Sympatric speciation is contentious and convincing examples from nature remain very scarce. Here we present multiple lines of molecular evidence supporting a sympatric origin for one of the most enigmatic birds in the world, the São Tomé grosbeak Crithagra concolor. This endemic from São Tomé Island, Gulf of Guinea, was initially placed in a monotypic genus, Neospiza. Mitochondrial data placed it as sister to the Príncipe seedeater Crithagra rufobrunneus, a Gulf of Guinea endemic with populations on the islands of São Tomé, Príncipe and Boné de Jóquei. For 29-30 seedeaters (9-10 per population), three grosbeak samples and outgroups we sequenced mitochondrial markers (c. 1000 bp), 27 nuclear introns and 6 exons (c. 20,000 bp), and we genotyped 33 microsatellite loci. All molecular markers inferred a closer relationship between the São Tomé grosbeak and the seedeater population from São Tomé, than between all three allopatric seedeater populations. Mitochondrial markers had a strong phylogenetic signal (no shared haplotypes, c. 2.5% sequence divergence between the sympatric grosbeak and seedeater). In contrast, lineage sorting of the nuclear markers was incomplete (albeit unique grosbeak mutations were also present). This pattern is consistent with sympatric speciation followed by incomplete lineage sorting of the nuclear markers rather than with past hybridization events (that would have homogenised the mtDNA faster than the nDNA).
Author(s): Videvall, E, Sletvold N, Hagenblad J, Ågren J, Hansson B
Understanding how hybridization influences phenotype is of major importance for evolutionary biology. Theoretically, phenotypic differences are expected to be controlled mainly by gene regulation, which can be environmentally or genetically determined. However, until recently it has been difficult to accurately measure expression levels and determine whether hybrid gene expression are caused by additive (intermediate expression levels between the parents), dominance (expression levels equal to one of the parents) or parental effects (gene expression similar to that of either the mother or the father). Here we use high throughput RNA-sequencing (Illumina) to test the different expression hypotheses and evaluate gene expression in hybrids between two differentiated populations of Arabidopsis lyrata. Our results showed (i) broad differential expression between populations (9573 significant genes) and (ii) a very strong maternal effect on hybrid gene expression (94.7% of genes followed the expression of the mother rather than the father plant). In plants, where seed dispersal is limited and the developing seedlings will experience similar environmental conditions as their mothers, such maternal effects are expected to be highly beneficial as they can facilitate local adaptation.
In migratory species, timing of arrival on the breeding grounds in spring is crucial for the reproductive success. Thus, in the face of a changing environment, it is important to be able to adjust the timing accordingly. However, little is known about the genetic background and hence evolutionary potential of arrival date. We have used a multi-level approach to investigate the evolutionary potential of arrival date combining data from a multigenerational pedigree and novel migration tracking techniques of a natural population of great reed warblers (Acrocephalus arundinaceus). We found that selection favours early arrival both in male and female great reed warblers, and that the trait is both repeatable and heritable. Further, arrival date in the population has advanced during the two decades of this study, a pattern that is in accordance with the response attributed to climate change reported in other migrant birds but also in accordance to the directional selection acting on the trait. Tracking the full migratory annual cycle of individual great reed warblers show that departure date from the wintering site determines arrival date and that spring migration is faster than autumn migration, corroborating the selection for earlier arrival. Our study is a first step towards dissecting the genetic and environmental factors that contribute to shape arrival date in long-distance migrant birds. Such analyses are essential if we want to understand how migratory species are able to cope with a rapidly changing environment.