Cover image

Cover The illustration depicts a dynamic generative process in which simple geometric motifs (left side) evolve into a diverse array of forms with increasing complexity by addition and modification of new elements (right side). At each step, each existing macrostructure evolves into a new macrostructure whose form depends on the spatial layout and articulation of its basic subunits. Macrostructures share common structural elements that are conserved throughout all the generative cycles and are recognizable in subsequent stages. In this issue, a conceptually similar case is described wherein closely physically linked groups of genes, which have been retained in many species since early animal ancestors, constitute foundational ancestral structural features of modern genomes even after hundreds of millions of years. (Cover illustration by David Cantarero Tomás. [For details, see Irimia et al., pp. 2356–2367.])