In the last two decades, the widespread application of genetic and genomic approaches has revealed a bacterial world astonishing in its ubiquity and diversity. This review examines how a growing knowledge of the vast range of animal–bacterial interactions, whether in shared ecosystems or intimate symbioses, is fundamentally altering our understanding of animal biology. Specifically, we highlight recent technological and intellectual advances that have changed our thinking about five questions: how have bacteria facilitated the origin and evolution of animals; how do animals and bacteria affect each other’s genomes; how does normal animal development depend on bacterial partners; how is homeostasis maintained between animals and their symbionts; and how can ecological approaches deepen our understanding of the multiple levels of animal–bacterial interaction. As answers to these fundamental questions emerge, all biologists will be challenged to broaden their appreciation of these interactions and to include investigations of the relationships between and among bacteria and their animal partners as we seek a better understanding of the natural world

One of the most pervasive challenges in molecular phylogenetics is the incongruence between phylogenies obtained using different data sets, such as individual genes. To systematically investigate the degree of incongruence, and potential methods for resolving it, we screened the genome sequences of eight yeast species and selected 106 widely distributed orthologous genes for phylogenetic analyses, singly and by concatenation. Our results suggest that data sets consisting of single or a small number of concatenated genes have a significant probability of supporting conflicting topologies. By contrast, analyses of the entire data set of concatenated genes yielded a single, fully resolved species tree with maximum support. Comparable results were obtained with a concatenation of a minimum of 20 genes; substantially more genes than commonly used but a small fraction of any genome. These results have important implications for resolving branches of the tree of life.

Choanoflagellates are the closest known relatives of metazoans. To discover potential molecular mechanisms underlying the evolution of metazoan multicellularity, we sequenced and analysed the genome of the unicellular choanoflagellate Monosiga brevicollis. The genome contains approximately 9,200 intron-rich genes, including a number that encode cell adhesion and signalling protein domains that are otherwise restricted to metazoans. Here we show that the physical linkages among protein domains often differ between M. brevicollis and metazoans, suggesting that abundant domain shuffling followed the separation of the choanoflagellate and metazoan lineages. The completion of the M. brevicollis genome allows us to reconstruct with increasing resolution the genomic changes that accompanied the origin of metazoans.Choanoflagellates have long fascinated evolutionary biologists for their marked similarity to the 'feeding cells' (choanocytes) of sponges and the possibility that they might represent the closest living relatives of metazoans 1,2 . Over the past decade or so, evidence supporting this relationship has accumulated from phylogenetic analyses of nuclear and mitochondrial genes [3][4][5][6] , comparative genomics between the mitochondrial genomes of choanoflagellates, sponges and other metazoans 7,8 , and the finding that choanoflagellates express homologues of metazoan signalling and adhesion genes 9-12 . Furthermore, species-rich phylogenetic analyses demonstrate that choanoflagellates are not derived from metazoans, but instead represent a distinct lineage that evolved before the origin and diversification of metazoans (Fig. 1a, Supplementary Fig. 1 and Supplementary Note 3.1) 8,13 . By virtue of their position on the tree of life, studies of choanoflagellates provide an unparallelled window into the nature of the unicellular and colonial progenitors of metazoans 14 .Choanoflagellates are abundant and globally distributed microbial eukaryotes found in marine and freshwater environments 15,16 . Like sponge choanocytes, each cell bears an apical flagellum surrounded by a distinctive collar of actin-filled microvilli, with which choanoflagellates trap bacteria and detritus (Fig. 1b). Using this highly effective means of prey capture, choanoflagellates link bacteria to higher trophic levels and thus have critical roles in oceanic carbon cycling and in the microbial food web 17,18 .More than 125 choanoflagellate species have been identified, and all species have a unicellular life-history stage. Some can also form simple colonies of equipotent cells, although these differ substantially from the obligate associations of differentiated cells in metazoans 19 . Studies of basal metazoans indicate that the ancestral metazoan was multicellular and had differentiated cell types, an epithelium, a body plan and regulated development including gastrulation. In contrast, the last common ancestor of choanoflagellates and metazoans was unicellular or possibly capable of forming simple colonies, underscoring the abundant biologi...