Large global effective population sizes in Paramecium.

The genetic effective population size (N(e)) of a species is an important parameter for understanding evolutionary dynamics because it mediates the relative effects of selection. However, because most N(e) estimates for unicellular organisms are derived either from taxa with poorly understood species boundaries or from host-restricted pathogens and most unicellular species have prominent phases of clonal propagation potentially subject to strong selective sweeps, the hypothesis that N(e) is elevated in single-celled organisms remains controversial. Drawing from observations on well-defined species within the genus Paramecium, we report exceptionally high levels of silent-site polymorphism, which appear to be a reflection of large N(e).

Who came first--larvae or adults? origins of bilaterian metazoan larvae.

There is a classic controversy in zoology over whether the common ancestor of living bilaterian phyla was a benthic animal with a bilaterian body plan, or was a pelagic larva-like animal similar to what we see today in the primary larvae of indirect-developing bilaterians. We examine the current larva-like adult hypothesis, and present an alternate model for the evolution of complex life histories by intercalation of larval features into the ontogeny of an ancestral direct-developing bilaterian. This gradual accumulation of larval features results in a developmental regulatory program that produces a larva distinct in body plan from the adult. The evolution of a rapid and complete metamorphosis is made possible by the convergent evolution of set aside cells in the final stages of the emergence of indirect developing larval forms. Although convergences abound either hypothesis for the evolution of developmental pathways and life histories, the bilaterian first hypothesis is consistent with all stages of evolution of a complex life history being selectively advantageous, with the rapid evolution of larval forms, and with the frequent co-option of genes from the adult phase of the life cycle prevalent in the evolution of embryos and larvae.