Strong selection on male plumage in a hybrid zone between a hybrid bird species and one of its parents.

Homoploid hybrid speciation (HHS) requires reproductive barriers between hybrid and parent species, despite incomplete reproductive isolation (RI) between the parents. Novel secondary sexual trait values in hybrids may cause prezygotic isolation from both parents, whereas signals inherited by the hybrid from one parent species may cause prezygotic isolation with the other. Here we investigate whether differences in male plumage function as a premating barrier between the hybrid Italian sparrow and one of its parent species, the house sparrow, in a narrow Alpine hybrid zone. Italian sparrow male plumage is a composite mosaic of the parental traits, with its head plumage most similar to its other parent, the Spanish sparrow. We use geographical cline analysis to examine selection on three plumage traits, 75 nuclear single nucleotide polymorphisms (SNPs) and hybrid indices based on these SNPs. Several SNPs showed evidence of restricted introgression in the Alps, supporting earlier findings. Crown colour exhibited the narrowest plumage cline, representing a 37% (range 4-65%) drop in fitness. The cline was too narrow to be due to neutral introgression. Only crown colour was significantly bimodal in the hybrid zone. Bimodality may be due to RI or a major QTL, although fitness estimates suggest that selection contributes to the pattern. We discuss the implications with respect to HHS and the species status of the Italian sparrow.

Local adaptation within a hybrid species.

Ecological divergence among populations may be strongly influenced by their genetic background. For instance, genetic admixture through introgressive hybridization or hybrid speciation is likely to affect the genetic variation and evolvability of phenotypic traits. We studied geographic variation in two beak dimensions and three other phenotypic traits of the Italian sparrow (Passer italiae), a young hybrid species formed through interbreeding between house sparrows (P. domesticus) and Spanish sparrows (P. hispaniolensis). We found that beak morphology was strongly influenced by precipitation regimes and that it appeared to be the target of divergent selection within Italian sparrows. Interestingly, however, the degree of parental genetic contribution in the hybrid species had no effect on phenotypic beak variation. Moreover, beak height divergence may mediate genetic differentiation between populations, consistent with isolation-by-adaptation within this hybrid species. The study illustrates how hybrid species may be relatively unconstrained by their admixed genetic background, allowing them to adapt rapidly to environmental variation.

Phylogenetic isolation of host trees affects assembly of local Heteroptera communities.

A host may be physically isolated in space and then may correspond to a geographical island, but it may also be separated from its local neighbours by hundreds of millions of years of evolutionary history, and may form in this case an evolutionarily distinct island. We test how this affects the assembly processes of the host's colonizers, this question being until now only invoked at the scale of physically distinct islands or patches. We studied the assembly of true bugs in crowns of oaks surrounded by phylogenetically more or less closely related trees. Despite the short distances (less than 150 m) between phylogenetically isolated and non-isolated trees, we found major differences between their Heteroptera faunas. We show that phylogenetically isolated trees support smaller numbers and fewer species of Heteroptera, an increasing proportion of phytophages and a decreasing proportion of omnivores, and proportionally more non-host-specialists. These differences were not due to changes in the nutritional quality of the trees, i.e. species sorting, which we accounted for. Comparison with predictions from meta-community theories suggests that the assembly of local Heteroptera communities may be strongly driven by independent metapopulation processes at the level of the individual species. We conclude that the assembly of communities on hosts separated from their neighbours by long periods of evolutionary history is qualitatively and quantitatively different from that on hosts established surrounded by closely related trees. Potentially, the biotic selection pressure on a host might thus change with the evolutionary proximity of the surrounding hosts.

Speciation through evolution of sex-linked genes.

Identification of genes involved in reproductive isolation opens novel ways to investigate links between stages of the speciation process. Are the genes coding for ecological adaptations and sexual isolation the same that eventually lead to hybrid sterility and inviability? We review the role of sex-linked genes at different stages of speciation based on four main differences between sex chromosomes and autosomes; (1) relative speed of evolution, (2) non-random accumulation of genes, (3) exposure of incompatible recessive genes in hybrids and (4) recombination rate. At early stages of population divergence ecological differences appear mainly determined by autosomal genes, but fast-evolving sex-linked genes are likely to play an important role for the evolution of sexual isolation by coding for traits with sex-specific fitness effects (for example, primary and secondary sexual traits). Empirical evidence supports this expectation but mainly in female-heterogametic taxa. By contrast, there is clear evidence for both strong X- and Z-linkage of hybrid sterility and inviability at later stages of speciation. Hence genes coding for sexual isolation traits are more likely to eventually cause hybrid sterility when they are sex-linked. We conclude that the link between sexual isolation and evolution of hybrid sterility is more intuitive in male-heterogametic taxa because recessive sexually antagonistic genes are expected to quickly accumulate on the X-chromosome. However, the broader range of sexual traits that are expected to accumulate on the Z-chromosome may facilitate adaptive speciation in female-heterogametic species by allowing male signals and female preferences to remain in linkage disequilibrium despite periods of gene flow.

Premating barriers to gene exchange and their implications for the structure of a mosaic hybrid zone between Chorthippus brunneus and C. jacobsi (Orthoptera: Acrididae).

Many hybrid zones contain a deficit of hybrid genotypes relative to expectations from tension zone models. This is often associated with separation of parental genotypes into distinct habitats (mosaicism), but sometimes parentals can be found co-occuring in the same local population (bimodality). In both cases, prezygotic isolation may play an important role in determining the genotypic composition of the zone. Chorthippus brunneus and C. jacobsi (Orthoptera: Acrididae) meet and form a complex hybrid zone in northern Spain. Analysis of stridulatory peg numbers reveals partial spatial and seasonal isolation in a 25 km2 area of the zone: C. jacobsi phenotypes predominate in June and July and are present in both valley and mountain habitats; C. brunneus phenotypes predominate in August and are restricted to valley habitats, always in sympatry with C. jacobsi. Strong assortative mating was observed in laboratory mating experiments. Spatial, seasonal and behavioural isolation combine to produce strong premating isolation in the study area. These results suggest a role for premating isolation in maintaining both the mosaic structure and bimodality of this hybrid zone.

The management of phobic disorders in general practice.

Over two thousand patients have been treated with a flexible drug regime, having presented with a wide spectrum of disorders. Common aetiology was found in these patients and all could be said to be phobic, and as such many produced "side-effects" which could by predicted with accuracy--and overcome. A method is described for inducing increased patient compliance and decreased drop-out rate.

A preliminary report on the use of clomipramine in general practice.

The use of clomipramine in a large suburban general practice is reviewed. Three hundred and fifty patients have been treated to date out of a total practice population of twenty-one thousand. It is argued that phobic anxiety states are much commoner than is normally supposed and that they are usually associated with a history of separation or rejection in childhood. A combined treatment regime is employed for one month thereafter clomipramine alone is used. Side-effects may initially present a problem although they may not all be truly drug induced. Some patients use side-effects to manipulate the clinical situation. However proper interpretative management of side effects can assist the clinicians in persuading patients to continue therapy. Some impressive results have been obtained with clomipramine therapy. Illustrative case histories are provided.