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.

No abstract

Selection pressures proposed to account for the convergent evolution of self/not-self recognition systems in lower organisms include defense against microbial parasites and somatic cell variants. Direct support for the existence of somatic cell parasites in natural populations has been lacking. I here report the occurrence of a somatic cell parasite in the cellular slime mold Dictyostelium mucoroides and discuss the implications of this phenomenon to the evolution of mechanisms of somatic tissue compatibility.Somatic mutations may arise in any proliferating cell lineage. In many organisms, such variants may become incorporated into gametes or asexual propagules. An organism that cannot control the proliferation of somatic variants could actually be parasitized by a member of its own species. I here report the occurrence of one such somatic cell parasite in natural populations of the cellular slime mold Dictyostelium mucoroides, review the potential fitness costs and benefits of somatic variation, and argue that somatic cell parasitism may be an important selective force underlying the convergent evolution of self/ not-self compatibility mechanisms in lower organisms. METHODSThere are three possible fates of any somatic variant (1). The mutant may be incapable ofincreasing in frequency in the presence of the cell lineage from which it arose and hence will be eliminated. This is presumably the fate of most mutations. Alternatively, the mutant lineage may be capable of increasing when rare in the presence ofthe original lineage and eventually establishing an independent existence by incorporation into gametes or asexual propagules. However, if such a form is incapable ofmaintaining itselfwithout the presence ofthe original form, the variant could still persist in natural populations by acting as a somatic cell parasite on competent forms (1). Demonstration that such a condition occurs requires (a) collection of chimeric individuals from natural populations and (b) demonstration that the mutant individual can increase when rare in the presence of the original cell line and persist within the soma of susceptible individuals for extended intervals. The following methods were developed to determine whether these conditions are realized in natural populations of cellular slime molds.Field Sampling. The detection of individual fruiting bodies in natural populations of the cellular slime mold D. mucoroides has never been reported, to my knowledge. The small size of fruiting bodies, the free-living nature of the amoebae, and the cryptic habit of occupying the interstices of the soil render direct methods of field sampling ineffective (2). Hence, an indirect method was developed which exploits the fact that amoebae will travel a limited, species-specific distance to produce an aggregation (i.e., the aggregation territory) (3). If a sample obtained on this scale yields a large number of slime mold isolates, the conclusion is inescapable that the sampling includes either a large number of amoebae, an aggregation cen...

Species of ectoprocts and solitary encrusting animals were subjected in aquaria to homogenates of 11 sympatric species of sponges and colonial ascidians. Five of the nine sponge species and one of the two ascidian species exhibited species-specific allelochemical effects. Evidence suggests that allelochemical interactions provide a widespread, specific, and complex mechanism for interference competition for space among natural populations of coral reef organisms. The existence of such species-specific mechanisms may provide a basis for maintenance of diversity in space-limited systems in the absence of high levels of predation and physical disturbance.Space on which to live is often the most important limiting resource in marine hard-substrate environments. Maintenance of a given level of diversity in these environments has been attributed to the interacting roles of competition, predation, and disturbance (1, 2

No abstract