Genome evolution in allotetraploid Nicotiana

The nuclear cytoplasmic interaction (NCI) hypothesis of genome evolution and speciation in plants states that newly formed allopolyploids pass through a bottleneck of sterility and the fertile plants that emerge are fixed for species‐specific chromosome translocations. These translocations restore fertility and reduce negative effects of the maternal cytoplasm on an alien paternal genome. Using fluorescent in situ hybridization and genomic in situ hybridization and by reviewing published data, we test the NCI hypothesis using three natural Nicotiana allotetraploids (all 2n = 4x = 48, N. arentsii, N. rustica and several genotypes, including a feral plant and cultivars, of N. tabacum (tobacco)). We compare these data with three synthetic tobacco plants (Th37) that are F3 descendent progeny of an allotetraploid formed from ♀N. sylvestris (2n = 24) ×♂N. tomentosiformis (2n = 24). No intergenomic translocations were observed in N. arentsii and N. rustica. An analysis of subtelomeric tandem repeats in these allotetraploids and their putative parents shows minimal genetic changes; those that do occur may reflect evolution in the diploids or the polyploids subsequent to allopolyploidy. All natural N. tabacum genotypes have intergenomic translocations. This may reflect a large ‘genomic‐shock’ generated by allopolyploidy involving widely diverged parental species. Two of three synthetic tobacco plants had a translocation similar to that found in all cultivars of tobacco. This translocation may be significant in tobacco fertility and may have been fixed early in tobacco's evolution. But it is lacking in the feral tobacco, which might indicate a polyphyletic origin or early divergence from all cultivars examined. Overall, only in tobacco is there any evidence that NCI may have influenced genome evolution, and here further data are required to verify chromosome identity. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 82 , 599–606.     

Importance of colour in the reaction of passerine predators to aposematic prey: experiments with mutants of Pyrrhocoris apterus (Heteroptera)

Persistent questions concerning the warning coloration of unpalatable insects address whether the bright aposematic colour itself or its combination with a species-specific dark pattern is the key factor in their protection against insectivorous birds, and how chromatic polymorphism originates and is maintained in aposematics. In the present study, these questions were tested experimentally, using the birds Parus major , Parus caeruleus , Erithacus rubecula , and Sylvia atricapilla as predators, and chromatically polymorphic firebug Pyrrhocoris apterus : red wild form, white, yellow, and orange mutants (all four of them with the same black melanin pattern, the mutants differing in colour of pteridine pigments only) and the nonaposematic brown-painted wild form as prey. The results show that a specific colour is essential for the birds to recognize the specific aposematic prey; the melanin pattern is not sufficient. White mutants were no better protected than nonaposematic firebugs; red wild-type and orange mutants were equally well protected against all bird species; and the reaction of birds to yellow mutants was species-specific. An evolutionary scenario of 'recurrent recessive mutations' is formulated to explain the origin of colour polymorphism in some aposematics.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 88 , 143–153.     

Mechanisms of reinforcement in natural and simulated polymorphic populations

Reinforcement speciation is the process whereby selection against hybrids drives the evolution of enhanced pre‐mating reproductive isolation. Work has focused on divergent mating preferences (assortative mating) but pre‐mating isolation can also arise via various migration modification behaviours, such as divergent habitat preferences. The relative importance of these two different mechanisms of reinforcement remains unclear. A recent theoretical model (Yukilevich–True model) found that relative fixation probabilities between these mechanisms can vary. Additionally, natural populations of Timema cristinae walking‐sticks exhibit variation (polymorphism) in both mechanisms, generating questions about the patterns expected for allele frequencies prior to fixation, during the early stages of the speciation process. In the present study, we report: (1) new analyses examining the correlation between fixation probabilities for assortative mating and migration modification in the Yukilevich–True model; (2) novel simulations examining allele frequencies in polymorphic populations; and (3) empirical patterns of reinforcement in T. cristinae in the context of theoretical predictions. Simulations of both types yielded congruent results, revealing that the outcome of reinforcement was dependent on the strength of selection. Under weak selection, reinforcement by either mechanism is unlikely. Under intermediate selection, the conditions favoring the rise and fixation of one mechanism favored the rise and fixation of the other. However, assortative mating evolved somewhat more readily than migration modification. Populations of T. cristinae, which experience such intermediate selection, supported these predictions. Under strong selection, the evolution of migration modification generally interfered with the evolution of assortative mating by decreasing migration between populations, thereby reducing selection for assortative mating. Congruence of the results for allele frequencies versus fixation probabilities suggests that similar patterns of reinforcement are expected during different stages of the speciation process. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 305–319.     

Growth and phenology of mature temperate forest trees in elevated CO2

Are mature forest trees carbon limited at current CO₂ concentrations? Will ‘mid‐life’, 35 m tall deciduous trees grow faster in a CO₂‐enriched atmosphere? To answer these questions we exposed ca. 100‐year‐old temperate forest trees at the Swiss Canopy Crane site near Basel, Switzerland to a ca. 540 ppm CO₂ atmosphere using web‐FACE technology. Here, we report growth responses to elevated CO₂ for 11 tall trees (compared with 32 controls) of five species during the initial four treatment years. Tested across all trees, there was no CO₂ effect on stem basal area (BA) increment (neither when tested per year nor cumulatively for 4 years). In fact, the 4th year means were almost identical for the two groups. Stem growth data were standardized by pretreatment growth (5 years) in order to account for a priori individual differences in vigor. Although this experiment was not designed to test species specific effects, one species, the common European beech, Fagus sylvatica, showed a significant growth enhancement in the first year, which reoccurred during a centennial drought in the third year. None of the other dominant species (Quercus petraea, Carpinus betulus) showed a growth response to CO₂ in any of the 4 years or for all years together. The inclusion or exclusion of single individuals of Prunus avium and Tilia platyphyllos did not change the picture. In elevated CO₂, lateral branching in terminal shoots was higher in Fagus in 2002, when shoots developed from buds that were formed during the first season of CO₂ enrichment (2001), but there was no effect in later years and no change in lateral branching in any of the other species. In Quercus, there was a steady stimulation of leading shoot length in high‐CO₂ trees. Phenological variables (bud break, leaf fall, leaf duration) were highly species specific and were not affected by elevated CO₂ in any consistent way. Our 4‐year data set reflects a very dynamic and species‐specific response of tree growth to a step change in CO₂ supply. Stem growth after 4 years of exposure does not support the notion that mature forest trees will accrete wood biomass at faster rates in a future CO₂‐enriched atmosphere.     

Reactions of passerine birds to aposematic and non-aposematic firebugs (Pyrrhocoris apterus ; Heteroptera)

In spite of the existence of many experimental studies on the function of warning coloration in insects, little is known about the universality of reactions of different predators towards a particular warning signal. Reactions of nine passerine bird species, namely Parus major , Parus caeruleus , Aegithalos caudatus , Erithacus rubecula , Turdus merula , Sylvia atricapilla , Fringilla coelebs , Carduelis chloris and Emberiza citrinella , to the firebug Pyrrhocoris apterus wildtype (brachypterous adults) and its artificially obtained (painted) brown non-aposematic variant were compared. Most insectivorous birds (great tits, blue tits, robins and blackcaps) distinguished between aposematic and non-aposematic bugs, attacking the former less often. Partly granivorous buntings and finches did not distinguish between them, and attacked both variants equally. As all the birds were caught in the wild, the results can be interpreted in terms of the presence of a higher proportion of experienced individuals among insectivorous than among omnivorous species. Two insectivorous species differed from others. The heaviest blackbird attacked and killed aposematic as well as non-aposematic firebugs, and, in contrast, the lightest long-tailed tit avoided both variants. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 78 , 517–525.