Yale Bulletin and Calendar

November 17, 2006|Volume 35, Number 11|Two-Week Issue

Trichoplax adherens represent descendents
of the oldest multi-celled animal.

Scientists trace evolution of one of
the earliest multi-cellular life forms

Yale scientists have offered new evidence that placozoans, animals that have only four body cell types and no structured organs, represent descendents of the oldest multi-celled animal, perhaps older even than sponges.

A recent paper in Proceedings of the National Academy of Sciences presents the complete sequence of the mitochondrial genome of Trichoplax, one of the first identified placozoans. The research was led by Stephen Dellaporta, professor of molecular, cellular and developmental biology, and Leo Buss, professor of ecology and evolutionary biology.

The scientists compared the placozoans' size, genes and gene arrangement to that of other mitochondrial genomes from "simple" multi-cellular animal life forms -- like cnidarians and sponges.

Mitochondria, often referred to as the "powerhouses" of the cell, are organelles that are characteristic of cells in higher organisms. Mitochondrial genomes have become more compact as evolution has progressed and animals have become more complex.

"The mitochondrial genome of Trichoplax, to date, is the largest found in multi-cellular animals and its structure, organization and sequence provides biomarkers that can be used to reconstruct pathways of the evolution of the earliest forms of animal life and relatedness between extant lower animal species," says Dellaporta. "It is over twice the size of mitochondrial genomes found in most animals and it contains genes, introns and spacer sequences more like those of single-celled organisms and fungi than of complex animals."

As primitive as Placozoa are, research from members of this group led by Ana Signorovitch, a recent graduate student of Buss, also showed that they contain signatures for sexual mode of reproduction not yet observed in placozoans.

The research -- an interdisciplinary effort enhanced by the expertise of investigators in fields of molecular evolution, invertebrate biology and ecology -- is part of a collaborative project on the origins of multi-cellularity that is supported by a National Science Foundation Genome-Enabled Environmental Science and Engineering Program grant and a Human Frontiers Science Program grant. Co-authors were Anthony Xu and Maria A. Moreno of Yale and Bernd Schierwater, Sven Sagasser and Wolfgang Jakob of Tieraerztliche Hochschule Hannover, Germany.

-- By Janet Rettig Emanuel

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Campus Notes