As arguably the simplest free-living animals, placozoans may represent a primitive metazoan form, yet their biology is poorly understood. Here we report the sequencing and analysis of the approximately 98 million base pair nuclear genome of the placozoan Trichoplax adhaerens. Whole-genome phylogenetic analysis suggests that placozoans belong to a 'eumetazoan' clade that includes cnidarians and bilaterians, with sponges as the earliest diverging animals. The compact genome shows conserved gene content, gene structure and synteny in relation to the human and other complex eumetazoan genomes. Despite the apparent cellular and organismal simplicity of Trichoplax, its genome encodes a rich array of transcription factor and signalling pathway genes that are typically associated with diverse cell types and developmental processes in eumetazoans, motivating further searches for cryptic cellular complexity and/or as yet unobserved life history stages.

Sex determination in maize is controlled by a developmental cascade leading to the formation of unisexual florets derived from an initially bisexual floral meristem. Abortion of pistil primordia in staminate florets is controlled by a tasselseed-mediated cell death process. We positionally cloned and characterized the function of the sex determination gene tasselseed1 (ts1). The TS1 protein encodes a plastid-targeted lipoxygenase with predicted 13-lipoxygenase specificity, which suggests that TS1 may be involved in the biosynthesis of the plant hormone jasmonic acid. In the absence of a functional ts1 gene, lipoxygenase activity was missing and endogenous jasmonic acid concentrations were reduced in developing inflorescences. Application of jasmonic acid to developing inflorescences rescued stamen development in mutant ts1 and ts2 inflorescences, revealing a role for jasmonic acid in male flower development in maize.

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Mitochondrial genomes of multicellular animals are typically 15-to 24-kb circular molecules that encode a nearly identical set of 12-14 proteins for oxidative phosphorylation and 24 -25 structural RNAs (16S rRNA, 12S rRNA, and tRNAs). These genomes lack significant intragenic spacers and are generally without introns. Here, we report the complete mitochondrial genome sequence of the placozoan Trichoplax adhaerens, a metazoan with the simplest known body plan of any animal, possessing no organs, no basal membrane, and only four different somatic cell types. Our analysis shows that the Trichoplax mitochondrion contains the largest known metazoan mtDNA genome at 43,079 bp, more than twice the size of the typical metazoan mtDNA. The mitochondrion's size is due to numerous intragenic spacers, several introns and ORFs of unknown function, and protein-coding regions that are generally larger than those found in other animals. Not only does the Trichoplax mtDNA have characteristics of the mitochondrial genomes of known metazoan outgroups, such as chytrid fungi and choanoflagellates, but, more importantly, it shares derived features unique to the Metazoa. Phylogenetic analyses of mitochondrial proteins provide strong support for the placement of the phylum Placozoa at the root of the Metazoa.animal evolution ͉ phylogenetics T richoplax adhaerens [Shulze 1883] is a marine invertebrate distributed in tropical waters worldwide (1-3). It is the simplest known free-living animal, displaying no axis of symmetry, lacking a basal membrane, possessing only four somatic cell types (4-6), and having one of the smallest known animal genomes (7-9). Until recently, T. adhaerens was the sole representative of the phylum Placozoa, but recent field studies and molecular analyses indicate genetic diversity underlying apparent morphological uniformity within the Placozoa (3, 10). In the laboratory, placozoans reproduce asexually by either binary fission or budding dispersive propagules called swarmers. Eggs have been observed, and recent DNA polymorphism analysis has provided evidence for sexual reproduction within the Placozoa (10).The phylogenetic placement of Placozoa among the metazoans, i.e., the animals, remains unresolved. In particular, its placement among lower metazoans, that is, the phyla Cnidaria, Ctenophora, and Porifera, has been controversial. Most studies place Porifera at the base of the metazoan tree of life (11-15), but others support placozoans as one of the earliest branching lineages of . Conflicting data, including 18S, 28S, and 16S analysis, have suggested that Placozoa form a sister clade to all bilaterians or a sister clade to both cnidarians and bilaterians (14,(21)(22)(23)(24)(25)(26)(27).Comparative mitochondrial genomics is becoming an effective tool to resolve phylogenetic placements because of several unique properties of mitochondrial genomes, including uniparental inheritance, orthologous genes, and lack of substantial intermolecular recombination (reviewed in refs. 28-30). Although some have questioned th...

Summary Colonial marine invertebrates, such as sponges, corals, bryozoans, and ascidians, often live in densely-populated communities where they encounter other members of their species as they grow over their substratum. Such encounters typically lead to a natural histocompatibility response in which colonies either fuse to become a single, chimeric colony or reject and aggressively compete for space. These allorecognition phenomena mediate intraspecific competition [1–3], support allotypic diversity [4], control the level at which selection acts [5–8], and resemble allogeneic interactions in pregnancy and transplantation [9–12]. Despite the ubiquity of allorecognition in colonial phyla, however, its molecular basis has not been identified beyond what is currently known about histocompatibility in vertebrates and protochordates. We positionally cloned an allorecognition gene using inbred strains of the cnidarian, Hydractinia symbiolongicarpus, which is a model system for the study of invertebrate allorecognition. The gene identified encodes a putative transmembrane receptor expressed in all tissues capable of allorecognition that is highly polymorphic and predicts allorecognition responses in laboratory and field-derived strains. This study reveals that a previously undescribed hypervariable molecule bearing three extracellular domains with greatest sequence similarity to the immunoglobulin superfamily is an allodeterminant in a lower metazoan.