Microglia play key roles in brain development, homeostasis, and function, and it is widely assumed that the adult population is long lived and maintained by self-renewal. However, the precise temporal and spatial dynamics of the microglial population are unknown. We show in mice and humans that the turnover of microglia is remarkably fast, allowing the whole population to be renewed several times during a lifetime. The number of microglial cells remains steady from late postnatal stages until aging and is maintained by the spatial and temporal coupling of proliferation and apoptosis, as shown by pulse-chase studies, chronic in vivo imaging of microglia, and the use of mouse models of dysregulated apoptosis. Our results reveal that the microglial population is constantly and rapidly remodeled, expanding our understanding of its role in the maintenance of brain homeostasis.

Hybridization between species can lead to introgression of genes from one species to another, providing a potential mechanism for preserving and recombining key traits during evolution. To determine the molecular basis of such transfers, we analyzed a natural polymorphism for flower-head development in Senecio. We show that the polymorphism arose by introgression of a cluster of regulatory genes, the RAY locus, from the diploid species S. squalidus into the tetraploid S. vulgaris. The RAY genes are expressed in the peripheral regions of the inflorescence meristem, where they promote flower asymmetry and lead to an increase in the rate of outcrossing. Our results highlight how key morphological and ecological traits controlled by regulatory genes may be gained, lost, and regained during evolution.

Genomic scans for selection are a useful tool for identifying genes underlying phenotypic transitions. In this article, we describe the results of a genome scan designed to identify candidates for genes targeted by selection during the evolution of cultivated sunflower. This work involved screening 492 loci derived from ESTs on a large panel of wild, primitive (i.e., landrace), and improved sunflower (Helianthus annuus) lines. This sampling strategy allowed us to identify candidates for selectively important genes and investigate the likely timing of selection. Thirty-six genes showed evidence of selection during either domestication or improvement based on multiple criteria, and a sequence-based test of selection on a subset of these loci confirmed this result. In view of what is known about the structure of linkage disequilibrium across the sunflower genome, these genes are themselves likely to have been targeted by selection, rather than being merely linked to the actual targets. While the selection candidates showed a broad range of putative functions, they were enriched for genes involved in amino acid synthesis and protein catabolism. Given that a similar pattern has been detected in maize (Zea mays), this finding suggests that selection on amino acid composition may be a general feature of the evolution of crop plants. In terms of genomic locations, the selection candidates were significantly clustered near quantitative trait loci (QTL) that contribute to phenotypic differences between wild and cultivated sunflower, and specific instances of QTL colocalization provide some clues as to the roles that these genes may have played during sunflower evolution.

Although the evolutionary importance of natural hybridization has been debated for decades, it has become increasingly clear that hybridization plays a fundamental role in the evolution of many plant and animal taxa, sometimes resulting in the formation of entirely new species. Although some hybrid species retain the base chromosome number of their parents, others combine the full chromosomal complements of their progenitors. Hybrid speciation can thus produce two fundamentally different types of evolutionary lineages, yet relatively little is known about the factors influencing ploidy level in hybrid neospecies. We estimated genetic divergence between species pairs that have given rise to homoploid and polyploid hybrid species and found that divergence is significantly greater for the parents of polyploids, even after controlling for potentially confounding factors. Our data thus provide the first direct evidence in support of the notion that the extent of genomic divergence between hybridizing species influences the likelihood of diploid versus polyploid hybrid speciation.

The domestication and improvement of crop plants have long fascinated evolutionary biologists, geneticists, and anthropologists. In recent years, the development of increasingly powerful molecular and statistical tools has reinvigorated this now fast‐paced field of research. In this paper, we provide an overview of how such tools have been applied to the study of crop evolution. We also highlight lessons that have been learned in light of a few long‐standing and interrelated hypotheses concerning the origins of crop plants and the nature of the genetic changes underlying their evolution. We conclude by discussing compelling evolutionary genomic approaches that make possible the efficient and unbiased identification of genes controlling crop‐related traits and provide further insight into the actual timing of selection on particular genomic regions.