A volvox inversionless mutant highlights the importance of the extracellular matrix in morphogenesis.

Volvox is a multicellular spherical green alga that is closely related to the more widely studied unicellular Chlamydomonas. Volvox is a fascinating creature that serves as a useful model system for investigating the evolution of multicellularity and the regulation of cell differentiation (reviewed in Kirk and Nishii, 2001). An adult Volvox, called a spheroid, consists of several thousand small biflagellate somatic cells and ;16 large asexual reproductive cells (gonidia) regularly positioned in a transparent sphere of glycoprotein-rich extracellular matrix (ECM). The name Volvox (fierce roller) refers to the characteristic forward rolling motion resulting from the strict orientation of the somatic cells within the ECM. A striking characteristic of Volvox development is that new embryos produced from the gonidia begin life inside out and must undergo inversion to achieve the proper adult conformation. The inversion process is similar to gastrulation, a dramatic restructuring that occurs early in the development of animal embryos. Ueki and Nishii (pages 1166–1181) characterize an inversionlessmutantofV. carteri called InvB (see figure). The disrupted gene in this mutant, invB, encodes a nucleotide-sugar transporter involved in synthesis of glycosylated proteins. Early development and inversion of the V. carteri embryo occurs within a glycoprotein-rich gonidial vesicle. The gonidial vesicle surrounding normal embryos expands during early embryogenesis, and the authors show that expansion is crucial for inversion; vesicles in the mutant fail to enlarge and physically restrict the ability of the embryo to invert. This was confirmed in microsurgery experiments wherein mutant embryos with the gonidial vesicles removed were subsequently able to undergo inversion. The authors show that the heterologous InvB protein expressed in Saccharomyces cerevisiae exhibits GDP-mannose transporter activity and that the gonidial vesicles of the V. carteri InvB mutant appear to contain much less mannose than wild-type vesicles. This suggests that InvB functions in the transport of GDP-mannose for glycosylation of proteins necessary for the development and expansion of the gonidial vesicle. This work highlights the importance of developmental regulation of the ECM (which is also critical during embryogenesis and organogenesis of higher plants and animals) and identifies a GDP-mannose transporter required for normal embryogenesis in V. carteri.