC class II DRB exon 2 of free living bank voles, Clethrionomys glareolus by single-strand conformation polymorphism and direct sequencing. Unlike other arvicolid species, the DRB locus of the bank vole is at least quadruplicated. No evidence for gene conversion events in the Clgl-DRB sequences was observed. We found not only high allelic polymorphism with 26 alleles in 36 individuals but also high rates of silent polymorphism. Exceptional for MHC class II genes is a purifying selection pressure upon the majority of MHC-DRB sequences. Further, we analysed the association between certain DRB alleles and the parasite burden with gastrointestinal trichostrongyle nematodes Heligmosomum mixtum and Heligmosomoides glareoli and found significant quality differences between specific alleles with respect to infection intensity. Our findings suggest a snapshot in an evolutionary process of ongoing birth-and-death evolution. One allele cluster has lost its function and is already silenced, another is loosing its adaptive value in terms of gastrointestinal nematode resistance, while a third group of alleles indicates all signs of classical functional MHC alleles.     

Phylogenetic analysis of floral integration in Schizanthus (Solanaceae): does pollination truly integrate corolla traits?

To assess whether floral integration patterns result from the action of pollinator selection on functionally related traits, we compared corolla integration patterns in eight Schizanthus species differing in pollination systems and in their degree of pollinator dependence across a molecular phylogeny. Integration patterns differed among species and these differences were not related to their phylogenetic relatedness. When the putative original function of some corolla traits was lost in pollinator-dependent species, the integration among nonfunctional characters and the rest of the corolla traits was disrupted. This pattern was not presented in species adapted for late autonomous selfing, which exhibited higher corolla integration than their pollinator-dependent relatives. These results suggest that corolla integration in pollinator-dependent species was shaped by pollinator-mediated selection. Decoupling of nonfunctional traits in these species may result from a relaxation of correlational selection or from selection acting against a default covariation provided by genetic and developmental connections.     

A tale of two matrices: multivariate approaches in evolutionary biology

Two symmetric matrices underlie our understanding of microevolutionary change. The first is the matrix of nonlinear selection gradients (gamma) which describes the individual fitness surface. The second is the genetic variance-covariance matrix (G) that influences the multivariate response to selection. A common approach to the empirical analysis of these matrices is the element-by-element testing of significance, and subsequent biological interpretation of pattern based on these univariate and bivariate parameters. Here, I show why this approach is likely to misrepresent the genetic basis of quantitative traits, and the selection acting on them in many cases. Diagonalization of square matrices is a fundamental aspect of many of the multivariate statistical techniques used by biologists. Applying this, and other related approaches, to the analysis of the structure of gamma and G matrices, gives greater insight into the form and strength of nonlinear selection, and the availability of genetic variance for multiple traits.     

Testing the evolutionary constraints of metamorphosis: The ontogeny of head shape in Triturus newts

In vertebrates with complex, biphasic, life cycles, larvae have a distinct morphology and ecological preferences compared to metamorphosed juveniles and adults. In amphibians, abrupt and rapid metamorphic changes transform aquatic larvae to terrestrial juveniles. The main aim of this study is to test whether, relative to larval stages, metamorphosis (1) resets the pattern of variation between ontogenetic stages and species, (2) constrains intraspecific morphological variability, and (3) similar to the “hour‐glass” model reduces morphological disparity. We explore postembryonic ontogenetic trajectories of head shape (from hatching to completed metamorphosis) of two well‐defined, morphologically distinct Triturus newts species and their F1 hybrids. Variation in head shape is quantified and compared on two levels: dynamic (across ontogenetic stages) and static (at a particular stage). Our results show that the ontogenetic trajectories diverge early during development and continue to diverge throughout larval stages and metamorphosis. The high within‐group variance and the largest disparity level (between‐group variance) characterize the metamorphosed stage. Hence, our results indicate that metamorphosis does not canalize head shape variation generated during larval development and that metamorphosed phenotype is not more constrained relative to larval ones. Therefore, metamorphosis cannot be regarded as a developmental constraint, at least not for salamander head shape.     

Identification of the amino acids in the Major Histocompatibility Complex class II region of Scottish Blackface sheep that are associated with resistance to nematode infection

Lambs with the Major Histocompatibility Complex DRB1*1101 allele have been shown to produce fewer nematode eggs following natural and deliberate infection. These sheep also possess fewer adult Teladorsagia circumcincta than sheep with alternative alleles at the DRB1 locus. However, it is unclear if this allele is responsible for the reduced egg counts or merely acts as a marker for a linked gene. This study defined the MHC haplotypes in a population of naturally infected Scottish Blackface sheep by PCR amplification and sequencing, and examined the associations between MHC haplotypes and faecal egg counts by generalised linear mixed modelling. The DRB1*1101 allele occurred predominately on one haplotype and a comparison of haplotypes indicated that the causal mutation or mutations occurred in or around this locus. Additional comparisons with another resistant haplotype indicated that mutations in or around the DQB2*GU191460 allele were also responsible for resistance to nematode infections. Further analyses identified six amino acid substitutions in the antigen binding site of DRB1*1101 that were significantly associated with reductions in the numbers of adult T. circumcincta.     

Glycoprotein 63 (gp63) genes show gene conversion and reveal the evolution of Old World Leishmania

Species of the subgenus Leishmania (Leishmania) cause the debilitating disease leishmaniasis on four continents. Species grouped within the Leishmania donovani complex cause visceral leishmaniasis, a life-threatening disease, often associated with poverty, and affecting some 0.5 million people each year. The Leishmania glycoprotein GP63, or major surface protease, is a metalloprotease involved in parasite survival, infectivity and virulence. Here, we show that evolution of the gp63 multigene family is influenced by mosaic or fragmental gene conversion. This is a major evolutionary force for both homogenisation and for generating diversity, even in the absence of sexual reproduction. We propose here that the high GC content at the third codon position in the gp63 family of Old World Leishmania may be higher in multicopy regions, under the biased gene conversion model, because increased copy numbers may lead to increased rates of recombination. We confirm that one class of gp63 genes with an extended 3'end signal, gp63(EXT), reveals genetic groups within the complex and gives insights into evolution and host associations. Gp63(EXT) genes can also provide the basis for rapid and reliable genotyping of strains in the L. donovani complex. Our results confirmed that a more stringent definition of Leishmania infantum is required and that the species Leishmania archibaldi should be suppressed.     

Behavioural syndromes differ predictably between 12 populations of three-spined stickleback

Animals often differ in suites of correlated behaviours, comparable with how humans differ in personality. Constraints on the architecture of behaviour have been invoked to explain why such 'behavioural syndromes' exist. From an adaptationist viewpoint, however, behavioural syndromes should evolve only in those populations where natural selection has favoured such trait covariance, and they should therefore exist only in particular types of population. A comparative approach was used to examine this prediction of the adaptive hypothesis. We measured behavioural correlations in 12 different populations of three-spined stickleback (Gasterosteus aculeatus) and assessed whether they indeed varied consistently according to the selective environment, where population was unit of analysis. For a sample of fry from each population, we measured five different behaviours within the categories of (i) aggression (towards conspecifics); (ii) general activity; and (iii) exploration-avoidance (of novel foods, novel environments and altered environments). We show that behavioural syndromes are not always the same in different types of stickleback population: the often-documented syndrome between aggressiveness, activity and exploratory behaviour existed only in large ponds where piscivorous predators were present. In small ponds where predators were absent, these behaviours were not (or only weakly) associated. Our findings imply that population variation in behavioural syndromes does not result from stochastic evolutionary processes, but may result instead from adaptive evolution of behaviour favouring what should prove to be optimal trait combinations.     

Early ontogenetic patterns in chemical defense in Plantago (Plantaginaceae): genetic variation and trade-offs

Predictions based on the plant age and growth-differentiation balance hypotheses of defense were tested in two congeneric species, Plantago lanceolata and P. major, by quantifying iridoid glycosides, defensive chemicals, in seeds and leaves during the first 6 wk of growth. Concentrations decreased from the seed to 2-wk-old seedling stage in P. lanceolata, but increased during this period in P. major. In both species, levels were similar for 2- and 4-wk-old plants, then significantly increased from 4 to 6 wk. Genetic variation in the ontogeny of iridoid glycoside production was significant in both species at the maternal family level and at the population level. To examine whether allocation costs could explain the low production of iridoid glycosides in seedlings, relationships between growth and defense (iridoid glycosides) were characterized. Growth and defense had a positive or null relationship in all age groups, indicating that there was no trade-off in these plants at any age. This study provides some support for the growth-differentiation balance hypothesis, but offers no support for the plant age hypothesis. Measuring how herbivory affects plant fitness at different ontogenetic stages may shed light on these patterns in Plantago and on the evolution of the ontogeny of defense.